Annite

phyllosilicate

almandine

Formula: KFe2+3(AlSi3O10)(OH)2
Phyllosilicate (sheet silicate) mica group
Specific gravity: 3.3
Hardness: 2½ to 3
Streak: Brownish white
Colour: Black, brown
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)

Common impurities: Ti,Mn,Mg,Ca,Na,K,Cl

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