Minerals

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• What is a mineral, really?

  Olivine is not a mineral. Neither is feldspar, mica, amphibole or biotite. All five are names in constant scientific use, and none of them names an accepted species — a distinction that matters as soon as anything is written down.

• Crystal structure and substitution

  A crystal is a set of sites of particular sizes and charges. Which ions can occupy them decides what the mineral is, how its composition varies, and — as it turns out — where elements go in the Earth.

• What is a mineral?

  The definition is short and it does real work: it is what separates a mineral from a rock, and it is why a rock’s identity depends on how its minerals are arranged, not only on which ones are present.

Silicates by structure

Grouped by how their tetrahedra link, because that is what governs their form and their behaviour.

• Mineral

  Forsterite

  The magnesium end member of the olivine series and the dominant mineral of the upper mantle. Its structure is isolated silicate tetrahedra bound by magnesium and iron, which is why its composition varies continuously toward the iron end member.

• Mineral

  Fayalite

  The iron end member of the olivine series. Natural olivines are almost never pure fayalite or pure forsterite but sit somewhere between them.

• Mineral

  Almandine

  The iron–aluminium garnet, common in metamorphosed shales and a standard indicator of metamorphic conditions.

• Mineral

  Pyrope

  The magnesium–aluminium garnet, characteristic of high-pressure rocks and of the mantle.

• Mineral

  Grossular

  The calcium–aluminium garnet, typical of metamorphosed impure carbonate rocks.

• Mineral

  Diopside

  A calcium–magnesium clinopyroxene: a single-chain silicate and one of the principal minerals of the mantle and of calcium-rich metamorphic rocks.

• Mineral

  Enstatite

  The magnesium orthopyroxene, and with olivine one of the main constituents of mantle peridotite.

• Mineral

  Augite

  The common clinopyroxene of basalts and gabbros, whose approved formula is written with variable cations because its composition genuinely varies.

• Mineral

  Muscovite

  The common white mica: a sheet silicate whose perfect basal cleavage follows the planes of its structure, and whose platy grains align under directed pressure to make foliation.

• Mineral

  Annite

  The iron end member of the mica series commonly called biotite — a name the IMA list does not recognise as a species.

• Mineral

  Phlogopite

  The magnesium end member of the series commonly called biotite, forming a solid solution with annite.

• Mineral

  Albite

  The sodium end member of the plagioclase feldspar series, and one of the most abundant minerals of the continental crust.

• Mineral

  Anorthite

  The calcium end member of the plagioclase series. The coupled substitution that links it to albite exchanges sodium and silicon for calcium and aluminium together, keeping charge balanced.

• Mineral

  Microcline

  A potassium feldspar with an ordered aluminium–silicon distribution, and one of three species sharing the same formula but differing in structural state.

• Mineral

  Orthoclase

  A potassium feldspar of intermediate structural order — the same composition as microcline and sanidine, distinguished from them by how aluminium and silicon are arranged.

• Mineral

  Sanidine

  The high-temperature potassium feldspar, with aluminium and silicon disordered across its tetrahedral sites. Its presence records rapid cooling.

• Mineral

  Quartz

  A hard, chemically simple silicate mineral (silicon dioxide) that is among the most abundant minerals of the continental crust and the index mineral for hardness 7 on the Mohs scale.

• Mineral

  Zircon

  A zirconium silicate that accepts uranium into its structure but excludes lead, so that any lead it contains accumulated by decay. That property, plus its durability, makes it the most important single mineral in geochronology.

• Mineral

  Kaolinite

  A clay mineral produced by the hydrolysis of feldspar. It is the residue of intense chemical weathering, retaining aluminium and silicon after the soluble cations have gone.

• Mineral

  Montmorillonite

  A swelling clay of the smectite family, whose interlayer sites take up water and exchangeable cations.

Oxides, carbonates and sulfides

Non-silicate species whose chemistry carries the redox and carbon stories.

• Mineral

  Magnetite

  A strongly magnetic iron oxide. Its presence in ocean-floor basalt is what allowed the sea floor to record the polarity of Earth’s magnetic field as it cooled — and so what made seafloor spreading detectable.

• Mineral

  Hematite

  The ferric iron oxide, and the mineral in which iron released by oxidation is most commonly fixed.

• Mineral

  Calcite

  The commonest carbonate mineral and the principal constituent of limestone. Its solubility depends on the acidity of the water it meets, which makes it the mineral through which carbon moves between rock and ocean.

• Mineral

  Dolomite

  A calcium–magnesium carbonate with the two cations ordered into alternate layers — a structural distinction, not merely a compositional one.

• Mineral

  Pyrite

  Iron disulfide, the commonest sulfide mineral. Its formation requires reducing conditions and its breakdown on exposure to oxygen and water drives acid generation.

• Mineral

  Chalcopyrite

  A copper–iron sulfide, and the standing illustration of chalcophile behaviour: copper in a sulfide, not a silicate.

Groups and series

Names in constant use that the IMA list does not accept as species.

• Mineral group

  Olivine

  A group of orthosilicate minerals, not a species. Natural olivine is a solid solution between forsterite and fayalite, and it is those end members — not “olivine” — that the IMA list accepts.

• Mineral group

  Feldspar

  The most abundant mineral group of the crust — and a group, not a species. Its members split into the plagioclase series and the alkali feldspars, three of which share one formula and differ only in structural order.

• Mineral group

  Plagioclase

  The continuous solid-solution series between albite and anorthite. A plagioclase composition is a position along that series, which is why the series and not a single formula is the right unit of description.

• Mineral group

  Alkali feldspar

  The potassium- and sodium-rich feldspars, including microcline, orthoclase and sanidine — one composition, three structural states, three species.

• Mineral group

  Pyroxene

  Single-chain silicates divided into orthopyroxenes and clinopyroxenes. Like olivine, a group name rather than a species.

• Mineral group

  Mica

  Sheet silicates whose structure produces perfect basal cleavage. Muscovite is a species within it; “biotite” is not.

• Mineral group

  Biotite

  A name in universal scientific use that the IMA list does not accept as a species: it denotes the series between annite and phlogopite. Lithoen records it as a series so that the name remains usable without being mistaken for a species.

• Mineral group

  Garnet

  Cubic orthosilicates sharing one structure type across a wide range of compositions, including almandine, pyrope and grossular.

• Mineral group

  Amphibole

  Double-chain silicates. “Hornblende” — the name most often used for them in the field — is not an accepted species either, and Lithoen does not record it as one.

• Mineral group

  Clay minerals

  A functional grouping of fine-grained hydrous sheet silicates defined as much by grain size and behaviour as by structure. It is not an IMA grouping, and Lithoen marks it informal rather than implying an authority that does not exist.

• Mineral group

  Silicates

  The mineral class built from silicon–oxygen tetrahedra, subdivided by how those tetrahedra link: isolated, in chains, in sheets, or in three-dimensional frameworks.