Stilpnomelane Category Phyllosilicates Smectite groupFormula (repeating unit) K(Fe2+ ,Mg,Fe3+ )8 (Si,Al)12 (O,OH)27 ·n(H2 O) IMA symbol Stp[ 1] Strunz classification 9.EG.40 Crystal system Triclinic Crystal class Pinacoidal (1 ) (same H-M symbol ) Space group P 1 Unit cell a = 21.72 Å , b = 21.72 Å Color Black, greenish black, yellowish bronze, greenish bronze Crystal habit Platey, scaly and fibrous with comb structures; radiating groups Cleavage Perfect on {001}, imperfect on {010} Tenacity Brittle Mohs scale hardness 3–4 Luster Vitreous to dull Streak Gray white Diaphaneity Subtranslucent to opaque Specific gravity 2.77 – 2.96 Optical properties Biaxial (−) Refractive index nα = 1.543 – 1.634 nβ = 1.576 – 1.745 nγ = 1.576 – 1.745 Birefringence δ = 0.033 – 0.111 Pleochroism X: bright golden yellow to pale yellow
Y and Z: deep reddish brown, to deep green to nearly black 2V angle 0–40 measured Dispersion None References [ 2] [ 3] [ 4]
Stilpnomelane is a phyllosilicate mineral. It has the chemical formula K(Fe2+ ,Mg,Fe3+ )8 (Si,Al)12 (O,OH)27 ·n(H2 O) .[ 4]
Stilpnomelane occurs associated with banded iron formations . It is a metamorphic mineral associated with the blueschist and greenschist facies .[ 2]
It was first described in 1827 for an occurrence in Moravia in the Czech Republic . The name is derived from the Greek stilpnos melanos [ 3]
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