Mineral Classification

Mineral Classification

kacimo samy   2:24 ص

Mineral Classification 

The 4,000 known minerals can be separated into a small number of groups, or mineral classes. You may think, “Why bother?” Classification schemes are useful because they help organize information and streamline discussion. Biologists, for example, classify animals into groups based on how they feed their young and on the architecture of their skeletons, and botanists classify plants according to the way they reproduce and by the shape of their leaves. In the case of minerals, a good means of classification eluded researchers until it became possible to determine the chemical makeup of minerals. A Swedish chemist, Baron Jöns Jacob Berzelius (1779–1848), analyzed minerals and noted chemical similarities among many of them. Berzelius, along with his students, established that most minerals can be classified by specifying the principal anion (negative ion) or anionic group (negative molecule) within the mineral. We now take a look at principal mineral classes, focusing especially on silicates, the class that constitutes most of the rock in the Earth.

The Mineral Classes 

Mineralogists distinguish several principal classes of minerals. Here are some of the major ones.

Physical characteristics of minerals.

• Silicates: The fundamental component of most silicates in the Earth’s crust is the SiO44– anionic group. A well-known example, quartz (figure above a), has the formula SiO2. 
• Oxides: Oxides consist of metal cations bonded to oxygen anions. Typical oxide minerals include hematite (Fe2O3; figure above b) and magnetite (Fe3O4; figure above g). 
• Sulfides: Sulfides consist of a metal cation bonded to a sulfide anion (S2–). Examples include galena (PbS) and pyrite (FeS2; figure above c). 
• Sulfates: Sulfates consist of a metal cation bonded to the SO42– anionic group. Many sulfates form by precipitation out of water at or near the Earth’s surface. An example is gypsum (CaSO4s (2O).
• Halides: The anion in a halide is a halogen ion (such as chloride [Cl–] or fluoride [F –]), an element from the second column from the right in the periodic table (see Appendix). Halite, or rock salt (NaCl; Fig. 3.8d), and fluorite (CaF2), a source of fluoride, are common examples. 
• Carbonates: In carbonates, the molecule CO32– serves as the anionic group. Elements such as calcium or magnesium bond to this group. The two most common carbonates are calcite (CaCO3; Fig. 3.8e) and dolomite (CaMg[CO3]2). 
• Native metals: Native metals consist of pure masses of a single metal. The metal atoms are bonded by metallic bonds. Copper and gold, for example, may occur as native metals. 

The nature of mineral cleavage and fracture.

Silicates: The Major Rock-Forming Minerals 

 The structure of silicate minerals.

Silicate minerals, or silicates, make up over 95% of the continental crust and almost 100% of the oceanic crust and of the Earth’s mantle consist almost entirely of silicates. Thus, silicates are the most common minerals on Earth. As we've noted, silicates in the Earth’s crust and upper mantle contain the SiO44– anionic group. In this group, four oxygen atoms surround a single silicon atom, thereby defining the corners of a tetrahedron, a pyramid-like shape with four triangular faces (figure above a). We refer to this anionic group as the silicon oxygen tetrahedron (or, informally, as the silica tetrahedron), and it acts, in effect, as the building block of silicate minerals. 

Mineralogists distinguish among several groups of silicate minerals based on the way in which silica tetrahedra are arranged (figure above b). The arrangement, in turn, determines the degree to which tetrahedra share oxygen atoms. Note that the number of shared oxygen determines the ratio of silicon (Si) to oxygen (O) in the mineral. Here are the groups, in order from fewer shared oxygen to more shared oxygen: 

• Independent tetrahedra: In this group, the tetrahedra are independent and do not share any oxygen atoms. The attraction between the tetrahedra and positive ions holds such minerals together. This group includes olivine, a glassy green mineral, and garnet ( 2nd figure above f). 
• Single chains: In a single-chain silicate, the tetrahedra link to form a chain by sharing two oxygen atoms. The most common of the many different types of single-chain silicates are pyroxenes (2nd figure above b). 
• Double chains: In a double-chain silicate, the tetrahedra link to form a double chain by sharing two or three oxygen atoms. Amphiboles are the most common type (2nd figure above c). 
• Sheet silicates: The tetrahedra in this group share three oxygen atoms and therefore link to form two- dimensional sheets. Other ions and, in some cases, water molecules fit between the sheets in some sheet silicates. Because of their structure, sheet silicates have cleavage in one direction, and they occur in books of very thin sheets. In this group, we find micas (2nd figure above a)  and clays. Clays occur only in extremely tiny flakes. 
• Framework silicates: In a framework silicate, each tetrahedron shares all four oxygen atoms with its neighbours, forming a three-dimensional structure. Examples include feldspar and quartz. The two most common feldspars are plagioclase, which tends to be white, gray, or blue; and orthoclase (also called potassium feldspar, or K-feldspar), which tends to be pink (1st figure above d). 

Credits: Stephen Marshak (Essentials of Geology)

How Can You Tell One Mineral From Another?

How Can You Tell One Mineral From Another?

kacimo samy   7:19 ص

How Can You Tell One Mineral From Another? 

Amateur and professional mineralogists get a kick out of recognizing minerals. They might hover around a display case in a museum and name specimens without bothering to look at the labels. How do they do it? The trick lies in learning to recognize the basic physical properties (visual and material characteristics) that distinguish one mineral from another. Some physical properties, such as shape and colour, can be seen from a distance. Others, such as hardness and magnetization, can be determined only by handling the specimen or by performing an identification test on it. Identification tests include scratching the mineral by another object, placing it near a magnet, weighing it, tasting it, or placing a drop of acid on it. Let’s examine some of the physical properties most commonly used in basic mineral identification.

Physical characteristics of minerals.

• Colour: Colour results from the way a mineral interacts with light. Sunlight contains the whole spectrum of colours; each colour has a different wavelength. A mineral absorbs certain wavelengths, so the colour you see when looking at a specimen represents the wavelengths the mineral does not absorb.  Certain minerals always have the same colour, but many display a range of colours (figure above a). Colour variations in a mineral are due to the presence of impurities. For example, trace amounts of iron may give quartz a reddish colour. 
• Streak: The streak of a mineral refers to the colour of a powder produced by pulverizing the mineral. You can obtain a streak by scraping the mineral against an unglazed ceramic plate (figure above b). The colour of a mineral powder tends to be less variable than the colour of a whole crystal, and thus  provides a fairly reliable clue to a mineral’s identity. Calcite, for example, always yields a white streak even though pieces of calcite may be white, pink, or clear. 
• Luster: Luster refers to the way a mineral surface scatters light. Geoscientists describe luster by comparing the appearance of the mineral with the appearance of a familiar substance. For example, minerals that look like metal have a metallic luster, whereas those that do not have a nonmetallic luster the adjectives are self-explanatory  (figure above c, d). Terms used for types of nonmetallic luster include silky, glassy, satiny, resinous, pearly, or earthy. 
• Hardness: Hardness is a measure of the relative ability of a mineral to resist scratching, and it therefore represents the resistance of bonds in the crystal structure to being broken. The atoms or ions in crystals of a hard mineral are more strongly bonded than those in a soft mineral. Hard minerals can scratch soft minerals, but soft minerals cannot scratch hard ones. Diamond, the hardest mineral known, can scratch most anything, which is why it is used to cut glass. In the early 1800s, a mineralogist named Friedrich Mohs listed some minerals in sequence of relative hardness; a mineral with a hardness of 5 can scratch all minerals with a hardness of 5 or less. This list, the Mohs hardness scale, helps in mineral identification. To make the scale easy to use, common items such as your fingernail, a penny, or a glass plate have been added (Table below). 
• Specific gravity: Specific gravity represents the density of a mineral, as represented by the ratio between the weight of a volume of the mineral and the weight of an equal volume of water at 4°C. For example, one cubic centimetre of quartz has a weight of 2.65 grams, whereas one cubic centimetre of water has a weight of 1.00 gram. Thus, the specific gravity of quartz is 2.65. In practice, you can develop a “feel” for specific gravity by hefting minerals in your hands. A piece of galena (lead ore) feels heavier than a similar-sized piece of quartz. 
• Crystal habit: The crystal habit of a mineral refers to the shape of a single crystal with well-formed crystal faces, or to the character of an aggregate of many well-formed crystals that grew together as a group (figure above e). The habit depends on the internal arrangement of atoms in the crystal.  A description of habit generally includes adjectives that highlight the shape of the crystal. For example, crystals that are roughly the same length in all directions are called equant or blocky, those that are much longer in one dimension than in others are columnar or needle-like, those shaped like  sheets of paper are platy, and those shaped like knives are bladed. 
• Special properties: Some minerals have distinctive properties that readily distinguish them from other minerals. For example, calcite (CaCO3) reacts with dilute hydrochloric acid (HCl) to produce carbon dioxide (CO2) gas (figure above f). Dolomite (CaMg[CO3]2) also reacts with acid, but not as strongly. Graphite makes a gray mark on paper, magnetite attracts a magnet (figure above g), halite tastes salty, and plagioclase has striations (thin parallel corrugations or stripes) on its surface.
• Fracture and cleavage: Different minerals fracture (break) in different ways, depending on the internal arrangement of atoms. If a mineral breaks to form distinct planar  surfaces that have a specific orientation in relation to the crystal structure, then we say that the mineral  has cleavage and we refer to each surface as a cleavage plane. Cleavage forms in directions where the bonds holding atoms together in the crystal are the weakest (figure below a–e). Some minerals have one direction of cleavage. For example, mica has very weak bonds in one direction but strong bonds in the other two directions. Thus, it easily splits into parallel sheets; the surface of each sheet is a cleavage plane. Other minerals have two or three directions of cleavage that intersect at a specific angle. For example, halite has three sets of cleavage planes that intersect at right angles, so halite crystals break into little cubes. Materials that have no cleavage at all (because bonding is equally strong in all directions) break either by forming irregular fractures or by forming conchoidal fractures (figure below f). Conchoidal fractures are smoothly curving, clamshell-shaped surfaces; they typically form in glass. Cleavage planes are sometimes hard to distinguish from crystal faces (figure below g).

