Stilpnomelane

minerals

chlorite

epidote

lawsonite

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Formula: (K,Ca,Na)(Fe,Mg,Al)8(Si,Al)12(O,OH)36.nH2O phyllosilicate (sheet silicate)
Specific gravity: 2.59 to 2.96
Hardness: 3 to 4
Streak: White, grey
Colour: Red, brown, black, dark green, dark reddish brown, golden brown
Common impurities: Ti,Mn,Ca,Na,K,H2O
Environments:

Metamorphic environments

Stilpnomelane can be an important constituent of low-grade, regionally metamorphosed slate and schist, and it also occurs in banded iron formations. It is a mineral of the prehnite-pumpellyite, greenschist and blueschist facies.

In the Kanto mountains, Japan, the minerals associated with stilpnomelane depend on the metamorphic grade.
At low grade metamorphism stilpnomelane is associated with quartz, albite, calcite, graphite and hematite.
At medium grade metamorphism the assemblages are (1) stilpnomelane and chlorite, and (2) lawsonite, stilpnomelane and muscovite variety sericite.
At the highest grade metamorphism the assemblages are (1) muscovite, chlorite and stilpnomelane, (2) chlorite, epidote, garnet and stilpnomelane, (3) chlorite, garnet, muscovite and stilpnomelane.

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