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Laytonville Stilpnomelane.jpg
Stilpnomelane with quartz, collected from a road cut near Laytonville, California
Category Phyllosilicates
Smectite group
(repeating unit)
Strunz classification 9.EG.40
Crystal system Triclinic
Crystal class Pinacoidal (1)
(same H-M symbol)
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
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

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 [1][2][3]

Stilpnomelane is a phyllosilicate mineral. It has the formula K(Fe2+,Mg,Fe3+)8(Si,Al)12(O,OH)27·n(H2O).[3]

Stilpnomelane occurs associated with banded iron formations. It is a metamorphic mineral associated with the blueschist and greenschist facies.[1]

It was first described in 1827 for an occurrence in Moravia in the Czech Republic. The name is derived from the Greek stilpnos for shining, and melanos for black.[2]