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Orthoclase thin section

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About Orthoclase
Orthoclase – from Greek for straight and fracture, in reference to the cleavage angle, which is 90°.
Feldspar (mineral group name), from Swedish names for field (feldt or falt) and spar (spat), in reference to the spar in tilled fields overlying granite; in German the word became Feldspat.
Orthoclase hand-specimen
Formula: KAlSi3O8
System: Monoclinic
Color: Colorless to white, Greenish white, Grayish yellow, Pale pink
Lustre: Vitreous, Sub-Vitreous, Resinous
Hardness: 6
Density: 2.55–2.63
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Orthoclase #1 thin section (hFOV 2mm)
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Orthoclase #2 thin section (hFOV 2mm)
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Orthoclase (perthitic texture) #1 thin section (hFOV 2mm)
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Orthoclase (perthitic texture) #2 thin section (hFOV 2mm)
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Orthoclase (graphic texture) #1 thin section (hFOV 2mm)
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Orthoclase (graphic texture) #2 thin section (hFOV 2mm)
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Orthoclase PPL properties
Relief: Low negative
Habit/Form: Common as anhedral and euhedral grains in igneous rocks. Crystals are commonly elongated parallel to c or a, and roughly tabular parallel to (010). Some orthoclase is perthitic, and much of it is cryptoperthitic.
Color: Colorless, frequently cloudy due to incipient alteration (kaolinization)
Pleochroism:
Cleavage: Sometimes after 1 or 2 directions; but are difficult to see in thin section due to very low relief; perfect on {001} and good/distinct on {010}, intersecting essentially at right angles; {110} cleavage is poor.
Orthoclase XPL properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order I gray to white; feldspars are very useful in determining the thickness of any slide in which it occurs.
Extinction angle: On (001) parallel, on (010) from 5° to 12°, increasing with the Na content (to 20°).
Twins: Carlsbad twinning with two individuals is common (simple lamellar twinning). Manebach and Braveno types are less common.
Uniaxial/Biaxial: Biaxial (-)
Optic axial angle (2V): 2V measured: 35 – 75°, calculated: 52 – 70°
Orthoclase 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