Mineral Classification

All minerals except the native elements are systematically classified according to the identity of their dominant anion or anionic group. ^[1] This approach mirrors standard practice in inorganic chemistry, where compounds are grouped by their anion, and it works well for minerals because minerals sharing the same anion or anionic group tend to display close family resemblances in crystal structure, physical properties, and chemical behavior. ^[1] Additionally, while the cation content of many minerals varies considerably from sample to sample, the anion content is typically far more restricted - making anion identity a stable and reliable basis for classification. ^[1]

This anion-based approach also finds justification in Pauling’s rules for ionic solids. The first, third, and fourth rules together establish that anions define the fundamental architecture of a mineral structure, so it is natural that the anion should also define the classification. ^[1]

The Two Classification Systems

Two formal classification systems are widely used in mineralogy: the Dana system and the Strunz-Nickel system. Both follow the anion-based arrangement, but they differ in how finely they subdivide the mineral kingdom. ^[1]

The modern iteration of the Dana system organizes the mineral kingdom into 78 overarching classes, which are subsequently broken down into specific types and groups. ^[1] The Strunz-Nickel system uses a coarser top-level grid of 10 mineral classes, each of which is subdivided into divisions, families, and groups. ^[1] In both systems, a mineral’s classification is expressed as a series of numbers or letters that encodes its position in the hierarchy - a compact address within the classification tree.

Anion Groups Used in Classification

The table below lists the major mineral groups together with their defining anion or anionic group, as used in both classification systems. ^[1]

| Mineral Group | Anion or Anionic Group | Mineral Group | Anion or Anionic Group | | ----------------- | -------------------------- | ----------------- | ------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | Native elements | N/A | Carbonates | CO₃ | ^[1] | | Oxides | O | Nitrates | NO₃ | ^[1] | | Hydroxides | OH | Borates | BO₃, BO₄ | ^[1] | | Halides | Cl, Br, F | Chromates | CrO₄ | ^[1] | | Sulfides | S | Tungstates | WO₄ | ^[1] | | Arsenides | As | Molybdates | MoO₄ | ^[1] | | Antimonides | Sb | Phosphates | PO₄ | ^[1] | | Selenides | Se | Arsenates | AsO₄ | ^[1] | | Tellurides | Te | Vanadates | VO₄ | ^[1] | | Sulfates | SO₄ | Silicates | SiO₄ | ^[1] |

The silicates form the largest and most varied class, reflecting the abundance of Si and O in Earth’s crust and the remarkable structural flexibility of the silicon tetrahedron.

The Role of Temperature: Order, Disorder, and Mineral Identity

The classification of many minerals is not purely compositional - structural state matters too, and structural state is controlled by crystallization temperature and cooling history. As a general rule, high temperatures favor crystallization in a disordered state, with cations distributed randomly across available structural sites. Low temperatures favor ordered arrangements, where each cation type preferentially occupies specific sites. Slow cooling provides time for cations to migrate through the structure and parse themselves into their preferred sites; rapid cooling freezes the disordered high-temperature arrangement in place. ^[1]

The three potassium feldspar polymorphs - sanidine, orthoclase, and microcline - illustrate this vividly, all sharing the composition KAlSi₃O₈ but differing in the degree to which Al is ordered into specific tetrahedral sites. Sanidine is the disordered, high-temperature form found in volcanic rocks, which combine the two conditions most favorable for disorder: high temperature at crystallization and rapid cooling that prevents subsequent re-ordering. ^[1] K-feldspar in plutonic igneous rocks begins as sanidine at high temperature, but slow cooling allows Al to progressively migrate into preferred sites. At the deepest, most slowly cooled intrusive environments, ordering proceeds far enough to produce microcline, the fully ordered triclinic polymorph. ^[1] The mineralogical identity of the feldspar present in a rock therefore serves as a direct record of the thermal history of that rock - not just its bulk chemistry.

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