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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
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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