 Mohs hardness scale. Mohs’ numbers are relative in reality, diamond is 3.5 times harder than corundum, as the graph shows.

The nature of mineral cleavage and fracture.

Credits: Stephen Marshak (Essentials of Geology)

Crystals and Their Structure

Crystals and Their Structure

kacimo samy   6:50 ص

Crystals and Their Structure 

What Is a Crystal? 

Some characteristics of crystals.

The word crystal brings to mind sparkling chandeliers, elegant wine goblets, and shiny jewels. But, as is the case with the word mineral, geologists have a more precise definition. A crystal is a single, continuous (that is, uninterrupted) piece of a crystalline solid, typically bounded by flat surfaces, called crystal faces, that grow naturally as the mineral forms. The word comes from the Greek krystallos, meaning ice. Many crystals have beautiful shapes that look like they belong in the pages of a geometry book. The angle between two adjacent crystal faces of one specimen is identical to the angle between the corresponding faces of another specimen. For example, a perfectly formed quartz crystal looks like an obelisk  (figure above a, b); the angle between the faces of the columnar part of a quartz crystal is always exactly 120°. This rule, discovered by one of the first geologists, Nicolas Steno (1638– 1686) of Denmark, holds regardless of whether the whole crystal is big or small and regardless of whether all of the faces are the same size. Crystals come in a great variety of shapes, including cubes, trapezoids, pyramids, octahedrons, hexagonal columns, blades, needles, columns, and obelisks (figure above c).

Because crystals have a regular geometric form, people have always considered them to be special, perhaps even a source of magical powers. For example, shamans of some cultures relied on talismans or amulets made of crystals, which supposedly brought power to their wearer or warded off evil spirits. Scientists have concluded, however, that crystals have no effect on health or mood. For millennia, crystals have inspired awe because of the way they sparkle, but such behaviour is simply a consequence of how crystal structures interact with light.

Looking Inside Crystals 

Patterns and symmetry in minerals.

What makes crystals have regular geometric forms? This problem was the focus of study for centuries. An answer finally came from the work of a German physicist, Max von Laue, in 1912. He showed that an X-ray beam passing through a crystal breaks up into many tiny beams to create a pattern of dots on a screen (figure above a). Physicists refer to this phenomenon as diffraction; it occurs when waves interact with regularly spaced objects whose spacing is close to the wavelength of the waves you can see diffraction of ocean waves when they pass through gaps in a seawall. Von Laue concluded that, for a crystal to cause diffraction, atoms within it must be regularly spaced and the spacing must be comparable to the wavelength of X-rays. Eventually, Von Laue and others learned how to use X-ray diffraction patterns as a basis for defining the specific arrangement of atoms in crystals. This arrangement defines the crystal structure of a mineral. 

If you've ever examined wallpaper, you've seen an example of a pattern (figure above b). Crystal structures contain one of nature’s most spectacular examples of such a pattern. In crystals, the 

pattern is defined by the regular spacing of atoms and, if the crystal contains more than one element, by the regular alternation of atoms (figure above c). (Mineralogists refer to a 3-D geometry of points representing this pattern as a lattice.) The pattern of atoms in a crystal may control the shape of a crystal. For example, if atoms in a crystal pack into the shape of a cube, the crystal may have faces that intersect at 90° angles galena  (PbS) and halite (NaCl) have such a cubic shape. Because of the pattern of atoms in a crystal structure, the structure has symmetry, meaning that the shape of one part of the structure is the mirror image of the shape of a neighbouring part. For example, if you were to cut a halite crystal or a water crystal (snowflake) in half, and place the half against a mirror, it would look whole again (figure above d). 

The nature of crystalline structure in minerals. The arrangement of atoms can be portrayed by a ball and stick model, or by a packed ball model.

To illustrate crystal structures, we look at a few examples. Halite (rock salt) consists of oppositely charged ions that stick together because opposite charges attract. In halite, six chloride (Cl–) ions surround each sodium (Na+) ion, producing an overall arrangement of atoms that defines the shape of a cube (figure above a, b). Diamond, by contrast, is a mineral made entirely of carbon. In diamond, each atom bonds to four neighbours arranged in the form of a tetrahedron; some naturally formed diamond crystals have the shape of a double tetrahedron  (figure above c). Graphite, another mineral composed entirely of carbon, behaves very differently from diamond. In contrast to diamond, graphite is so soft that we use it as the “lead” in a pencil; when a pencil moves across paper, tiny flakes of graphite peel off the pencil point and adhere to the paper.  This behaviour occurs because the carbon atoms in graphite  are not arranged in tetrahedra, but rather occur in sheets  (figure above d). The sheets are bonded to each other by weak bonds and thus can separate from each other easily. Of note, two different minerals (such as diamond and graphite) that have the same composition but different crystal structures are polymorphs.

The Formation and Destruction of Minerals 

New mineral crystals can form in five ways. First, they can form by the solidification of a melt, meaning the freezing of a liquid to form a solid. For example, ice crystals, a type of mineral, are made by solidifying water, and many different minerals form by solidifying molten rock. Second, they can form by precipitation from a solution, meaning that atoms, molecules, or ions dissolved in water bond together and separate out of the water. Salt crystals, for example, precipitate when you evaporate salt water. Third, they can form by solid-state diffusion, the movement of atoms or ions through a solid to arrange into a new crystal structure, a process that takes place very slowly. For example, garnets  grow by diffusion in solid rock. Fourth, minerals can form at interfaces between the physical and biological components of the Earth System by a process called biomineralization. This occurs when living organisms cause minerals to precipitate either within or on their bodies, or immediately adjacent to their bodies. For example, clams and other shelled organisms extract ions from water to produce mineral shells. Fifth, minerals can precipitate directly from a gas. This process typically occurs around volcanic vents or around geysers, for at such locations volcanic gases or steam enter the atmosphere and cool abruptly. Some of the bright yellow sulphur deposits found in volcanic regions form in this way.

The growth of crystals.

