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Sericite thin section

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About Sericite
From Greek word for silky, in allusion to its silky luster.
Sericite hand-specimen
Formula: KAl2(AlSi3O10)(OH)2
System: Monoclinic
Color: White to colorless, silvery-white
Lustre: Vitreous, Silky, Pearly
Hardness:
Density: 2.77–2.88
XPL
XPL
XPL
PPL
PPL
Sericite #1 thin section (hFOV 2mm)
XPL
XPL
XPL
PPL
PPL
Sericite #2 thin section (hFOV 2mm)
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Sericite PPL properties
Relief: Low-Moderate positive
Habit/Form: Well-formed tabular crystals with a roughly hexagonal outline are rare. Usually found as micaceous flakes or tablets with irregular outlines, tabular parallel to {001}. Sericite is name given to very fine, ragged grains and aggregates of white mica – usually muscovite – produced by the alteration of feldspars or other minerals. Included grains of zircon or other radioactive minerals may form pale pleochroic halos in surrounding muscovite.
Color: Colorless
Pleochroism:
Cleavage: Perfect in one direction {001}; in basal sections (z-axis) has no cleavage.
Sericite XPL properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order II – III
Extinction angle: 0 – 3° (bird’s eye extinction)
Twins: Absent
Uniaxial/Biaxial: Biaxial (-)
Optic axial angle (2V): 2V 28 – 47°
Sericite 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