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Material World
Mineral · Fe2SiO4

Fayalite

The iron end of the olivine series — dark, dense, low-melting, and a common product of iron smelting slag.

Fayalite is iron olivine, the opposite end of the series from forsterite, and almost every property that makes forsterite useful is reversed in it. It is denser, darker, softer at temperature and melts nearly seven hundred degrees lower — which is why an olivine deposit is valued on its magnesium content and why iron-rich olivine is largely a curiosity industrially.

It is much less common than forsterite in nature, because the Earth's mantle is magnesium-rich, and it occurs mostly in unusual iron-rich igneous rocks and in some metamorphosed ironstones.

How it forms

Fayalite forms in iron-rich, silica-poor igneous rocks and in metamorphosed iron formations. It is also one of the commonest minerals in a place nobody thinks of as geological: iron smelting slag.

That makes it an archaeological marker. Fayalite in slag identifies a bloomery or blast furnace site and can be used to date and characterise early ironworking, since the slag composition records the ore and the process that produced it.

Uses

Almost none directly. Fayalite is too iron-rich for refractory use — the property that makes forsterite valuable — and it is not concentrated enough anywhere to be an iron ore.

Its significance is as an indicator. In igneous rocks it signals unusual iron-rich conditions; in slag it identifies iron production; and in laboratory work the fayalite-magnetite-quartz assemblage is a standard reference for the oxygen conditions under which a rock formed.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

belongs to the group

  • Olivine — mineral · the iron end member Wikidata

contains

is a component of

  • Granite — rock · only in the ferroan and alkaline granites, where iron-rich olivine is stable alongside quartz; absent from ordinary granite entirely

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • Crystallography Open Database
    COD Advisory Board / Vilnius University · CC0 — contributors place data in the public domain

Questions this page answers

Where it comes from, and what it becomes

Follow Fayalite back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Downstream — what it becomes

  • Fayalite → is a component of (only in the ferroan and alkaline granites, where iron-rich olivine is stable alongside quartz; absent from ordinary granite entirely) → Granite → is a source for (quarried in blocks for cladding, kerbs and monuments) → Dimension stone → is used in (quarried rather than mined, and the distinction is legal and technical rather than merely a word) → Mining and quarrying → is associated with (steam pumping allowed mines below the water table, which is what made deep coal possible) → Industrial Revolution complete chain
  • Fayalite → is a component of (only in the ferroan and alkaline granites, where iron-rich olivine is stable alongside quartz; absent from ordinary granite entirely) → Granite → is an input to (cut with diamond wire for dimension stone, and crushed elsewhere for aggregate) → Quarrying → produces (cut rather than blasted, because a quarry that shatters its stone has destroyed what it came for) → Dimension stone → is used in (cladding, flooring, kerbs and monuments) → Construction
  • Fayalite → is a component of (only in the ferroan and alkaline granites, where iron-rich olivine is stable alongside quartz; absent from ordinary granite entirely) → Granite → is used in → Construction
  • Fayalite → is a component of (only in the ferroan and alkaline granites, where iron-rich olivine is stable alongside quartz; absent from ordinary granite entirely) → Granite → is a source for (the last fraction of a granite to crystallise, not a separate magma) → Pegmatite
  • Fayalite → is a component of (only in the ferroan and alkaline granites, where iron-rich olivine is stable alongside quartz; absent from ordinary granite entirely) → Granite → is a source for (the roof of a granite, chemically rebuilt by vapour escaping from the granite below it) → Greisen
  • Fayalite → is a component of (only in the ferroan and alkaline granites, where iron-rich olivine is stable alongside quartz; absent from ordinary granite entirely) → Granite → is a source for (quarried in blocks for cladding, kerbs and monuments) → Dimension stone → is used in (cladding, flooring, kerbs and monuments) → Construction

These are the most distinct paths onward. Fayalite ends up in others besides.