Formula: Al4+2xSi2-2xO10-x (x≈ 0.4)
Nesosilicate (insular SiO4 groups), andalusite group
Crystal System: Orthorhombic
Specific gravity: 3.11 to 3.26 measured, 3.17 calculated
Hardness: 6 to 7
Streak: White
Colour: Colourless, white, yellow, pink, red, gray
Solubility: Insoluble in acids (AM 9.211)
Common impurities: Ti,Fe,Na,K

Metamorphic environments

Mullite occurs in high temperature thermally metamorphosed argillaceous (clay-sized particles) rocks, formed by heating any of the Al2O(SiO4) paramorphs (andalusite, sillimanite, kyanite) (Dana)

At ValSissone, Italy, mullite occurs as microscopic inclusions in sillimanite in pelitic (derived by metamorphism of a fine-grained sediment) inclusions in a tonalite, associated with sillimanite and kyanite (HOM).

At Eddy Creek quarry, Weld river, southern Tasmania, Australia, mullite has been identified in a skarn deposit which also contains artinite, foshagite, fukalite, hillebrandite, plombièrite and thaumasite. Apart from artinite, none of these species is visually identifiable (AJM 18.1.65).

In the Bushveld complex, Maandagshoek, South Africa, mullite occurs in a banded mullite - sillimanite - corundum - spinel - cordierite graphitic xenolith (Lauf p186).

At the Roberts Victor mine, South Africa, mullite occurs with corundum at the margin of kyanite in a partially melted eclogite nodule in kimberlite (Lauf p186).

At Sithean Sluaigh, Argyllshire, Scotland, mullite occurs with corundum, hercynite, cordierite, magnetite, spinel, sanidine and pseudobrookite in rocks adjacent to a dolerite plug (Lauf p186, HOM). The high temperature of the doleritic magma fused the original sedimentary rocks (Lauf p186).

At the type locality, Seabank Villa, Isle of Mull, Argyll and Bute, Scotland, UK, Seabank Villa, mullite occurs in fused argillaceous (clay-sized particles) inclusions in Tertiary eruptive rocks (HOM, AM9.212) associated rarely with corundum (AM 9.212, Lauf p186).

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