Minerals are naturally occurring inorganic solids with an ordered chemical composition and ordered atomic structure, each mineral possessing unique physical characteristics that distinguish it from others; such as color, luster, hardness, cleavage strength, specific gravity solubility magnetism fracture resistance.
Identification of minerals is typically done through their crystal shapes; however, other ways can include fluorescence, odor and other characteristics.
Color
Color is one of the primary characteristics that define minerals; its color depends on how it absorbs and reflects light. Minerals which absorb all wavelengths of visible light appear black; those reflecting most wavelengths appear white.
Birefringence refers to how certain minerals change colors depending on how they’re oriented; an example would be ZnS’ sphalerite which changes from yellow, brownish green, to dark purple depending on its orientation.
Other minerals possess color centers where particular elements cause them to absorb specific wavelengths of light. Aluminum, for instance, may give calcite a blue streak but is usually not responsible for its overall hue. Finding out why certain minerals are colored usually requires extensive investigation that involves precision spectroscopy, impurity analysis to ppm levels, radiation testing and laboratory synthesis techniques.
Luster
Luster describes how minerals reflect light when light hits their surfaces, with metallic minerals reflecting like polished metals, while non-metallics may display glassy, dull, pearly or silky surfaces depending on how light interacts with their surfaces. Although luster can help distinguish minerals, other properties have less potential for error and provide quicker identification processes.
Glassy (vitreous) minerals like quartz and tourmaline exhibit vitreous sheen that resembles window glass, while pearly minerals have thin transparent co-planar sheets which reflect light similarly to mother-of-pearl; examples include mica and talc. Greasy minerals like opal and cordierite showcase an optical play that looks similar to grease or fat while resinous minerals such as amber and spessartine garnet display an aromatic resinous sheen.
Cleavage
Cleavage, fracture, and parting are properties resulting from repeated repetition of fundamental atomic groupings within a mineral’s crystal lattice; their primary characteristic being repeated symmetry repetition of unit cells within this lattice. Although not easily visible as colors, streaks, hardness levels or crystal growth forms, cleavage can distinguish one mineral from others by showing whether its crystal lattice fractures into smooth planes aligned with zones of weak bonding or not.
Perfect cleavage gives broken specimens an almost mirror-like surface; imperfect cleavage tends to be less smooth, leading to fragments being separated with greater frequency. Minerals with three right-angle cleavage directions at right angles to one another are classified as having cubic cleavage while those with two cleavage directions at right angles from each other are classified as having rhombohedral cleavage.
Hardness
Hardness of minerals is an integral aspect of their classification and identification. Geologists use hardness measurements to compare similar minerals by their softness or hardness levels; additionally, geologists use hardness comparisons as a measure for other physical attributes like cleavage or luster comparisons.
Friedrich Mohs’ Mohs scale has long been used as the standard method for measuring mineral hardness. This ranking system ranks minerals on a scale from one (talc) to ten (diamond), with talc being softest and diamond being hardest.
Hardness of minerals is determined by their arrangement of atoms, as well as the strength of their bonds; covalent bonds generally create harder minerals than ionic or van der Waals bonds, while many minerals exhibit anisotropy–meaning their hardness varies slightly in different directions.
Density
Geologists often use density as a way of distinguishing minerals; its ratio between mass and volume allows geologists to make this determination. When measuring density of minerals, geologists look for evidence of compact structures; the higher its density the denser its structure is likely to be.
A rock’s bulk (or dry) density includes its mineral assemblage and pore spaces while grain density only refers to individual mineral particles that make up its composition. Porosity affects bulk density more than composition does and may even be determined more by geology than composition.
Typically, basic igneous and extrusive rocks typically possess low bulk densities while sedimentary and metamorphic ones have greater densities. Some metal ore-bearing rocks tend to have much higher bulk densities due to the presence of high concentrations of metal ores particles.
