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Andalusite


Properties
Chemical Composition: 
Aluminium silicate with trace elements of iron or manganese. Al2SiO5
Classification / Type: 
Polymorphous with Kyanite and Sillimanite.
Colors / Varieties: 
  • Brown, green, yellow common. Rarely seen in pink and violet. (Green known as viridine)
  • Chiastolite is an opaque variety of andalusite which contains a black cross-like core made up of carbonaceous material in a grey - brown background.
Crystal System / Forms: 
Orthorhombic System / Vertically striated prisms capped with pyramids; much of the gem material as water-worn pebbles.
Hardness: 
7 - 7.5
Specific Gravity: 
3.18 ± 0.04
Cleavage / Fracture: 
Easy prismatic cleavage / Conchoidal fracture.
Optic Character: 
Anisotropic, D.R.; Biaxial negative.
Lustre: 
Vitreous.
Refractive Index / Birefringence: 
1.634 - 1.643 / 0.008 - 0.013 (birefringence is highest when the indices are lowest). Range: 1.629 - 1.664
Pleochroism: 
Very strong. Generally light yellow green and dark brownish red and brownish green.
Dispersion: 
0.016
Magnification: 
Rutile needles, fingerprints, crystals. Chiastolite exhibits a dark-cross.
U.V. Fluorescence: 
May fluoresce dark green to yellow green under shortwave.
Spectrum: 
  • Manganese spectrum: seen at 553.5nm and fine lines at 550.5nm and 547.5nm, band at 455nm.
  • Rare earth spectrum at 580nm in some yellow, green andalusite.
Cause of Color: 
  • Green (viridine) andalusite: manganese
  • Brownish Green andalusite: charge transfer
Treatment (Enhancement): 
-
Specific Tests & Remarks: 
The strong pleochroism is often mistaken for a color change effect.
Synthesis: 
-
Simulants (with key separation tests): 
Tourmaline (optic figure, birefringence, pleochroism), Epidote (R.I., S.G.), Alexandrite (R.I., S.G., pleochroism, inclusions), Apatite (optic figure, pleochroism).
Geological Occurrence: 
In metamorphic rocks, within mica schist or gneiss
Sources: 
Sri Lanka, Spain, Brazil, Myanmar, Russia (Ural Mountains), U.S.A. (Arizona, California).
Cuts & Uses: 
Facetted cuts, cabochons, beads. Generally in fancy cuts to bring out its strong pleochroic property.
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