Mineral

Besides the essential chemical composition and crystal structure, the description of a mineral species usually includes its common physical properties such as habit, hardness, lustre, diaphaneity, colour, streak, tenacity, cleavage, fracture, system, zoning, parting, specific gravity, magnetism, fluorescence, radioactivity, as well as its taste or smell and its reaction to acid.

The IMA is also reluctant to accept minerals that occur naturally only in the form of nanoparticles a few hundred atoms across, but has not defined a minimum crystal size.

[21] Recent advances in high-resolution genetics and X-ray absorption spectroscopy are providing revelations on the biogeochemical relations between microorganisms and minerals that may shed new light on this question.

[22] The group's scope includes mineral-forming microorganisms, which exist on nearly every rock, soil, and particle surface spanning the globe to depths of at least 1600 metres below the sea floor and 70 kilometres into the stratosphere (possibly entering the mesosphere).

These eight elements, summing to over 98% of the crust by weight, are, in order of decreasing abundance: oxygen, silicon, aluminium, iron, magnesium, calcium, sodium and potassium.

[47] For example, in most igneous rocks, the aluminium and alkali metals (sodium and potassium) that are present are primarily found in combination with oxygen, silicon, and calcium as feldspar minerals.

Other examples are the aluminosilicates kyanite, andalusite, and sillimanite (polymorphs, since they share the formula Al2SiO5), which differ by the coordination number of the Al3+; these minerals transition from one another as a response to changes in pressure and temperature.

Physical properties applied for classification include crystal structure and habit, hardness, lustre, diaphaneity, colour, streak, cleavage and fracture, and specific gravity.

Other less general tests include fluorescence, phosphorescence, magnetism, radioactivity, tenacity (response to mechanical induced changes of shape or form), piezoelectricity and reactivity to dilute acids.

[61] These families can be described by the relative lengths of the three crystallographic axes, and the angles between them; these relationships correspond to the symmetry operations that define the narrower point groups.

For example, halite (NaCl), galena (PbS), and periclase (MgO) all belong to the hexaoctahedral point group (isometric family), as they have a similar stoichiometry between their different constituent elements.

Silicon, as a general rule, is in four-fold coordination in all minerals; an exception is a case like stishovite (SiO2, an ultra-high pressure quartz polymorph with rutile structure).

Common habits include acicular, which describes needle-like crystals as in natrolite; dendritic (tree-pattern) is common in native copper or native gold with a groundmass (matrix); equant, which is typical of garnet; prismatic (elongated in one direction) as seen in kunzite or stibnite; botryoidal (like a bunch of grapes) seen in chalcedony; fibrous, which has fibre-like crystals as seen in wollastonite; tabular, which differs from bladed habit in that the former is platy whereas the latter has a defined elongation as seen in muscovite; and massive, which has no definite shape as seen in carnallite.

Iridescence is a variety of the play of colours where light scatters off a coating on the surface of crystal, cleavage planes, or off layers having minor gradations in chemistry.

Cleavage is not a universal property among minerals; for example, quartz, consisting of extensively interconnected silica tetrahedra, does not have a crystallographic weakness which would allow it to cleave.

[98] As the composition of the Earth's crust is dominated by silicon and oxygen, silicates are by far the most important class of minerals in terms of rock formation and diversity.

[99][100] Non-silicate minerals are subdivided into several other classes by their dominant chemistry, which includes native elements, sulfides, halides, oxides and hydroxides, carbonates and nitrates, borates, sulfates, phosphates, and organic compounds.

Most non-silicate mineral species are rare (constituting in total 8% of the Earth's crust), although some are relatively common, such as calcite, pyrite, magnetite, and hematite.

The base structure becomes either [AlSi3O8]− or [Al2Si2O8]2− There are 22 mineral species of feldspars, subdivided into two major subgroups – alkali and plagioclase – and two less common groups – celsian and banalsite.

Exsolution can be on a scale from microscopic to readily observable in hand-sample; perthitic texture forms when Na-rich feldspar exsolve in a K-rich host.

[113] The kaolinite-serpentine group consists of T-O stacks (the 1:1 clay minerals); their hardness ranges from 2 to 4, as the sheets are held by hydrogen bonds.

[114] Micas are also T-O-T-stacked phyllosilicates, but differ from the other T-O-T and T-O-stacked subclass members in that they incorporate aluminium into the tetrahedral sheets (clay minerals have Al3+ in octahedral sites).

Mica T-O-T layers are bonded together by metal ions, giving them a greater hardness than other phyllosilicate minerals, though they retain perfect basal cleavage.

These asbestos minerals form long, thin, flexible, and strong fibres, which are electrical insulators, chemically inert and heat-resistant; as such, they have several applications, especially in construction materials.

[127] Other examples of sorosilicates include lawsonite, a metamorphic mineral forming in the blueschist facies (subduction zone setting with low temperature and high pressure), vesuvianite, which takes up a significant amount of calcium in its chemical structure.

The remaining Al3+ can be in six-fold coordination (kyanite), five-fold (andalusite) or four-fold (sillimanite); which mineral forms in a given environment is depend on pressure and temperature conditions.

Bauxites are the chief aluminium ore, and are a heterogeneous mixture of the hydroxide minerals diaspore, gibbsite, and bohmite; they form in areas with a very high rate of chemical weathering (mainly tropical conditions).

The reaction of acid with carbonates, most commonly found as the polymorph calcite and aragonite (CaCO3), relates to the dissolution and precipitation of the mineral, which is a key in the formation of limestone caves, features within them such as stalactite and stalagmites, and karst landforms.

While hydrated calcium oxalate can be found in coal seams and other sedimentary deposits involving organic matter, the hydrothermal occurrence is not considered to be related to biological activity.

[154][155] According to these new rules, "mineral species can be grouped in a number of different ways, on the basis of chemistry, crystal structure, occurrence, association, genetic history, or resource, for example, depending on the purpose to be served by the classification.