• 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 (also called Bucholzite) 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 fibrous massive form.

Sillimanite is the official state mineral of Delaware.


  • Color is white, brown and green.
  • Luster is silky when fibrous 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 fibrous masses.
  • Cleavage is good in one direction lengthwise.
  • Fracture is splintery.
  • Hardness is 7.5 in large crystals but more fibrous 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 minerals.
  • Notable Occurrences include Brazil, New England area and many European sites.
  • Best Field Indicators are crystal habit, color, brittleness and hardness if not fibrous.
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Available SILLIMANITE specimens:
see this List of ALL specimens including SOLD ones


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