Andalusite

andalusite
Andalusite cluster ring

Andalusite varies in colour from pale yellowish brown to some dark bottle-green, dark brown, or the most popular greenish red. It has very strong and distinctive pleochroism, so that, when turned, the same stone may appear yellow, green and red. Large crystals might be vertically striated prisms having a square cross-section and pyramidal ends, but they are rare. More usual are opaque, rod-like aggregates of crystals or waterworn pebbles. It is the pebbles which are usually cut as gemstones.

Andalusite is generally found in pegmatites. Pebbles occur within the gem gravels of Sri Lanka and Brazil. Other localities include Spain, Canada, Russia, Australia and the United States.

An opaque, yellowish grey variety, chiastolite, occurs as long prisms, which make a cross when cut and polished.

Cause of Color :
  • Green (viridine) andalusite: manganese
  • Brownish Green andalusite: charge transfer
Chemical Composition :

Aluminium silicate with trace elements of iron or manganese. Al2SiO5

Classification / Type :

Polymorphous with Kyanite and Sillimanite.

Crystal System / Forms :

Orthorhombic System / Vertically striated prisms capped with pyramids; much of the gem material as water-worn pebbles.

Cuts & Uses :

Facetted cuts, cabochons, beads. Generally in fancy cuts to bring out its strong pleochroic property.

Hardness : 7 - 7.5
Lustre : Vitreous.
Magnification :

Rutile needles, fingerprints, crystals. Chiastolite exhibits a dark-cross.

Optic Character :

Anisotropic, D.R.; Biaxial negative.

Pleochroism : Very strong. Generally light yellow green and dark brownish red and brownish green.
Refractive Index / Birefringence :

1.634 – 1.643 / 0.008 – 0.013 (birefringence is highest when the indices are lowest). Range: 1.629 – 1.664

Simulants (with separation tests) :

Tourmaline (optic figure, birefringence, pleochroism), Epidote (R.I., S.G.), Alexandrite (R.I., S.G., pleochroism, inclusions), Apatite (optic figure, pleochroism).

Sources :

Sri Lanka, Spain, Brazil, Myanmar, Russia (Ural Mountains), U.S.A. (Arizona, California).

Specific Tests :

The strong pleochroism is often mistaken for a color change effect.

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.

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