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Formula: Fe2+2Al9Si4O23(OH)
Nesosilicate (insular SiO4 groups)
Crystal System: Monoclinic
Specific gravity: 3.7 to 3.8 measured, 3.686 calculated
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
Common impurities: Ti,Cr,Mn,Co,Zn,Li,H2O
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).
Localities
At Dusky Sound, Fiordland National Park, Southland District, Southland Region, New Zealand, staurolite with up to 6.5% MgO and
magnesium as the dominant divalent cation is recorded from an incompletely reconstituted
meta-troctolite. It occurs in complex assemblages including
pyrope, hornblende,
spinel and corundum, and is estimated to have
crystallized at about 12 kbar and 750°C. A green staurolite with 2% Cr2O3 is also recorded from a
mafic meta-tuff from Fiordland
(AM 69.531–540).
In Patrick and Henry counties, Virginia, USA, staurolite occurs in schist.
The crystals
are wholly or partially altered to 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 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).
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