The first step in forming a crystal is the chance formation of a seed, or an extremely small crystal (figure above a). Once the seed exists, other atoms in the surrounding material attach themselves to the face of the seed. As the crystal grows, crystal faces move outward but maintain the same orientation (figure above b). The youngest part of the crystal is at its outer edge.
In the case of crystals formed by the solidification of a melt, atoms begin to attach to the seed when the melt becomes so cool that thermal vibrations can no longer break apart the attraction between the seed and the atoms in the melt. Crystals formed by precipitation from a solution develop when the solution becomes saturated, meaning the number of dissolved ions per unit volume of solution becomes so great that they can get close enough to each other to bond together.
As crystals grow, they develop their particular crystal shape, based on the geometry of their internal structure. The shape is defined by the relative dimensions of the crystal (needle- like, sheet-like, etc.) and the angles between crystal faces. Typically, the growth of minerals is restricted in one or more directions, because existing crystals act as obstacles. In such cases, minerals grow to fill the space that is available, and their shape is controlled by the shape of their surroundings. Minerals without well-formed crystal faces are anhedral grains (figure above c). If a mineral’s growth is unimpeded so that it displays well-formed crystal faces, then it is a euhedral crystal. The surface crystals of a geode, a mineral-lined cavity in rock, may be euhedral (figure above d).
A mineral can be destroyed by melting, dissolving, or some other chemical reaction. Melting involves heating a mineral to a temperature at which thermal vibration of the atoms or ions in the lattice break the chemical bonds holding them to the lattice. The atoms or ions then separate, either individually or in small groups, to move around again freely. Dissolution occurs when you immerse a mineral in a solvent, such as water. Atoms or ions then separate from the crystal face and are surrounded by solvent molecules. Chemical reactions can destroy a mineral when it comes in contact with reactive materials. For example, iron-bearing minerals react with air and water to form rust. The action of microbes in the environment can also destroy minerals. In effect, some microbes can “eat” certain minerals; the microbes use the energy stored in the chemical bonds that hold the atoms of the mineral together as their source of energy for metabolism.

Credits: Stephen Marshak (Essentials of Geology)

What Is a Mineral?

What Is a Mineral?

kacimo samy   6:16 ص

What Is a Mineral? 

To a geologist, a mineral is a naturally occurring solid, formed by geologic processes, that has a crystalline structure and a definable chemical composition. Almost all minerals are inorganic. Let’s pull apart this mouthful of a definition and examine its meaning in detail.

• Naturally occurring: True minerals are formed in nature, not in factories. We need to emphasize this point because in recent decades, industrial chemists have learned how to synthesize materials that have characteristics virtually identical to those of real minerals. These materials are not minerals in a geologic sense, though they are referred to in the  commercial world as synthetic minerals. 
• Formed by geologic processes: Traditionally, this phrase implied processes, such as solidification of molten rock or direct precipitation from a water solution, that did not involve living organisms. Increasingly, however, geologists recognize that life is an integral part of the Earth System. So, some  geologists consider solid, crystalline materials produced by organisms to be minerals too. To avoid confusion, the term “biogenic mineral” may be used when discussing such  materials. 
• Solid: A solid is a state of matter that can maintain its shape indefinitely, and thus will not conform to the shape of its container. Liquids (such as oil or water) and gases (such as air) are not minerals (Some Basic Concepts from Chemistry). 
• Crystalline structure: The atoms that make up a mineral are not distributed randomly and cannot move around easily. Rather, they are fixed in a specific, orderly pattern. A material in which atoms are fixed in an orderly pattern is called a crystalline solid. 
• Definable chemical composition: This simply means that it is possible to write a chemical formula for a mineral (Some Basic Concepts from Chemistry). Some minerals contain only one element, but most are compounds of two or more elements. For example, diamond and graphite have the formula C, because they consist entirely of carbon. Quartz has the formula SiO2 it contains the elements silicon and oxygen in the proportion of one silicon atom for every two oxygen atoms. Calcite has the formula CaCO3, meaning it consists of a calcium (Ca ) ion and a carbonate (CO3 ) ion. Some formulas are more complicated: for example, the formula for biotite is K(Mg,Fe)3(AlSi3O10)(OH)2. 
• Inorganic: Organic chemicals are molecules containing some carbon-hydrogen bonds. Sugar (C12H22O11), for example, 
• is an organic chemical. Almost all minerals are inorganic. Thus, sugar and protein are not minerals. But, we have to add the qualifier “almost all” because mineralogists do consider about 30 organic substances formed by “the action of geologic processes on organic materials” to be minerals. Examples include the crystals that grow in ancient deposits of bat guano.

The nature of crystalline and noncrystalline materials.

With these definitions in mind, we can make an important distinction between minerals and glass. Both minerals and glass are solids, in that they can retain their shape indefinitely. But a mineral is crystalline, and glass is not. Whereas atoms, ions, or molecules in a mineral are ordered into a crystal lattice, like soldiers standing in formation, those in a glass are arranged in a semi-chaotic way, like people at a party, in small clusters or chains that are neither oriented in the same way nor spaced at regular intervals (figure above a, b). 

If you ever need to figure out whether a substance is a mineral or not, just check it against the criteria listed above. Is motor oil a mineral? No it’s an organic liquid. Is table salt a mineral? Yes it’s a solid crystalline compound with the formula NaCl. Is the hard material making up the shell of an oyster considered to be a mineral? Microscopic examination of  an oyster shell reveals that  it consists of calcite, so it can be called a biogenic mineral. Is rock candy a mineral? No. Even though it is solid and crystalline, it’s made by people and it consists of sugar (an organic chemical). 

Some Basic Concepts from Chemistry 

To describe minerals, we need to use several terms from chemistry. To avoid confusion, terms are listed in an order that permits each successive term to utilize previous terms. 

Examples of states of matter and chemical bonds.

• Element: A pure substance that cannot be separated into other materials. 
• Atom: The smallest piece of an element that retains the characteristics of the element. An atom consists of a nucleus surrounded by a cloud of orbiting electrons; the nucleus is made up of protons and neutrons (except in hydrogen, whose nucleus contains only one proton and no neutrons). Electrons have a negative charge, protons have a positive charge, and neutrons have a neutral charge. An atom that has the same number of electrons as protons is said to be neutral, in that it does not have an overall electrical charge. 
• Atomic number: The number of protons in an atom of an element. 
• Atomic weight: Approximately the number of protons plus neutrons in an atom of an element. 
• Ion: An atom that is not neutral. An ion that has an excess negative charge (because it has more electrons than protons) is an anion, whereas an ion that has an excess positive charge (because it has more protons than electrons) is a cation. We indicate the charge with a superscript. For example, Cl has a single excess electron; Fe2 is missing two electrons.
• Chemical bond: An attractive force that holds two or more atoms together (figure above a–c). For example, covalent bonds form when atoms share electrons. Ionic bonds form when a cation and anion (ions with opposite charges) get close together and attract each other. In materials with metallic bonds, some of the electrons can move freely. 
• Molecule: Two or more atoms bonded together. The atoms may be of the same element or of different elements. 
• Compound: A pure substance that can be subdivided into two or more elements. The smallest piece of a compound that retains the characteristics of the compound is a molecule. 
• State of matter: The form of a substance, which reflects the degree to which the atoms or molecules comprising the matter are bonded together. figure above d–f defines three of the states solid, liquid, and gas. There are more bonds in a solid than in a liquid, and more in a liquid than in a gas. Which state exists at a given location depends on pressure and temperature, as indicated by a phase diagram (figure above g). A fourth state, plasma, exists only at very high temperatures. 
• Chemical: A general name used for a pure substance (either an element or a compound). 
• Chemical formula: A shorthand recipe that itemizes the various elements in a chemical and specifies their relative proportions. For example, the formula for water, H2O, indicates that water consists of molecules in which two hydrogens bond to one oxygen.
• Chemical reaction: A process that involves the breaking or forming of chemical bonds. Chemical reactions can break molecules apart or create new molecules and/or isolated atoms. 
• Mixture: A combination of two or more elements or compounds that can be separated without a chemical reaction. For example, a cereal composed of bran flakes and raisins is a mixture you can separate the raisins from the flakes without destroying either. 
• Solution: A type of material in which one chemical (the solute) dissolves in another (the solvent). In solutions, a solute may separate into ions during the process. For example, when salt (NaCl) dissolves in water, it separates into sodium (Na ) and chloride (Cl ) ions. In a solution, atoms or molecules of the solvent surround atoms, ions, or molecules of the solute. 
• Precipitate: A compound that forms when ions in liquid solution join together to create a solid that settles out of the solution; (verb) the process of forming solid grains by separation and settling from a solution. For example, when saltwater evaporates, solid salt crystals precipitate.

Credits: Stephen Marshak (Essentials of Geology)

Mineral Identification

Mineral Identification

kacimo samy   11:47 ص

How to identify minerals?

