Cronstedtite

minerals

clinochlore

siderite

fayalite

Formula:(Fe2+,Fe3+)3(Si,Fe3+)2O5(OH)4
Phyllosilicate (sheet silicate) serpentine group

Formula: Fe3Si2O5(OH)4
Specific gravity: 3.34 to 3.35
Common impurities: Al,Ca
Hardness: 3½
Streak: Dark olive green
Colour: Black, dark brown-black, green-black
Environments:

Hydrothermal environments

Cronstedtite is a low temperature hydrothermal product in ore veins, commonly associated with clinochlore, pyrite, quartz, siderite and sphalerite.

Alteration

fayalite, H2O and O2 to cronstedtite and magnetite
6Fe2+2(SiO4) + 6H2O + ½O2 = 3Fe3Si2O5(OH)4 + Fe2+Fe3+2O4
(DHZ 1A p262)

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