• 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.
SILLIMANITE specimens:
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SILLIMANITE specimen sil-1
$ 40.00
Dims: 4.2 x 2.7 x 1.0" (10.7 x 6.9 x 2.5 cm)
Wt: 9.8 oz. (279 g)
This specimen consists of a chunk of compact, fibrous Sillimanite, which is sometimes called fibrolite. It shows obvious crystalline tendencies, but no definite orthorhombic form. The fibers are not straight- they take on an undulating, slightly serpentine curve. The material has a pale gray-green coloration and a pearly luster and is translucent, though the piece as a whole is essentially opaque. There are traces of a black mineral embedded in the Sillimanite, but I cannot identify it. There is no host rock present, however.
no photo
sil-1 ($ 40.00)
SILLIMANITE specimen sil-2
$ 35.00
Dims: 3.2 x 1.3 x 1.0" (8.1 x 3.4 x 2.5 cm)
Wt: 3.6 oz. (103 g)
Minas Gerais, Brazil
This hand specimen consists of a chunk of water-worn Sillimanite. The Sillimanite shows no definable crystal form, being water-worn, but has the compact, fibrous habit that is a primary characteristic of the specie. Its color is a deep, violet-brown and is streaked with white, and viewing the white areas, one can see the dull, silky luster caused by the fibers. There is no appreciable host rock present. Such a piece might polish to a very beautiful appearance.
no photo
sil-2 ($ 35.00)
Minas Gerais, Brazil
SILLIMANITE specimen sil-3
$ 25.00
Dims: 2.4 x 1.3 x 0.5" (5.9 x 3.3 x 1.3 cm)
Wt: 1.5 oz. (42 g)
Minas Gerais, Brazil
This flat hand specimen consists of a water-worn chunk of intensely compact, fibrous Sillimanite. It shows no crystal form, no fresh damage and orange, violet, brown and white hues. When viewed from certain angles, the piece exhibits a silky luster that is caused by light reflecting off of its fibers. There is no host rock present. This piece might be very attractive if it is polished.
no photo
sil-3 ($ 25.00)
Minas Gerais, Brazil
SILLIMANITE specimen sil-4
$ 25.00
Dims: 2.1 x 1.1 x 0.8" (5.3 x 2.7 x 1.9 cm)
Wt: 1.4 oz. (39 g)
Minas Gerais, Brazil
A worn, rounded nugget of fibrous Sillimanite makes up this hand specimen. It shows no discernable crystal form and is rounded and smooth, with a dull pearly luster. The material has colors ranging from creamy-white to dark brown with a hint of violet, and is completely opaque. A few areas show a faint schiller. There is no host material present.
no photo
sil-4 ($ 25.00)
Minas Gerais, Brazil
SILLIMANITE specimen sil-5
$ 25.00
Dims: 2.1 x 1.2 x 0.7" (5.4 x 3.1 x 1.9 cm)
Wt: 1.6 oz. (46 g)
Minas Gerais, Brazil
This rounded, partly tumbled Sillimanite piece is in very good condition, showing no damage, and has no visible crystal form. The patterns of pale cream and dark, violet-brown colors on its surface suggest both massive and fibrous, compact habits. At the moment, it has a dull, only slightly glossy luster, but it could easily be polished.
no photo
sil-5 ($ 25.00)
Minas Gerais, Brazil
SILLIMANITE specimen sil-6
$ 20.00
Dims: 1.8 x 1.3 x 0.6" (4.5 x 3.2 x 1.5 cm)
Wt: 1.0 oz. (29 g)
Minas Gerais, Brazil
This small hand specimen consists of a rounded, worn piece of compact, fibrous and massive Sillimanite. It shows no crystal form but one can see a few fibrous tendencies. The material is mostly a pale brown in color, though some darker material with a violet tinge is present. It has a dull, waxy luster, but could likely be polished to a high gloss.
no photo
sil-6 ($ 20.00)
Minas Gerais, Brazil


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