Open Adit™ - Mineral Variations

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The structure of a mineral determines how much variation is possible. For instance, the mineral quartz (SiO₂) has very little compositional variation; it is essentially composed of one atom of silicon (Si) for every two oxygen (O) atoms, and its structure makes it difficult for other sorts of atoms to become included. Other minerals are much more variable, and some materials, such as limonite, are so extremely variable that they are considered to be mineraloids, rather than true minerals.

Variations in mineral composition can produce differences in structure, and are reflected in differences in the color, density, and other physical and optical properties. These differences are used to define different "varieties" of a particular mineral. For example, red or pink tourmaline is called rubellite, blue tourmaline is indicolite and iron-rich and black tourmaline is schorl.

Minerals crystallize from complex chemical solutions so there is ample opportunity for the substitution of one ion for another. Practically all minerals display some variation in their composition. We call this process ionic substitution.

There are several factors that determine how much variation will take place:

Size of the ion. Ions of 2 elements can readily substitute for each other only if their sizes differ by <15%
Temperature at which the crystal is grown. The higher the temperature, the greater the amount of disorder, and the less strigent are the space requirements of the crystal lattice for ionic substitution

Usually, when the elements in a mineral vary, a series is formed. A series consists of a minerals in which one (or more) of the elements varies. Most mineral series form solid solutions. In a solid solution, there are intermediate members between the two end members.; such is the case with olivine. The formula for olivine is written as (Mg, Fe)₂SiO₄ (If a chemical formula with two elements in parenthesis is separated by a comma, the number of those elements vary). This is the formula for common olivine.The end members (all Mg or all Fe) are forsterite: Mg₂SiO₄ and fayalite: Fe₂SiO₄. The intermediary member is chrysolite, which is a combination of the two (Mg,Fe)₂SiO₄.

Other examples of solid-solution series are the plagioclase feldspars, in which the end members are albite Na(AlSi₃O₈) and anorthite, Ca(Al₂Si₂O₈), and the garnets in which complete solid-solution relationships exist between most of the named varieties.

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