Amateur and professional mineralogists get a kick out of identify minerals. They might hover around a display case in a museum and name specimens without bothering to look at the labels. How do they do it? The trick lies in learning to recognize the basic physical properties (visual and material characteristics) that distinguish one mineral from another. Some physical properties, such as shape and colour, can be seen from a distance. Others, such as hardness and magnetization, can be determined only by handling the specimen or by performing an identification test on it. Identification tests include scratching the mineral by another object, placing it near a magnet, weighing it, tasting it, or placing a drop of acid on it. Let’s examine some of the physical properties most commonly used in basic mineral identification. 

1. Colour: Colour results from the way a mineral interacts with light. Sunlight contains the whole spectrum of colours; each colour has a different wavelength. A mineral absorbs certain wavelengths, so the colour you see when looking at a specimen represents the wavelengths the mineral does not absorb.  Certain minerals always have the same colour, but many display a range of colours. Colour variations in a mineral are due to the presence of impurities. For example, trace amounts of iron may give quartz a reddish colour. 
2. Streak: The streak of a mineral refers to the colour of a powder produced by pulverizing the mineral. You can obtain a streak by scraping the mineral against an unglazed ceramic plate. The colour of a mineral powder tends to be less variable than the colour of a whole crystal, and thus  provides a fairly reliable clue to a mineral’s identity. Calcite, for example, always yields a white streak even though pieces of calcite may be white, pink, or clear. 
3. Luster: Luster refers to the way a mineral surface scatters light. Geoscientists describe luster by comparing the appearance of the mineral with the appearance of a familiar substance. For example, minerals that look like metal have a metallic luster, whereas those that do not have a non-metallic luster. Terms used for types of non-metallic luster include silky, glassy, satiny, resinous, pearly, or earthy. 
4. Hardness: Hardness is a measure of the relative ability of a mineral to resist scratching, and it therefore represents the resistance of bonds in the crystal structure to being broken. The atoms or ions in crystals of a hard mineral are more strongly bonded than those in a soft mineral. Hard minerals can scratch soft minerals, but soft minerals cannot scratch hard ones. Diamond, the hardest mineral known, can scratch most anything, which is why it is used to cut glass. In the early 1800s, a mineralogist named Friedrich Mohs listed some minerals in sequence of relative hardness; a mineral with a hardness of 5 can scratch all minerals with a hardness of 5 or less. This list, the Mohs hardness scale, helps in mineral identification. To make the scale easy to use, common items such as your fingernail, a penny, or a glass plate have been added. 
5. Specific gravity: Specific gravity represents the density of a mineral, as represented by the ratio between the weight of a volume of the mineral and the weight of an equal volume of water at 4°C. For example, one cubic centimetre of quartz has a weight of 2.65 grams, whereas one cubic centimetre of water has a weight of 1.00 gram. Thus, the specific gravity of quartz is 2.65. In practice, you can develop a “feel” for specific gravity by hefting minerals in your hands. A piece of galena (lead ore) feels heavier than a similar-sized piece of quartz. 
6. Crystal habit: The crystal habit of a mineral refers to the shape of a single crystal with well-formed crystal faces, or to the character of an aggregate of many well-formed crystals that grew together as a group. The habit depends on the internal arrangement of atoms in the crystal.  A description of habit generally includes adjectives that highlight the shape of the crystal. For example, crystals that are roughly the same length in all directions are called equant or blocky, those that are much longer in one dimension than in others are columnar or needle-like, those shaped like  sheets of paper are platy, and those shaped like knives are bladed. 
7. Special properties: Some minerals have distinctive properties that readily distinguish them from other minerals. For example, calcite (CaCO3) reacts with dilute hydrochloric acid (HCl) to produce carbon dioxide (CO2) gas. Dolo mite (CaMg[CO3]2) also reacts with acid, but not as strongly. Graphite makes a gray mark on paper, magnetite attracts a magnet, halite tastes salty, and plagioclase has striations (thin parallel corrugations or stripes) on its surface.
8. Fracture and cleavage: Different minerals fracture (break) in different ways, depending on the internal arrangement of atoms. If a mineral breaks to form distinct planar  surfaces that have a specific orientation in relation to the crystal structure, then we say that the mineral  has cleavage and we refer to each surface as a cleavage plane. Cleavage forms in directions where the bonds holding atoms together in the crystal are the weakest. Some minerals have one direction of cleavage. For example, mica has very weak bonds in one direction but strong bonds in the other two directions. Thus, it easily splits into parallel sheets; the surface of each sheet is a cleavage plane. Other minerals have two or three directions of cleavage that intersect at a specific angle. For example, halite has three sets of cleavage planes that intersect at right angles, so halite crystals break into little cubes. Materials that have no cleavage at all (because bonding is equally strong in all directions) break either by forming irregular fractures or by forming conchoidal fractures. Conchoidal fractures are smoothly curving, clamshell-shaped surfaces; they typically form in glass. Cleavage planes are sometimes hard to distinguish from crystal faces.

Credits: Stephen Marshak (Essentials of Geology)

Minerals

Minerals

kacimo samy   9:29 ص

What is a mineral?

To a geologist, a mineral is a naturally occurring solid, formed by geologic processes, that has a crystalline structure and a definable chemical composition. Almost all minerals are inorganic. Let’s pull apart this mouthful of a definition and examine its meaning in detail.

• Naturally occurring: True minerals are formed in nature, not in factories. We need to emphasize this point because in recent decades, industrial chemists have learned how to synthesize materials that have characteristics virtually identical to those of real minerals. These materials are not minerals in a geologic sense, though they are referred to in the  commercial world as synthetic minerals.  
• Formed by geologic processes: Traditionally, this phrase implied processes, such as solidification of molten rock or direct precipitation from a water solution, that did not involve living organisms. Increasingly, however, geologists recognize that life is an integral part of the Earth System. So, some  geologists consider solid, crystalline materials produced by organisms to be minerals too. To avoid confusion, the term “biogenic mineral” may be used when discussing such  materials. 
• Solid: A solid is a state of matter that can maintain its shape indefinitely, and thus will not conform to the shape of its container. Liquids (such as oil or water) and gases (such as air) are not minerals.  
• Crystalline structure: The atoms that make up a mineral are not distributed randomly and cannot move around easily. Rather, they are fixed in a specific, orderly pattern. A material in which atoms are fixed in an orderly pattern is called a crystalline solid. 
• Definable chemical composition: This simply means that it is possible to write a chemical formula for a mineral. Some minerals contain only one element, but most are compounds of two or more elements. For example, diamond and graphite have the formula C, because they consist entirely of carbon. Quartz has the formula SiO2 it contains the elements silicon and oxygen in the proportion of one silicon atom for every two oxygen atoms. Calcite has the formula CaCO3, meaning it consists of a calcium (Ca ) ion and a carbonate (CO3 ) ion. Some formulas are more complicated: for example, the formula for biotite is K(Mg,Fe)3(AlSi3O10)(OH)2. 
• Inorganic: Organic chemicals are molecules containing some carbon-hydrogen bonds. Sugar (C12H22O11), for example, is an organic chemical. Almost all minerals are inorganic. Thus, sugar and protein are not minerals. But, we have to add the qualifier “almost all” because mineralogists do consider about 30 organic substances formed by “the action of geologic processes on organic materials” to be minerals. Examples include the crystals that grow in ancient deposits of bat guano.

With these definitions in mind, we can make an important distinction between minerals and glass. Both minerals and glass are solids, in that they can retain their shape indefinitely. But a mineral is crystalline, and glass is not. Whereas atoms, ions, or molecules in a mineral are ordered into a crystal lattice, like soldiers standing in formation, those in a glass are arranged in a semi-chaotic way, like people at a party, in small clusters or chains that are neither oriented in the same way nor spaced at regular intervals. If you ever need to figure out whether a substance is a mineral or not, just check it against the criteria listed above. Is motor oil a mineral? No it’s an organic liquid. Is table salt a mineral? Yes it’s a solid crystalline compound with the formula NaCl. Is the hard material making up the shell of an oyster considered to be a mineral? Microscopic examination of  an oyster shell reveals that  it consists of calcite, so it can be called a biogenic mineral. Is rock candy a mineral? No. Even though it is solid and crystalline, it’s made by people and it consists of sugar (an organic chemical).
Credits: Stephen Marshak (Essentials of Geology)

Non metal mineral resources

Non metal mineral resources

kacimo samy   11:30 م

Non metal mineral resources

Society uses many non-metallic mineral resources, also known as industrial minerals, as well. From the ground, we get the stone used to make roadbeds and buildings, the chemicals for fertilizers, the gypsum in drywall, the salt filling salt shakers, and the sand used to make glass the list is endless. This section looks at a few of these materials and explains where they come from.

