ORDER VI. PRISMATIC DISTHENE-SPAR. 



Compound Varieties. Twin-crystals: faces of 

 composition parallel, axis of revolution perpendicu- 

 lar to ?r + oo. Massive : composition broad co- 

 lumnar, sometimes straight lamellar, often curved 

 or divergent ; faces of composition in most cases ir- 

 regularly streaked. 



OBSERVATIONS. 



1. The two varieties formerly distinguished as particular 

 species, which the prismatic Disthene-spar comprehends, 

 are Kyanite and Rhcetizite, but they are so nearly allied to 

 each other, that the colour is the only property in which 

 they can be said to differ, the latter of them referring to 

 those varieties whose colour is white, without any deline- 

 ations of blue. 



2. Three varieties of the present species, analysed, the 

 first by SAUSSURE, the second by LAUGIER, the third by 

 KLAPROTH, have yielded, 



Alumina 54-50 55-50 55-50. 



Silica 30-62 38-50 43-00. 



.Lime 2-02 0-50 0-00. 



Magnesia 2-30 0-00 0-00. 



Oxide of Iron 6-00 2-?5 0-50. 



Water 4-56 0*75 0-00. 



Potash 0-00 0-00 a trace. 



It is not altered on being exposed to heat, and is infusible 

 even in very high degrees of temperature. The Rhaeti- 

 zite becomes first red, but again white, if further heated. 

 They are difficultly, but entirely soluble in borax. Some 

 crystals exhibit positive, others negative electricity, on be- 

 ing rubbed, and to this property in particular the name 

 Disthcne refers, given by HAUY to the present species. 



3. The varieties of prismatic Disthene-spar occur in crys- 

 tals, or massive, imbedded in rocks, as gneiss, mica slate, &c. 

 The former are often accompanied by prismatoidal Garnet, 

 united with them in a remarkable manner. It is found 



