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Riebeckite thin section

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About Riebeckite
For Emil Riebeck (1853-1885), German explorer.
Riebeckite hand-specimen
Formula: ▯[Na2][Fe32+Fe23+] Si8O22(OH)2
System: Monoclinic
Color: Usually black, also commonly light blue to blue-black, gray-blue, gray, brown
Lustre: Vitreous, Sub-Vitreous
Hardness: 5–5½
Density: 3.28–3.44
XPL
XPL
XPL
PPL
PPL
Riebeckite #1 thin section (hFOV 2mm)
XPL
XPL
XPL
PPL
PPL
Riebeckite #2 thin section (hFOV 2mm)
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Riebeckite PPL properties
Relief: High positive
Habit/Form: Riebeckite commonly forms slender bladed or prismatic crystals, usually without terminations. Cross sections show the typical diamond shape of amphiboles and the trace of the two cleavages. Grains may be irregular and poikilitic, enclosing associated minerals. The fibrous to asbestiform variety crocidolite is common in metamorphosed iron formations.
Color: Inky blue-black to muddy brown; blue, indigo, violet, yellowish green, yellowish brown, dark blue. Glaucophane tends to show more pale lavender, and riebeckite shows more dark blue-green.
Pleochroism: Intense; strong in blue, yellow-green, brown.
Cleavage: Good in two directions {110} – in two directions are at nearly obtuse angles (124° with 56°), and in one direction for longitudinal section. Sections parallel to (100) which display no cleavage since both cleavages are at too acute an angle to the section to be visible.
Riebeckite XPL properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order I red to order III green; the interference colors are masked by the deep blue color of the mineral.
Extinction angle: The maximum extinction angle in elongate sections is about 0-8°, but the fibrous variety, crocidolite, has parallel extinction.
Twins: Simple and lamellar twins on {100}
Uniaxial/Biaxial: Biaxial (+) (-)
Optic axial angle (2V): 2V measured: 68 – 85°, calculated: 62 – 78°
Riebeckite distinguishing features under the microscope
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References
  • Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming minerals (pp. 498). Mineralogical Society of Great Britain and Ireland, London.
  • mindat.org – The Mineral Database