THE MINERAL SILLIMANITE

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

Sillimanite is a polymorph with two other minerals; kyanite and andalusite. A polymorph is a mineral that shares the same chemistry but a different crystal structure with another, or other, minerals. Sillimanite is the rarest of the three trimorphs. A variety of sillimanite is called "fibrolite". It is a common fiberous massive form.

PHYSICAL CHARACTERISTICS:

Color is white, brown and green.
Luster is silky when fiberous or vitreous otherwise.
Transparency crystals are transparent to translucent.
Crystal System is Orthorhombic; 2/m2/m2/m
Crystal Habits include rare prismatic crystals but mostly fiberous masses.
Cleavage is good in one direction lengthwise.
Fracture is splintery.
Hardness is 7.5 in large crystals but more fiberous forms are softer.
Specific Gravity is approximately 3.2+ (above average)
Streak is white.
Associated Minerals are garnets, biotite, feldspars, quartz, kyanite and andalusite.
Other Characteristics: fibers are brittle distinguishing them from asbestos.
Notable Occurrences include Brazil, New England area and many European sites.
Best Field Indicators are crystal habit, color, brittlness and hardness if not fiberous.