Sillimanite

nesosilicate

kyanite

Formula: Al2OSiO4 nesosilicate (insular SiO4 groups). Polymorph (same formula, different structure) of andalusite and kyanite.
Specific gravity: 3.23 to 3.27
Hardness: 6½ to 7½
Streak: White
Colour: Colourless, white, yellow, brown, green, grey
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments (occasional)
Metamorphic environments (common)

The occasional occurrence of sillimanite in granitic rocks is normally the result of contamination by clay-rich xenoliths, but hydrothermal activity may also be responsible. Sillimanite usually occurs in high pressure and high temperature thermal aureoles around intrusive rocks, and in high pressure and temperature regionally metamorphosed rocks.
In contact metamorphosed rocks it may occur in sillimanite-cordierite gneiss or sillimanite-biotite hornfels.
In regionally metamorphosed rocks it is found in quartz-muscovite- biotite-oligoclase- muscovite-sillimanite schist. In silica-poor rocks it may be associated with corundum.
Sillimanite also may be found in gneiss and hornfels.

It is a characteristic mineral of the amphibolite and granulite facies, and it is also a mineral of the pyroxene-hornfels facies.

Alteration

Aluminium silicate stability diagram andalusite, sillimanite and kyanite are polymorphs (same formula, different structure); they are in equilibrium at a pressure of 3.75 kbar and temperature 504oC (amphibolite facies).
Sillimanite is unstable at low temperature and cannot form at temperature less than 504oC.

muscovite and garnet to biotite, sillimanite and quartz
KAl2(AlSi3O10)(OH)2 + (Fe2+,Mg)3Al2(SiO4)3 → K(Fe2+,Mg)3(AlSi3O10)(OH)2 + 2Al2SiO5 + SiO2
muscovite is unstable in combination with garnet (DHZ 3 p24).

muscovite and quartz to sillimanite, K-feldspar and H2O
KAl2(Si3Al)O10(OH)2 + SiO2 ⇌ Al2SiO5 + KAlSi3O8 + H2O
At 5 kbar pressure the equilibrium temperature is about 690oC (amphibolite facies) (SERC).
The forward reaction is strongly endothermic (absorbs heat) and the reverse reaction in exothermic (gives out heat), hence the forward reaction is favoured by high temperatures, as the system adjusts to bring the temperature back down (KB p17).
Although the muscovite-quartz assemblage is stable over a large part of the PT range of regional metamorphism, at temperatures around 600 to 650oC it is replaced by sillimanite and K-feldspar (DHZ 3 p24).

sillimanite, annite and H2O to staurolite, muscovite, SiO2 and O2
31Al2SiO5 + 4KFe2+3(AlSi3O10)(OH)2 + 6H2O → 34Fe2+2Al9si4O23(OH) + KAl2 (AlSi3O10)(OH)2 + 7 SiO2 + 1.5O2
Staurolite may occur as a product of retrograde metamorphism according to the above reaction (DHZ 1A p859).

spinel-hercynite, sillimanite and SiO2 to sapphirine
7(Mg,Fe2+)Al2O4 + 2Al2SiO5 + SiO2 → 4(Mg,Fe)1.75Al4.5Si0.75O10
(DHZ 2A p633)

staurolite and quartz to almandine, sillimanite and H2O
62Fe2+2Al9Si4O23(OH) + 11SiO2 ⇌ 4Fe2+3Al2(SiO4)3 + 23Al2OSiO4 + 3H2O
Increasing temperature favours the forward reaction. At higher pressure kyanite replaces sillimanite in the above reaction (AM61.699-709).

Common impurities: Fe

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