Stilpnomelane

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Stilpnomelane
Stilpnomelane-217448.jpg
General
CategoryPhyllosilicates
Smectite group
Formula
(repeating unit)
K(Fe2+,Mg,Fe3+)8
(Si,Al)12(O,OH)27·n(H2O)
Strunz classification9.EG.40
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupP1
Unit cella = 21.72 Å, b = 21.72 Å
c = 17.4 Å; α = 124.14°
β = 95.86°, γ = 120°; Z = 6
Identification
ColorBlack, greenish black, yellowish bronze, greenish bronze
Crystal habitPlatey, scaly and fibrous with comb structures; radiating groups
CleavagePerfect on {001}, imperfect on {010}
TenacityBrittle
Mohs scale hardness3 - 4
LusterVitreous to dull
StreakGray white
DiaphaneitySubtranslucent to opaque
Specific gravity2.77 - 2.96
Optical propertiesBiaxial (-)
Refractive indexnα = 1.543 - 1.634 nβ = 1.576 - 1.745 nγ = 1.576 - 1.745
Birefringenceδ = 0.033 - 0.111
PleochroismX: bright golden yellow to pale yellow Y and Z: deep reddish brown, to deep green to nearly black
2V angle0-40 measured
DispersionNone
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]

References[edit]