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Stilpnomelane

Stilpnomelane is a phyllosilicate mineral. It has the chemical formula K(Fe²⁺, Mg, Fe³⁺)₈(Si, Al)₁₂(O, OH)₂₇·n. Stilpnomelane occurs associated with banded iron formations. It is a metamorphic mineral associated with the blueschist and greenschist... Wikipedia
Crystal class: Pinacoidal (1); (same H-M symbol)
Formula (repeating unit): K(Fe2+,Mg,Fe3+)8(Si,Al)12(O,OH)27·n(H2O)
IMA symbol: Stp
Optical properties: Biaxial (−)
Space group: P1
Unit cell: a = 21.72 Å, b = 21.72 Å; c = 17.4 Å; α = 124.14°; β = 95.86°, γ = 120°; Z = 6