Stilpnomelane is a phyllosilicate mineral. It has the formula K(Fe2+,Mg,Fe3+)8(Si,Al)12(O,OH)27·n(H2O).[3]
| Stilpnomelane |
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| General |
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| Category | Phyllosilicates
Smectite group |
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Formula
(repeating unit) | K(Fe2+,Mg,Fe3+)8
(Si,Al)12(O,OH)27·n(H2O) |
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| Strunz classification | 9.EG.40 |
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| Crystal system | Triclinic |
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| Crystal class | Pinacoidal (1)
(same H-M symbol) |
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| Space group | P1 |
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| Unit cell | a = 21.72 Å, b = 21.72 Å
c = 17.4 Å; α = 124.14°
β = 95.86°, γ = 120°; Z = 6 |
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| Identification |
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| Color | Black, greenish black, yellowish bronze, greenish bronze |
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| Crystal habit | Platey, scaly and fibrous with comb structures; radiating groups |
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| Cleavage | Perfect on {001}, imperfect on {010} |
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| Tenacity | Brittle |
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| Mohs scale hardness | 3 - 4 |
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| Luster | Vitreous to dull |
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| Streak | Gray white |
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| Diaphaneity | Subtranslucent to opaque |
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| Specific gravity | 2.77 - 2.96 |
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| Optical properties | Biaxial (-) |
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| Refractive index | nα = 1.543 - 1.634 nβ = 1.576 - 1.745 nγ = 1.576 - 1.745 |
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| Birefringence | δ = 0.033 - 0.111 |
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| Pleochroism | X: bright golden yellow to pale yellow Y and Z: deep reddish brown, to deep green to nearly black |
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| 2V angle | 0-40 measured |
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| Dispersion | None |
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| References | [1][2][3] |
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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.