minerals
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.
About: Forsterite · Olivine · Feldspar · Biotite · Quartz
A mineral is a naturally occurring inorganic element or compound with an orderly internal structure and a characteristic chemical composition, crystal form and physical properties.[1]
“Characteristic composition” is doing careful work in that sentence. It does not mean a fixed formula in every sample: most minerals accept substitutions, and their natural compositions vary.[1]
The Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association approves new minerals and rules on the status of existing ones. Its list of accepted species describes itself as the primary and official source on minerals, and records 6,239 valid species.[1,2]
Each name carries a status: approved for species accepted since 1958, grandfathered for those described earlier and still considered valid, redefined, renamed, or questionable where a mineral is poorly characterised.[1]
Search that list for “olivine” and there is no entry. What exists are forsterite and fayalite, the magnesium and iron end members of a continuous series; a natural olivine sits somewhere between them.[1,2]
The same holds for feldspar, for mica, for amphibole and for hornblende. “Biotite” — as universal a word as mineralogy has — denotes the series between annite and phlogopite, both of which are accepted species while biotite is not.[1,2,3,4]
This is not a fault in ordinary usage. The USGS itself lists quartz, feldspar, mica, amphibole, olivine and calcite together as common minerals, and in a sentence about rocks that is exactly right. It is only when the distinction is written into a database that collapsing it does damage.[1]
Microcline, orthoclase and sanidine share one approved formula, K(AlSi3O8), and are three separate species. What distinguishes them is how aluminium and silicon are ordered across the tetrahedral sites — structure, not chemistry.[1]
So a formula alone does not identify a mineral, and a mineral is not simply "a chemical compound found in rocks".[1,2]
Often said
“Olivine, feldspar, mica and biotite are minerals.”
What the evidence supports
They are groups and series. The IMA list of accepted species contains no entry for any of them; it contains forsterite and fayalite, albite and anorthite, muscovite, annite and phlogopite. The group names remain useful — they are simply not species names.
Supporting records: olivine-group-not-a-species · feldspar-group-not-a-species · biotite-end-members · ima-is-authority
Often said
“A mineral has one fixed chemical formula.”
What the evidence supports
An approved formula is an idealised composition. Most species accept substitution, and some approved formulas are written with variable cations for that reason — augite is (Ca,Mg,Fe)2Si2O6.
Supporting records: formula-is-idealised · olivine-solid-solution · pyroxene-end-members
Bracketed references link to the structured entity records whose claims support each paragraph. Last updated 2026-09-06.