GUIDE TO THE MINBRALOGIC COLLECTIONS 103 



are consequently rarely terminated at both ends. Topaz occurs 

 also in columnar masses and rolled fragments. 



The luster is vitreous and resembles that of quartz; the crys- 

 tals are colorless, yellow, reddish, bluish, faintly green or pink; 

 massive varieties are often white. The hardness exceeds that 

 of quartz but is not as high as corundum. 



Topaz occurs in veins and cavities in the highly acid igneous 

 rocks such as granite, rhyolite, etc, and sometimes in gneiss 

 and schists. It is often found in alluvial deposits with stream 

 tin. It is commonly associated with fluorite, cassiterite and 

 tourmalin. 



It is found in Saxony, the Urals, Japan, Brazil, Mexico; and 

 in Maine, Colorado and Utah. 



Transparent varieties are cut for gems. 



Andalusite AL.SiO-, 



Andalusite is an orthosilicate of aluminium. 



It occurs in coarse orthorhombic crystals nearly square in 

 cross section or in tough columnar or granular masses. The 

 variety chiastolite occurs in rounded prisms which are charac- 

 terized by carbonaceous inclusions symmetrically arranged with 

 respect to the vertical axis; these show on a fracture, a cross 

 or tesselated figure as in pi. 30 2 . The luster is vitreous inclining 

 to pearly; the color varies from white or light gray through light 

 green or violet to rose-red or flesh-red. 



Andalusite occurs in imperfectly crystalline schist, in gneiss, 

 mica schist and other metamorphic rocks. Chiastolite is com- 

 monly a contact mineral in clay slates adjoining granite dikes. 

 It is found in Andalusia, Spain; Brazil and in many localities 

 in the New England states, Pennsylvania and California. 



Sillimanite (fibrolite) ALSiO- 



An orthosilicate of aluminium with the same composition as 

 andalusite. It occurs in long slender orthorhombic crystals, in 

 parallel groups passing into fibrous or columnar masses, brown 

 or gray in color and extremely tough in tenacity. Its mode of 

 occurrence is similar to that of andalusite. 