Dimension Stone 

Stone production in quarries.

The Parthenon, a colossal stone temple rimmed by 46 carved columns, has stood atop a hill overlooking the city of Athens for almost 2,500 years. No wonder“stone,” an architect’s word for rock, outlasts nearly all other construction materials. We use stone to make facades, roofs, curbs, steps, counter tops, and floors. We value stone for its visual appeal as well as its durability. The names that architects give to various types of stone may differ from the formal rock names that geologists use. For example, architects refer to any polished carbonate rock as “marble,” whether or not it has been metamorphosed. Likewise, they refer to any crystalline rocks containing feldspar and/or quartz as “granite,” regardless of whether the rock has an igneous or a metamorphic texture, or a felsic or mafic composition.
To obtain intact slabs and blocks of rock known as dimension stone in the trade for architectural purposes, workers must carefully cut rock out of the walls of quarries (a in figure above). (Note that a quarry provides stone, whereas a mine supplies ore.) To cut stone slabs, quarry operators split rock blocks from bedrock by hammering a series of wedges into the rock, or slice it off bedrock by using a wireline saw, a thermal lance, or a water jet. A wireline saw consists of a loop of braided wire moving between two pulleys. In some cases, as the wire moves along the rock surface, the quarry operator spills abrasive (sand or garnet grains) and water onto the wire. The movement of the wire drags the abrasive along the rock and grinds into it. Alternatively, the quarry operator may use a diamond-coated wire, cooled with pure water. A thermal lance looks like a long blowtorch: a flame of burning diesel fuel, stoked by high-pressure air, pulverizes rock and thereby cuts a slot. More recently, quarry operators have begun to use an abrasive water jet, which squirts out water and abrasives at very high pressure, to cut rock.

Crushed Stone and Concrete 

Crushed stone forms the substrate of highways and rail roads and serves as the raw material for manufacturing cement, concrete, and asphalt. In crushed-stone quarries (b in figure above), operators use high explosives to break up bedrock into rubble that they then transport by truck to a jaw crusher. This reduces the rubble into usable chunks.
Most of the buildings and highways constructed in the past two centuries consist of bricks attached to each other by mortar, or of walls, floors, columns, and roads made of concrete that has been spread into a layer or poured into a form. Both mortar and concrete start out as a slurry, but when allowed to set, they harden into a hard, rock-like substance. The slurry from which mortar and concrete form consists of “aggregate” (sand and/or gravel) mixed with water and cement. Before mixing, the cement in mortar and concrete is a powder that consists of lime (CaO), quartz (SiO2), aluminium oxide (Al2O3), and iron oxide (Fe2O3); typically, lime accounts for 66% of cement, silica for 25%, and the remaining chemicals for about 9%. When cement is mixed with water, the chemicals comprising it dissolve. Mortar and concrete set when these chemicals react and a complex assemblage of new mineral crystals grows and binds together pre-existing solid grains in effect, the “cement” in mortar or concrete serves the same purpose as the “cement” in a sandstone or conglomerate.
It appears that the ancient Romans were the first to use cement they made it from a mixture of volcanic ash and limestone. In the 18th and early 19th centuries, cement was produced by heating specific types of limestone (which happened to contain calcite, clay, and quartz in the correct proportions) in a kiln up to a temperature of about 1450nC; the heating releases CO2 gas and produces “clinker,” chunks consisting of lime and other oxide compounds. Manufacturers crushed the clinker into cement powder and packed it in bags for transport. But limestone with the exact composition necessary to make such “cement” is fairly rare, so most cement used today is Portland cement, made by mechanically mixing limestone, sandstone, and shale in just the right proportions, before heating in a kiln to provide the correct chemical make-up. Isaac Johnson, an English engineer, came up with the recipe for Portland cement in 1844; he named it after the town of Portland, England, because he thought that concrete made from it resembled rock exposed there. 

Nonmetallic Minerals for Homes and Farms 

We use an astounding variety of non-metallic geologic resources without ever realizing where they come from. Consider the materials in a typical house or apartment. The foundation consists of concrete, made from limestone mixed with sand or gravel. The bricks in the exterior walls originated as clay, formed from the chemical weathering of silicate rocks and perhaps dug from the floodplain of a stream. To make bricks, workers mould wet clay into blocks and then bake it. Baking drives out water and causes metamorphic reactions that recrystallize the clay. The glass used to glaze windows consists largely of silica, formed by first melting and then freezing pure quartz sand from a beach deposit or a sandstone formation. Gypsum board (drywall), used to construct interior walls, comes from a slurry of water and the mineral gypsum sandwiched between sheets of paper. Gypsum (CaSO4s (2O) occurs in evaporite strata precipitated from seawater or saline lake water. Evaporites provide other useful minerals as well, such as halite, and serve as the source for lithium, a key element in computer and camera batteries. Modern technological innovations have also greatly increased the demand for the rare earth elements, a group of 17 elements including the lanthanides, scandium, and yttrium. While the names of rare earth elements are unfamiliar to most people, the elements themselves have become essential in the production of lasers, magnets, X-ray tubes, night vision goggles, camera lenses, and high-tech lamps. Rare earth elements aren't actually that rare, in terms of their abundance relative to other elements in the crust. But localities where ores have a high enough concentration to be mined are rare.
Credits: Stephen Marshak (Essentials of Geology)

10 World's most deadly minerals

10 World's most deadly minerals

kacimo samy   1:30 م

World's most deadly minerals

World's most deadly minerals that are valuable make the present day world go 'round. They're utilized as a part of everything from circuit sheets to tableware. They're likewise the absolute most poisonous materials known not, and unearthing them has demonstrated so perilous throughout the years, some have been eliminated of mechanical generation inside and out. Recorded beneath are the 10 most savage minerals on earth. These stones don't should be tossed to hurt you.

Chalcanthite

Chalcanthite (hydrated copper sulphate) is the brilliant blue mineral developed from arrangement in science labs and home substance packs. Chalcanthite ought to never be taste tried by beginner researchers for salt substance, or a to a great degree genuine overdose of copper could come about. Simply discharging gems of the blue mineral has murdered whole lakes of green growth, and postured extraordinary ecological dangers. . It is water solvent and will take shape out again from arrangement. The copper in this mineral is exceptionally bio-accessible and is lethal to plants and in high amounts dangerous to people.

Hutchinsonite 

Hutchinsonite is a sulfosalt mineral of thallium, arsenic and lead with recipe (Tl,Pb)2As5S9. Hutchinsonite is an unsafe however emotional blend of thallium, lead and arsenic. Is an uncommon sulphosalt mineral that obliges taking care of with extraordinary consideration as every one of the three of its fundamental segments are toxic especially thallium as it can bring about male pattern baldness, genuine ailment and passing. The three toxic metals shape a deadly mineral mixed drink that ought to be taken care of just with incredible alert.

Galena

Galena is a stand out amongst the most bounteous and generally appropriated sulphide minerals. Galena is the standard metal of lead, and structures sparkling silver 3D squares with unnaturally flawless shapes. In spite of the fact that lead is ordinarily greatly adaptable, the sulphur substance of galena makes it phenomenally weak and receptive to compound treatment. It's not as awful as mercury, which will execute you promptly through and through, however lead doesn't get flushed out of your framework. It aggregates throughout the years, in the long run coming to harmful levels. When that happens both you and your children pay the cost, as lead danger is cancer-causing to you and is teratogenic (creating extreme conception deformities) to you're posterity.

Asbestos

Asbestos is not one mineral but rather six characterized separate minerals. One being a serpentine (chrysotile) and the other five being amphiboles (crocidolite, grunerite (amosite), tremolite, anthophyllite and actinolite). Dissimilar to alternate minerals in the main 10 deadliest. It was once broadly utilized for an assortment of business and mechanical applications on account of its solid, fireproof, and adaptable nature from roof tiles and roofing materials to ground surface and warm protection.

Torbernite

Torbernite is the mineral from hell fire. The crystal moulded green gems structure as auxiliary stores in granitic shakes, and are made out of uranium. Framed through a mind boggling response between phosphorous, copper, water and uranium, the shocking gem showcases have allured numerous mineral authorities into taking an example for a rack accumulation. The mineral is radioactive and emanates the disease shaping radon gas. This is one mineral you don't need on your showcase bureau rack.

