What Is a Mineral?

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)

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