HELLS CANYON
GEM CLUB, INC.
P.O. BOX 365, LEWISTON, ID 83501
SINCE 1956
SILLIMANITE
In the search for rock hound material that is of gem quality in our area the search area extends North of Lewiston, Idaho to Clarkia, Idaho.  Just to the east of that imaginary line are igneous formations that contain indicators.  One indicator not so often found is Sillimanite.  Sillimanite float is collected along some of the creeks and rivers.  We are sure this material was moved from the higher elevations by glacier action, covered up by sedimentation from erosion and washed out into the waterways over time.  Associated minerals that are found with Sillimanite are Kyanite and Andalusite all of which are aluminum silicate oxides (Al2SiO5). 
 
Sillimanite has the following characteristics:
Color:  white, colorless, gray, yellowish, brownish, greenish (dark green almost black looking), bluish
Composition:  (Al2SiO5)
Texture:  vitreous to silky luster.  Crystals are not found in our region, massive formations are common.  It is often mistaken for jade.
Crystal:  Orthorhombic
Formation:  high temperature regional metamorphic rocks, and in some igneous pegmatite�s.
Tests:  infusible and insoluble in acids
Hardness:  6 � - 7 �
Fracture:  uneven
Group:  Silicates
  

In addition to the profile of Sillimanite is a list of derivatives that provides gem deposit possibilities:      
Zircon (Zn SiO4
Staurolite (Fe+2, Mg, Zn)2 Al9(Si, Al)4 O22 (OH)2
Topaz  (Al2 SiO4 (F, OH)2)
Willemite  (Zn2 SiO4)
Olivine  (Mg2 SiO4 � Fe2 Si O4)  (Peridot, Forrsterite)
Pyrope Garnet  (Mg3 Al2 (Si O4)3)
Grossular Garnet  (Ca3 Al2 (Si O4)3)
Almandine Garnet  (Fe3 +2 Al2(Si O4)3)
Epidote
Zoisite
Idocrase (vesuvianite)
Beryl (Emerald, Aquamarine)
Tourmaline
Spodumene
Jadeite

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