Erionite 

Erionite (NaK2MgCa1.5)[Al8Si28]O72 28H2O) is an actually happening stringy mineral that fits in with a gathering of minerals called zeolites. It for the most part is found in volcanic fiery remains that has been modified by weathering and ground water. It looks a considerable measure like asbestos minerals and damages people much in the same way: mesothelioma. It's essentially an industry-particular infection or if nothing else it was, until we understood that it created super-malignancy and quit mining the stuff in the late 1980.

Cinnabar

Cinnabar (mercury sulphide - HgS) is the absolute most harmful mineral to handle on earth. It is the world's fundamental wellspring of mercury and has been mined subsequent to Neolithic times. In any case, when oxidized, this component will create methyl mercury and dimethyl mercury, two dangerous intensifies that cause un-salvageable damage to the sensory systems of kids. It is dangerous in little fixations and can be consumed through the respiratory tract, entrails, or skin. Much all the more unfathomably, some antiquated medicinal experts trusted cinnabar held recuperating powers, and recommended it for specific conditions.

Phenacite 

Phenacite is a genuinely uncommon nesosilicate mineral comprising of beryllium orthosilicate, Be2SiO4. Phenacite is mined both as a gemstone and for its important beryllium content. Beryllium was at one time a forerunner for some fired materials, until individuals made sense of that breathing in Beryllium dust brought about berylliosis otherwise known as interminable beryllium sickness. It's similar to silicosis however a great deal more extreme furthermore unending. You don't recoup from CBD essentially by minimizing your beryllium presentation. When you have it, you have it forever. Fundamentally what happens is the lungs get to be easily affected to berrylium, which causes an unfavourably susceptible response wherein the lungs from little knobs called granulomas. These granulomas make breathing amazingly troublesome and can go ahead to affect illnesses like tuberculosis.

Stibnite

Stibnite is a sulphide mineral with the equation Sb2S3. Hence, the tremendous, sparkling metallic gems of this shaky compound were once designed into superb eating utensils. Be that as it may, the sword moulded precious stones bore the forces of death to the individuals who utilized them. Treatment of this metalloid mineral can bring about harming.

Hydroxyapatite 

Hydroxyapatite (Ca5(PO4)3(OH)2 is a normally happening mineral type of calcium apatite. The phosphorous in your greenery enclosure manure and fluoride in your faucet water likely originated from a stone like this, called Apatite. These phosphate minerals come in three mixed bags, each separately containing hoisted levels of OH, F, or Cl particles the Hydroxyapatite variant being a noteworthy segment of your tooth finish and the Fluorapatite form constituting what's dumped into metro water supplies to avert depressions. Keeping in mind having solid teeth and bones is something worth mining so as to be thankful for, presentation to Hydroxyapatite (either or handling it) will store those same minerals on your heart valves, viably petrifying them.

Geology of Mineral Resources

Geology of Mineral Resources

kacimo samy   6:43 ص

The geology of mineral resources is intimately related to the entire geologic cycle, and nearly all aspects and processes of the cycle are involved to a lesser or greater extent in producing local concentrations of useful materials.

Local Concentrations of Metals 

The term ore is sometimes used for those useful metallic minerals that can be mined at a profit, and locations where ore is found have anomalously high concentrations of these minerals. The concentration of metal necessary for a particular mineral to be classified as an ore varies with technology, economics, and politics. Before smelting (extraction of metal by heating) was invented, the only metal ores were those in which the metals appeared in their pure form; gold, for example, was originally obtained as a pure, or native, metal. Now gold mines extend deep beneath the surface, and the recovery process involves reducing tons of rock to ounces of gold. Although the rock contains only a minute amount of gold, we consider it a gold ore because we can extract the gold profitably. The concentration factor of a metal is the ratio of its necessary concentration for profitable mining (that is, of its concentration in ore) to its average concentration in the earth's crust. Aluminum has an average concentration of about 8 percent in the Earth's crust and needs to be found at concentrations of about 35 percent to be mined economically, giving it a concentration factor of about 4. Mercury, on the other hand, has an average concentration of only a tiny fraction of 1 percent and must have a concentration factor of about 10,000 to be mined economically. Nevertheless, mercury ores are common in certain regions, where they and other metallic ores are deposited. The percentage of a metal in ore (and thus the concentration factor) is subject to change as the demand for the metal changes.

Igneous Processes 

Most ore deposits, caused by igneous processes, result from an enrichment process that concentrates an economically desirable ore of metals, such as copper, nickel, or gold. In some cases, however, an entire igneous rock mass contains disseminated crystals that can be recovered economically. Perhaps the best-known example is the occurrence of diamond crystals, found in a coarse-grained igneous rock called kimberlite, which characteristically occurs as a pipe shaped body of rock that decreases in diameter with depth. Almost the entire kimberlite pipe is the ore deposit, and the diamond crystals are disseminated throughout the rock. Diamonds, which are composed of carbon, form at very high temperatures and pressures, perhaps at depths as great as 150 km well below the crust of the earth and into the mantle. Some kimberlite pipes in South Africa are believed to be as old as 2 billion years. Near the surface, diamonds are not stable over geologic time and will eventually change to graphite (the mineral in lead pencils). The transformation will not happen at surface temperature and pressure, and, as a result, diamonds are metastable, remaining beautiful and mysterious for periods of time of interest to humans. The fact that the kimberlite pipes are so old suggests that they must be intruded (moved upward) from deep diamond forming depth to near the surface relatively quickly. If this were not the case, the diamonds would have been transformed to graphite.

Crystal Settling 

More concentrated ore deposits can result from igneous processes called crystal settling that segregate crystals formed earlier from those formed later. For example, as magma cools, heavy minerals that crystallize early may slowly sink or settle toward the lower part of the magma chamber, where they form concentrated layers. Deposits of chromite (ore of chromium) have formed by this process.

Late Magmatic Processes and Hydrothermal Replacement 

Late magmatic processes occur after most of the magma has crystallized, and rare and heavy metalliferous materials in water- and gas-rich solutions remain. This late-stage metallic solution may be squeezed into fractures or settle into interstices (empty spaces) between earlier-formed crystals. Other late-stage solutions form coarse-grained igneous rock known as pegmatite, which is rich in feldspar, mica, and quartz, as well as certain rare minerals. Pegmatites have been extensively mined for feldspar, mica, spodumene (lithium mineral), and clay that forms from weathered feldspar. Hydrothermal (hot-water) mineral deposits are a common type of ore deposit. They originate from late-stage magmatic processes and give rise to a variety of mineralization, including gold, silver, copper, mercury, lead, zinc, and other metals, as well as many non-metallic minerals. The hydrothermal solutions that form ore deposits are mineralizing fluids that migrate through a host rock, crystallizing as veins or small dikes. The mineral material is either produced directly from the igneous parent rock or altered by metamorphic processes, as magmatic solutions intrude into the surrounding rock (alteration by metamorphic processes, called contact metamorphism, is discussed under Metamorphic Processes). Many hydrothermal deposits cannot be traced to a parent igneous rock mass, however, and their origin remains unknown. It is speculated that circulating groundwater, heated and enriched with minerals after contact with deeply buried magma, might be responsible for some of these deposits.
Two types of hydrothermal deposits can be recognized: cavity-filling and replacement. Cavity-filling deposits are formed when hydrothermal solutions migrate along openings in rocks (such as fracture systems, pore spaces, or bedding planes) and precipitate (crystallize) ore minerals. Replacement deposits form as hydrothermal solutions react with the host rock, forming a zone in which ore minerals precipitate from the mineralizing fluids and replace part of the host rock. Although replacement deposits are believed to dominate at higher temperatures and pressures than cavity-filling deposits, both may be found in close association as one grades into the other; that is, the filling of an open fracture by precipitation from hydrothermal solutions may occur simultaneously with replacement of the rock that lines the fracture.
Hydrothermal replacement processes are significant because, excluding some iron and non-metallic deposits, they have produced some of the worlds largest and most important mineral deposits. Some of these deposits result from a massive, nearly complete replacement of host rock with ore minerals that terminate abruptly; others form thin replacement zones along fissures; and still others form disseminated replacement deposits that may involve huge amounts of relatively low-grade ore.
The actual sequence of geologic events leading to the development of a hydrothermal ore deposit is usually complex. Consider, for example, the tremendous, disseminated copper deposits of northern Chile. The actual mineralization is thought to be related to igneous activity, faulting, and folding that occurred 60 million to 70 million years ago. The ore deposit is an elongated, tabular mass along a highly sheared (fractured) zone separating two types of granitic rock.
The concentration of copper results from a number of factors:

• A source igneous rock supplied the copper. 
• The fissure zone tapped the copper supply and facilitated movement of the mineralizing fluids. 
• The host rock was altered and fractured, preparing it for deposition and replacement processes that produced the ore. 
• The copper was leached and redeposited again by meteoric water, which further concentrated the ore.

