Staurolite

nesosilicate

sillimanite

Formula: Fe2+2Al9Si4O23(OH)
Nesosilicate (insular SiO4 groups)
Specific gravity: 3.7 to 3.8
Hardness: 7 to 7½
Streak: White
Colour: Reddish brown to blackish brown
Solubility: Insoluble in water, hydrochloric and nitric acid, slightly soluble in sulphuric acid
Environments:

Metamorphic environments

Staurolite is typically a product of medium-grade regional metamorphism in which clay, shale, mudstone, siltstone or marl is converted to schist (Lauf p205).

In high-grade metamorphosed sedimentary rocks staurolite may be associated with sillimanite, andalusite or cordierite (R&M 93.4.349-350).

In medium grade mica schist staurolite is typically associated with almandine, muscovite, biotite, kyanite and quartz (R&M 93.4.349-350, Lauf p205).

In lower grade regional metamorphic rocks staurolite is often associated with chloritoid. During retrograde metamorphism staurolite may be altered to, or replaced by, chlorite, mica, magnetite, hercynite or quartz (R&M 93.4.349-350. Lauf p205).

Staurolite is resistant to weathering, and commonly found in detrital deposits (Lauf p205).
It may be found in mica schist, gneiss and phyllite.
Staurolite is a characteristic mineral of the amphibolite facies. It is often associated with almandine.
Pseudomorphs of muscovite after staurolite have been reported (Lauf p207).

In Patrick and Henry counties, Virginia, USA, staurolite occurs in schist. The crystals are wholly or partially altered to muscovite variety sericite, chlorite and limonite, and they contain numerous inclusions of small garnets, which weather out (Lauf p208).

In Zambia staurolite co-exists with magnetite, hematite, ilmenite and chlorite (Lauf p207).

Alteration

Staurolite is frequently altered to muscovite variety sericite (common) and chlorite (less common) (R&M 93.4.350).

chloritoid and andalusite to staurolite and quartz and H2O
4Fe2+Al2O(SiO4)(OH)2 + 5Al2OSiO4 ⇌ 22Fe2+2Al9Si4O23(OH) + SiO2 + 3H2O
Increasing temperature favours the forward reaction. At higher pressure kyanite replaces andalusite in the above reaction (AM61.699-709).

chloritoid and quartz to staurolite, almandine and H2O
23Fe2+Al2O(SiO4)(OH)2 + 8SiO2 ⇌ 4Fe2+2Al9Si4O23(OH) + 5Fe2+3Al2(SiO4)3 + 21H2O (DHZ 1A p844)

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).

staurolite, annite and O2 to hercynite, magnetite, muscovite,corundum, SiO2 and H2O
2Fe2+2Al9Si4O23(OH) + KFe2+3 (AlSi3O10)(OH)2 +2O2 → 4Fe2+Al2O4 + Fe2+Fe3+2O4 + KAl2 (AlSi3O10)OH)2 + 4Al2O3 + 8SiO2 + 2H2O (DHZ 1A p860)

staurolite and quartz to almandine and 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: Ti,Cr,Mn,Co,Zn,Li,H2O

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