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Omphacite thin section

About Omphacite
From the Greek for unripe grape, in allusion to its characteristic green color.
Omphacite hand-specimen
Formula: (Ca,Na)(Mg,Fe,Al)Si2O6
System: Monoclinic
Color: Dark green
Lustre: Vitreous
Hardness: 5–6
Density: 3.16–3.43
Omphacite #1 thin section (hFOV 2mm)
Omphacite #2 thin section (hFOV 2mm)
Omphacite PPL properties
Relief: High positive
Habit/Form: Omphacite crystals are stubby prisms with four- or eight-sided cross sections. Basal sections show both cleavages at about 87°, and longitudinal sections are roughly rectangular and show only one cleavage direction. Also commonly anhedral granular.
Color: Colorless, very pale green, blue-green
Pleochroism: Weak; very pale green to blue green
Cleavage: Good in two directions {110} (at right angles to the c-axis – on (001)) – the clivage angle in two directions are at nearly right angles 87° and 93°.
Prismatic sections parallel to (100) or (010) to which both cleavages are at too acute an angle to be always visible, especially in slightly thick sections.
Omphacite XPL properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order II
Extinction angle: 36 – 45°
Twins: Twins with {100} as twin-plane are known.
Uniaxial/Biaxial: Biaxial (+)
Optic axial angle (2V): 2V measured: 58 – 83°, calculated: 74 – 88°
Omphacite distinguishing features under the microscope
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  • 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