Metamorphic Processes 

Contact Metamorphism 

Ore deposits are often found along the contact between igneous rocks and the surrounding rocks they intrude. This area is characterized by contact metamorphism, caused by the heat, pressure, and chemically active fluids of the cooling magma interacting with the surrounding rock, called country, or host rock. The width of the contact metamorphic zone varies with the type of country rock. The zone is usually thickest in limestone because limestone is more reactive: The release of carbon dioxide increases the mobility of reactants. The zone is generally thinnest for shale because the fine-grained texture retards the movement of hot, chemically active solutions, and the zone is intermediate for sandstone. As we have already mentioned, some of the mineral deposits that form in contact areas originate from the magmatic fluids and some from reactions of these fluids with the country rock.

Regional Metamorphism 

Metamorphism can also result from regional increase of temperature and pressure associated with deep burial of rocks or tectonic activity. This regional metamorphism can change the mineralogy and texture of the pre-existing rocks, producing ore deposits of asbestos, talc, graphite, and other valuable non-metallic deposits. Metamorphism has been suggested as a possible origin of some hydrothermal fluids. It is a particularly likely cause in high-temperature, high-pressure zones, where fluids might be produced and forced out into the surrounding rocks to form replacement or cavity-filling deposits. For example, the native copper found along the top of ancient basalt flows in the Michigan copper district was apparently produced by metamorphism and alteration of the basalt, which released the copper and other materials that produced the deposits.
Our discussion of igneous and metamorphic processes has focused primarily on ore deposits. However, igneous and metamorphic processes are also responsible for producing a good deal of stone used in the construction industry. Granite, basalt, marble (metamorphosed limestone), slate (metamorphosed shale), and quartzite (metamorphosed sandstone), along with other rocks, are quarried to produce crushed rock and dimension stone in the United States. Stone is used in many aspects of construction work; but many people are surprised to learn that, in total value, with the exception of iron and steel, the stone industry is one of the largest non-fuel mineral industries in the United States.

Sedimentary Processes 

Sedimentary processes are often significant in concentrating economically valuable materials in sufficient amounts for extraction. As sediments are transported, wind and running water help segregate the sediment by size, shape, and density. Thus, the best sand or sand and gravel deposits for construction purposes are those in which the finer materials have been removed by water or wind. Sand dunes, beach deposits, and deposits in stream channels are good examples.

Sand and Gravel 

The U.S. sand and gravel industry amounts to about $8.5 billion per year, and, by volume mined (about 1300 million tons in 2006), it is one of the largest non-fuel mineral industries in the United States. Currently, most sand and gravel are obtained from river channels and water-worked glacial deposits. The United States now produces more sand and gravel than it needs, but demand is increasing. Environmental restrictions on extraction are causing sand and gravel operations to move away from areas with high population density, and shortages of sand and gravel are expected to increase as zoning and land development restrict locations where they may be extracted. Extraction from river channels and active floodplains can cause degradation to the river environment, and objections to river extraction operations are becoming more common.

Placer Deposits 

Stream processes transport and sort all types of materials according to size and density. Therefore, if the bedrock in a river basin contains heavy metals such as gold, streams draining the basin may concentrate heavy metals to form placer deposits (ore formed by deposit of sediments) in areas where there is reduced turbulence or velocity of flow, such as between particles on riffles, in open crevices

or fractures at the bottoms of pools, or at the inside curves of bends. Placer mining of gold known as a poor man method because a miner needed only a shovel, a pan, and a strong back to work the stream side claim helped to stimulate settlement of California, Alaska, and other areas of the United States. Furthermore, the gold in California attracted miners who acquired the expertise necessary to locate and develop other resources in the western conterminous United States and Alaska. Placer deposits of gold and diamonds have also been concentrated by coastal processes, primarily wave action. Beach sands and near-shore deposits are mined in Africa and other places.

Evaporite Deposits 

Rivers and streams that empty into the oceans and lakes carry tremendous quantities of dissolved material derived from the weathering of rocks. From time to time, geologically speaking, a shallow marine basin may be isolated by tectonic activity (uplift) that restricts circulation and facilitates evaporation. In other cases, climatic variations during the ice ages produced large inland lakes with no outlets, which eventually dried up. In either case, as evaporation progresses, the dissolved materials precipitate, forming a wide variety of compounds, minerals, and rocks called evaporite deposits that have important commercial value.
Most evaporite deposits can be grouped into one of three types:
Marine evaporites (solids) potassium and sodium salts, calcium carbonate, gypsum, and anhydrite; non-marine evaporites (solids) sodium and calcium carbonate, sulphate, borate, nitrate, and limited iodine and strontium compounds; and brines (liquids derived from wells, thermal springs, inland salt lakes, and seawaters) bromine, iodine, calcium chloride, and magnesium. Heavy metals (such as copper, lead, and zinc) associated with brines and sediments in the Red Sea, Salton Sea, and other areas are important resources that may be exploited in the future. Extensive marine evaporite deposits exist in the United States. The major deposits are halite (common salt, NaCl), gypsum anhydrite , and inter-bedded limestone Limestone, gypsum, and anhydrite are present in nearly all marine evaporite basins, and halite and potassium minerals are found in a few. Evaporite materials are widely used in industry and agriculture. Marine evaporites can form stratified deposits that may extend for hundreds of kilometres, with a thickness of several thousand meters. The evaporites represent the product of evaporation of seawater in isolated shallow basins with restricted circulation. Within many marine evaporite basins, the different deposits are arranged in broad zones that reflect changes in salinity and other factors controlling the precipitation of evaporites; that is, different materials may be precipitated at the same time in different parts of the evaporite basin. Halite, for example, is precipitated in areas where the brine is more saline, and gypsum where it is less saline. Economic deposits of potassium evaporite minerals are relatively rare but may form from highly concentrated brines. Non-marine evaporite deposits form by evaporation of lakes in a closed basin. Tectonic activity, such as faulting, can produce an isolated basin with internal drainage and no outlet. However, to maintain a favourable environment for evaporite mineral precipitation, the tectonic activity must continue to uplift barriers across possible outlets or lower the basin floor faster than sediment can raise it. Even under these conditions, economic deposits of evaporites will not form unless sufficient dissolved salts have washed into the basin by surface run off from surrounding highlands. Finally, even if all favourable environmental criteria are present, including an isolated basin with sufficient run off and dissolved salts, valuable non-marine evaporates, such as sodium carbonate or borate, will not form unless the geology of the highlands surrounding the basin is also favourable and yields run off with sufficient quantities of the desired material in solution. Some evaporite beds are compressed by overlying rocks and mobilized, then pierce or intrude the overlying rocks. Intrusions of salt, called salt domes, are quite common in the Gulf Coast of the United States and are also found in north western Germany, Iran, and other areas. Salt domes in the Gulf Coast are economically important because:

• They are a good source for nearly pure salt. Some have extensive deposits of elemental sulphur. 
• Some have oil reserves on their flanks. 

Salt domes are also environmentally important as possible permanent disposal sites for radioactive waste, although, because salt domes tend to be mobile, their suitability as disposal sites for hazardous wastes must be seriously questioned. Evaporites from brine resources of the United States are substantial, assuring that no shortage is likely for a considerable period of time. But many evaporites will continue to have a place value because transportation of these mineral commodities increases their price, so continued discoveries of high-grade deposits closer to where they will be consumed remains an important goal.

Biological Processes 

Organisms are able to form many kinds of minerals, such as the various calcium and magnesium carbonate minerals in shells and calcium phosphate in bones. Some of these minerals cannot be formed inorganically in the biosphere. Thirty-one different biologically produced minerals have been identified. Minerals of biological origin contribute significantly to sedimentary deposits. 

