Riebeckite |
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Black riebeckite in an alkaline pegmatite from Lindinosa near Évisa, Corsica, France |
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Category | Silicate mineral |
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Formula (repeating unit) | ☐Na2(Fe2+3Fe3+2)Si8O22(OH)2 |
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Crystal symmetry | Monoclinic 2/m |
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Unit cell | a = 9.76 Å, b = 18.04 Å, c = 5.33 Å; β = 103.59°; Z=2 |
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Color | Black, dark blue; dark blue to yellow-green in thin section |
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Crystal habit | As prismatic crystals, commonly fibrous, asbestiform; earthy, massive |
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Crystal system | Monoclinic - Prismatic |
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Twinning | Simple or multiple twinning parallel to {100} |
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Cleavage | Perfect on {110}, intersecting at 56° and 124°; partings on {100} and {010} |
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Fracture | Conchoidal to uneven |
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Tenacity | Brittle |
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Mohs scale hardness | 6 |
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Luster | Vitreous to silky |
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Streak | Pale to bluish gray |
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Diaphaneity | Semitransparent |
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Specific gravity | 3.28–3.44 |
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Optical properties | Biaxial (-) |
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Refractive index | nα = 1.680–1.698 nβ = 1.683–1.700 nγ = 1.685–1.706 |
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Birefringence | δ = 0.005–0.008 |
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Pleochroism | X = blue, indigo; Y = yellowish green, yellow brown; Z = dark blue |
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2V angle | Measured: 68° to 85°, Calculated: 62° to 78° |
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Dispersion | Strong |
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References | |
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