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Formula: KFe2+3(AlSi3O10)(OH)2
Phyllosilicate (sheet silicate), mica group, forms a series with
phlogopite
Specific gravity: 3.3
Hardness: 2½ to 3
Streak: Brownish white
Colour: Black, brown
Common impurities: Ti,Mn,Mg,Ca,Na,K,Cl
Environments:
Plutonic igneous environments
Metamorphic environments
Annite occurs in igneous and metamorphic rocks that are low in magnesium.
It may decompose
to one of the following assemblages:
hematite + sanidine,
magnetite + sanidine,
fayalite + leucite +
kalsilite,
iron + sanidine
Alteration
almandine and
phlogopite to
pyrope and annite
Fe2+3Al2(SiO4)3 +
KMg3AlSi3O12(OH)2 ⇌
Mg3Al2Si3O12 +
KFe3AlSi3O10(OH)2
(JVW p 179)
This assemblage is commonly formed during amphibolite facies
metamorphism of pelitic rocks
(KB p129).
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)
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