Andalusite

Group: Al₂SiO₅ There are 3 polymorphs in this group. Also found are sillimanite and kyanite.

The andalusite I examined were all reddish in color.

Chemical formula: Al₂SiO₅

Diagnostic Properties:

• Hardness: 7.5
• Specific Gravity: 3.13-3.16
• Cleavage: good
• Translucent to opaque
• Orthorhombic crystals; 2/m2/m2/m. Usually occurs in coarse square prisms.
• Color: red, reddish brown, olive green, pinkish white.
• Vitreous luster
• Optics (-), = 1.632, = 1.638, = 1.643; 2V = 85; X = c; Z = a.

Key Diagnostic properties:

• Hardness
• Nearly square prism

Interesting notes:

• In some xls there is a strong pleochroism; the X is red and the Y & Z are green!

Found in Andalusia, Spain; the Austrian Tyrol; Minas Gerais, Brazil; Bimbowrie, South Africa; Wite Mountains (near Laws), California; Delaware County, Pennsylvainia; Westford, Lancaster and Sterling Massachusetts.

Uses: used to manufacture spark plugs and other highly refractory porcelains. When transparent it may serve as a gemstone.

Named for Andalusia, Spain.