THE MINERAL ANDALUSITE

Chemistry: Al2 SiO5, Aluminum Silicate
Class: Silicates
Subclass: Nesosilicates
Uses: in the manufacture of spark plugs etc, as a gemstone and as mineral specimens.
Specimens

Andalusite is a polymorph with two other minerals; kyanite and sillimanite. A polymorph is a mineral that shares the same chemistry but a different crystal structure with another, or other, minerals. A unique variety of andalusite is called "chiastolite". It contains black or brown clay and/or carbonaceous material inclusioned in the crystal. These inclusions are arranged in regular symmetrical shapes. Usually they are in the form of a cross or X.

PHYSICAL CHARACTERISTICS:

Color is white, red, brown, orange and green.
Luster is vitreous.
Transparency crystals are transparent to translucent.
Crystal System is Orthorhombic; 2/m2/m2/m
Crystal Habits include prismatic crystals with a square cross section terminated by a pinacoid. also massive and granular.
Cleavage is good in two directions.
Fracture is splintery to subconchoidal.
Hardness is 7.5
Specific Gravity is approximately 3.15+ (above average)
Streak is white.
Associated Minerals are cordierite, biotite, feldspars, quartz, kyanite and sillimanite.
Other Characteristics: dark inclusions produce cruciform shapes in the variety, chiastolite. Index of refraction is 1.632-1.638.
Notable Occurrences include Andalusia, Spain; Austria; California, USA and China.
Best Field Indicators are crystal habit, color, inclusions (if present) and hardness.