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2011-03-12T15:46:00.000-08:00

a mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes.^[2]

The modern classifications include an organic class – in both the new Dana and the Strunz classification schemes.^[3]^[4]
The chemical composition may vary between end members of a mineral system. For example the plagioclase feldspars comprise a continuous series from sodium and silicon-rich albite (NaAlSi₃O₈) to calcium and aluminium-rich anorthite (CaAl₂Si₂O₈) with four recognized intermediate compositions between. Mineral-like substances that don't strictly meet the definition are sometimes classified as mineraloids. Other natural-occurring substances are nonminerals. Industrial minerals is a market term and refers to commercially valuable mined materials (see also Minerals and Rocks section below).
A crystal structure is the orderly geometric spatial arrangement of atoms in the internal structure of a mineral. There are 14 basic crystal lattice arrangements of atoms in three dimensions, and these are referred to as the 14 "Bravais lattices". Each of these lattices can be classified into one of the seven crystal systems, and all crystal structures currently recognized fit in one Bravais lattice and one crystal system. This crystal structure is based on regular internal atomic or ionic arrangement that is often expressed in the geometric form that the crystal takes. Even when the mineral grains are too small to see or are irregularly shaped, the underlying crystal structure is always periodic and can be determined by X-ray diffraction. Chemistry and crystal structure together define a mineral. In fact, two or more minerals may have the same chemical composition, but differ in crystal structure (these are known as polymorphs). For example, pyrite and marcasite are both iron sulfide, but their arrangement of atoms differs. Similarly, some minerals have different chemical compositions, but the same crystal structure: for example, halite (made from sodium and chlorine), galena (made from lead and sulfur) and periclase (made from magnesium and oxygen) all share the same cubic crystal structure.
Crystal structure greatly influences a mineral's physical properties. For example, though diamond and graphite have the same composition (both are pure carbon), graphite is very soft, while diamond is the hardest of all known minerals. This happens because the carbon atoms in graphite are arranged into sheets which can slide easily past each other, while the carbon atoms in diamond form a strong, interlocking three-dimensional network.

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2011-03-12T15:44:00.001-08:00

a mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes.^[2]

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2011-03-12T15:43:00.001-08:00

a mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes.^[2]