104 NEW YORK STATE MUSEUM 



Cyanite (disthene) Al 2 Si0 5 



Cyanite is probably a basic rneta silicate of aluminium with 

 the formula (A10) 2 Si0 3 . Dana, however, places it for conveni- 

 ence in the group with sillimanite, to which mineral.it bears a 

 close relation. 



Cyanite is found in long bladelike, triclinic crystals which are 

 rarely terminated and in coarsely bladed columnar masses usu- 

 ally of a grayish blue color (pi. 2 2 ). It cleaves easily parallel 

 to the three pinacoids. The luster is vitreous to pearly and the 

 color commonly blue along the center of the blades^ shading to 

 white on the edges; also gray, green to nearly black. 



It occurs in gneiss and mica schist with garnet and staurolite 

 and is often associated with corundum. It is found in the cor- 

 undum regions of Massachusetts, Pennsylvania, North Caro- 

 lina and Georgia; it has been noted in the rocks of New York 

 island. 



Cyanite is sometimes used as a gem. 



Datolite Ca(B0H)Si0 4 

 Datolite is a basic calcium and boron orthosilicate. 

 It crystallizes in monoclinic forms of varied habit but usually 

 short prismatic (fig. 226) and often highly 

 modified. The crystals are glassy, transpa- 

 rent or translucent and colorless, white or 

 pale green. A massive compact variety has 

 a dull luster resembling unglazed porcelain 

 and is gray or pinkish in color. 



Datolite occurs as a secondary mineral in 

 veins and cavities in basic eruptive rocks asso- 

 ciated with calcite, prehnite and the zeolites; also in metallic 

 veins as in the Lake Superior copper region where the massive 

 variety is quite common. It is also found in the vicinity of 

 Bergen Hill and Paterson N. J. and in other localities through- 

 out New York 1 and New England. 



Epidote HCa 2 (ALFe) 3 Si 3 13 



Epidote is a basic calcium, aluminium and iron silicate. 



It occurs in monoclinic crystals which are commonly elongated 



1 A few rare occurrences are noted in St Lawrence county. See Dana, 

 J. D. System of mineralogy. 1892. 