An interesting example of mineral deposits produced by biological processes are phosphates associated with sedimentary marine deposition. Phosphorus-rich sedimentary rocks are fairly common in some of the western states, as well as in Tennessee, North Carolina, and Florida. The common phosphorus-bearing mineral in these rocks is apatite, a calcium phosphate associated with bones and teeth. Fish and other marine organisms extract the phosphate from seawater to form apatite, and the mineral deposit results from sedimentary accumulations of the phosphate-rich fish bones and teeth. The richest phosphate mine in the world, known as Bone Valley, is located about 40 km east of Tampa, Florida. The deposit is marine sedimentary rocks composed in part of fossils of marine animals that lived 10 million to 15 million years ago, when Bone Valley was the bottom of a shallow sea. That deposit has supplied as much as one-third of the worlds phosphate production. Another important source of phosphorus is guano (bird faeces), which accumulates where there are large colonies of nesting sea birds and a climate arid enough for the guano to dry to a rock like mass. Thus, the formation of one of the major sources of phosphorus depends upon unique biological and geographical conditions.

Weathering Processes 

Weathering is responsible for concentrating some materials to the point that they can be extracted at a profit. Weathering processes can produce residual ore deposits in the weathered material and provide secondary enrichment of low-grade ore.

Residual Ore Deposits 

Intensive weathering of rocks and soils can produce residual deposits of the less soluble materials, which may have economic value. For example, intensive weathering of some rocks forms a type of soil known as laterite (a residual soil derived from aluminium- and iron-rich igneous rocks). The weathering processes concentrate relatively insoluble hydrated oxides of aluminium and iron, while more soluble elements, such as silica, calcium, and sodium, are selectively removed by soil and biological processes. If sufficiently concentrated, residual aluminium oxide forms an aluminium ore known as bauxite. Important nickel and cobalt deposits are also found in laterite soils, developed from ferromagnesian-rich igneous rocks. Insoluble ore deposits, such as native gold, are generally residual, meaning that, unless they are removed by erosion, they accumulate in weathered rock and soil. Accumulation of the insoluble ore minerals is favoured where the parent rock is a relatively soluble material, such as limestone. Care must be taken in evaluating a residual weathered rock or soil deposit because the near-surface concentration may be a much higher grade than ore in the parent, un-weathered rocks.

Secondary Enrichment 

Weathering is also involved in secondary enrichment processes to produce sulphide ore deposits from low-grade primary ore. Near the surface, primary ore containing such minerals as iron, copper, and silver sulphides is in contact with slightly acid soil water in an oxygen-rich environment. As the sulphides are oxidized, they are dissolved, forming solutions rich in sulphuric acid and in silver and copper sulphate; these solutions migrate downward, producing a leached zone devoid of ore minerals. Below the leached zone, oxidation continues, as the sulphate solutions continue to move toward the groundwater table. Below the water table, if oxygen is no longer available, the solutions are deposited as sulphides, increasing the metal content of the primary ore as much as tenfold. In this way, low-grade primary ore is rendered more valuable, and high-grade primary ore is made even more attractive.

The presence of a residual iron oxide cap at the surface indicates the possibility of an enriched ore below, but it is not always conclusive. Of particular importance to the formation of a zone of secondary enrichment is the presence in the primary ore of iron sulphide (for example, pyrite). Without it, secondary enrichment seldom takes place, because iron sulphide in the presence of oxygen and water forms sulphuric acid, which is a necessary solvent. Another factor favouring development of a secondary-enrichment ore deposit is the primary ore being sufficiently permeable to allow water and solutions to migrate freely downward. Given a primary ore that meets these criteria, the reddish iron oxide cap probably does indicate that secondary enrichment has taken place.

Several disseminated copper deposits have become economically successful because of secondary enrichment, which concentrates dispersed metals. For example, secondary enrichment of a disseminated copper deposit at Miami, Arizona, increased the grade of the ore from less than 1 percent copper in the primary ore to as much as 5 percent in some localized zones of enrichment.

Other Minerals from the Sea 

Mineral resources in seawater or on the bottom of the ocean are vast and, in some cases, such as magnesium, nearly unlimited. In the United States, magnesium was first extracted from seawater in 1940. By 1972, one company in Texas produced 80 percent of our domestic magnesium, using seawater as its raw material source. In 1992, three companies in Texas, Utah, and Washington extracted magnesium, respectively, from seawater, lake brines, and dolomite (mineral composed of calcium and magnesium carbonate). The deep-ocean floor may eventually be the site of a next mineral rush. Identified deposits include massive sulphide deposits associated with hydrothermal vents, manganese oxide nodules, and cobalt-enriched manganese crusts.

Sulfide Deposits 

Massive sulphide deposits containing zinc, copper, iron, and trace amounts of silver are produced at divergent plate boundaries (oceanic ridges) by the forces of plate tectonics. Pressure created by several thousand meters of water at ridges forces cold seawater deep into numerous rock fractures, where it is heated by up-welling magma to temperatures as high as The pressure of the heated water produces vents known as black smokers, from which the hot, dark-coloured, mineral-rich water emerges as hot springs. Circulating seawater leaches the surrounding rocks, removing metals that are deposited when the mineral-rich water is ejected into the cold sea. Sulphide minerals precipitate near the vents, forming massive tower like formations, rich in metals. The hot vents are of particular biologic significance because they support a unique assemblage of animals, including giant clams, tube worms, and white crabs. Ecosystems including these animals base their existence on sulphide compounds extruded from black smokers, existing through a process called chemosynthesis, as opposed to photosynthesis, which supports all other known ecosystems on earth. The extent of sulphide mineral deposits along oceanic ridges is poorly known, and, although leases to some possible deposits are being considered, it seems unlikely that such deposits will be extracted at a profit in the near future. Certainly, potential environmental degradation, such as decreased water quality and sediment pollution, will have to be carefully evaluated prior to any mining activity. Study of the formation of massive sulphide deposits at oceanic ridges is helping geologists understand some of the mineral deposits on land. For example, massive sulphide deposits being mined in Cyprus are believed to have formed at an oceanic ridge and to have been later uplifted to the surface.

Manganese Oxide Nodules 

Manganese oxide nodules cover vast areas of the deep-ocean floor. They contain manganese (24 percent) and iron (14 percent), with secondary copper (1 percent), nickel (1 percent), and cobalt (0.25 percent). Nodules are found in the Atlantic Ocean off Florida, but the richest and most extensive accumulations occur in large areas of the north eastern, central, and southern Pacific, where they cover 20 to 50 percent of the ocean floor.

Manganese oxide nodules are usually discrete, but are welded together locally to form a continuous pavement. Although they are occasionally found buried in sediment, nodules are usually surficial deposits on the seabed. Their size varies from a few millimetres to a few tens of centimetres in diameter (many are marble to baseball sized). Composed primarily of concentric layers of manganese and iron oxides, mixed with a variety of other materials, each nodule formed around a nucleus of a broken nodule, a fragment of volcanic rock, or, sometimes, a fossil. The estimated rate of nodular growth is 1 to 4 mm per million years. The nodules are most abundant in those parts of the ocean where sediment accumulation is at a minimum, generally at depths of 5 to 7 km. 

The origin of the nodules is not well understood; presumably, they might form in several ways. The most probable theory is that they form from material weathered from the continents and transported by rivers to the oceans, where ocean currents carry the material to the deposition site in the deep-ocean basins. The minerals from which the nodules form may also derive from submarine volcanism, or may be released during physical and biochemical processes and reactions that occur near the water sediment interface during and after deposition of the sediments. Mining of manganese oxide nodules involves lifting the nodules off the bottom and up to the mining ship; this may be done by using suction or scraper equipment. Although mining of the nodules appears to be technologically feasible, production would be expensive compared to mining manganese on land. In addition, there are uncertainties concerning ownership of the nodules, and nodule mining would cause significant damage to the sea-floor and local water quality, raising environmental concerns.

Cobalt-enriched Manganese Crusts 

Oceanic crusts rich in cobalt and manganese are present in the mid- and south-west Pacific, on flanks of sea-mounts, volcanic ridges, and islands. Cobalt content varies with water depth; the maximum concentration of about 2.5 percent is found at water depths of 1 to 2.5 km. Thickness of the crust averages about 2 cm. The processes of formation are not well understood. Both the nature and the extent of the crusts, which also contain nickel, platinum, copper, and molybdenum, are being studied by U.S. Geological Survey scientists.