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Andalusite thin section

About Andalusite
Locality in Andalusia, a province of Spain.
Andalusite hand-specimen
Formula: Al2SiO5
System: Orthorhombic
Color: Pink to red brown
Lustre: Vitreous, Sub-Vitreous, Greasy
Hardness: 6½–7½
Density: 3.13–3.21
Andalusite #1 thin section (hFOV 2mm)
Andalusite PPL properties
Relief: Moderate positive
Habit/Form: Crystals are usually elongate prisms with a nearly square cross section. A variety called chiastolite contains dark inclusions that form a cross along the diagonals of the prism. Andalusite also forms anhedral grains or highly irregular masses. Numerous inclusions of quartz, fine opaques, or other minerals are common.
Color: Usually colorless; can be weak reddish pink, weak greenish yellow
Pleochroism: Usually is absent but when is colored show weak pleochroism usually pink to colorless, others colors are green and yellow
Cleavage: Good prismatic cleavages {110}. In cross sections the cleavage traces are in two directions at approximately right angles.
Andalusite XPL properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order I gray and white; near that of quartz. Interference colors range up to first-order yellow.
Extinction angle: Parallel / 0° / straight; in basal sections it is symmetrical to intersecting cleavages.
Twins: Rare on {101}, may be cruciform.
Uniaxial/Biaxial: Biaxial (-)
Optic axial angle (2V): 2V measured: 73 – 86°, calculated: 80 – 84°
Andalusite 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