Mineral: "A homogeneous, naturally occurring, solid inorganic crystalline substance with a specific chemical composition." Thus:

The geometric regularity with which anions and cations arrange themselves in a lattice is reflected on a macroscopic scale in the geometry of the crystal faces of the growing crystal grain. The ions' packing arrangement determines the crystal's macroscopic cubical shape.


When do crystals form? Mineral complications and caveats

Cation substitution: Remember:

As a result, anions tend to occupy a much larger volume than cations of similar mass. Thus, the geometry of a crystal lattice is largely determined by the packing arrangement of anions. Different cations often substitute for one another in the lattice.
EG: the mineral olivine may have iron (Fe2SiO4) or magnesium (Mg2SiO4), or a combination ((Fe2Mg2)SiO4). Another common cation substitutions is aluminum (Al3+) and silicon (Si4+).

More


Polymorphs: Depending on the circumstances, the same ions might form crystals with different geometries. We have already seen the common example of diamond and graphite. Such groups are termed polymorphs of carbon. The former forms a three-dimensional lattice, the latter a series of two dimensional sheets. Calcite and aragonite are polymorphs of calcium carbonate (CaCO3).

More


Categories of rock-forming minerals:

  • Major physical properties of minerals:

  • Many other physical properties are used in mineral identification, albeit less frequently. These include:


    Key concepts and vocabulary: