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Cordierite thin section

About Cordierite
For Pierre Louis A. Cordier (1777-1861), French mining engineer and geologist.
Cordierite hand-specimen
Formula: Mg2Al3(AlSi5)O18
System: Orthorhombic
Color: Grey, blue, blue-violet, etc.
Lustre: Vitreous
Hardness: 7–7½
Density: 2.6–2.66
Cordierite #1 thin section (hFOV 2mm)
Cordierite #2 thin section (hFOV 2mm)
Cordierite PPL properties
Relief: Low positive
Habit/Form: Commonly found as anhedral grains or irregular porphyroblastic grains, often with numerous inclusions of quartz, opaque grains, or other minerals. Less commonly as euhedral hexagonal prismatic crystals.
Color: Colorless; may have pale yellow pleochroic haloes around inclusions of zicron and apatite. Other colors: green, violet, violet-blue, pale blue.
Pleochroism: None; sometimes weak
Cleavage: Fair cleavage on {010}; but it may not show in sections.
Cordierite XPL properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order I yellow to order I red; usually about the same as that of quartz – order I gray or white.
Extinction angle: Parallel / 0° / straight to all cleavages and crystal outlines.
Twins: Complex twinning – lamellar and cyclic twins are common. The usual twin planes are {110} and {130}. The twinning may resemble plagioclase twin, although basal sections may display a radial or cyclic pattern with roughly hexagonal symmetry. Those cordierite crystals formed at high temperatures tend to be most complexly twinned.
Uniaxial/Biaxial: Biaxial (+/-)
Optic axial angle (2V): 2V measured: 75 – 89°, calculated: 54 – 86°
Cordierite 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