TKISILICATES. 



261 



Vesuvianite. — Idocrase. 



Dimetric. : 1=142° 46' ; 1 Al = 129° 21', 1 : i-i=12T 

 14'. Cleavage not very distinct parallel with I. Also found 

 massive granular, and subcolumnar. 



1. 

 35^ 



Color brown ; sometimes passing into green. In some 

 varieties the color is oil-green in the direction of the axis 

 and yellowish green at right angles with it. Streak nn- 

 colored. Lustre vitreous. Subtransparent to nearly opaque. 

 H.=6o. G. = 3-33-3 4. 



Composition. (^Ca 3 -^A1) 2 12 Si 3 . A small part of the 

 Ca is usually replaced by magnesium, and part of the alu- 

 minum sometimes by iron in the sesquioxide state. Per- 

 centage of a common variety, Silica 37 - 3, alumina 16*1, 

 iron sesquioxide 3*7, lime 35 4, magnesia 2*1, iron protox- 

 ide 2*9, water 2 -1 = 99 -6. B.B. fuses easily with efferves- 

 cence to a greenish or brownish globule. 



J) iff. Resembles some brown varieties of garnet, tourma- 

 line and epidote, but besides its difference of crystallization, 

 it is much more fusible. 



Obs. Vesuvianite was first found in the lavas of Vesuvius, 

 and hence the name. It has since been obtained in Pied- 

 mont ; near Christiania, Norway ; in Siberia ; also in the 

 Fassa Valley. Cyprine includes blue crystals from Telle- 

 marken, Norway ; supposed to be colored by copper. 



In the United States, it occurs in fine crystals at 

 Phippsburg and Rumford, Sandford, Parsonsfield and 

 Poland, Me. ; Newton, N. J.; Amity, N. Y. In Canada at 

 Calumet Falls, and at Grenville. 



The name idocrase is from the Greek eiclo, to see, and 

 hrasis, mixture ; because its crystalline forms have much 

 resemblance to those of other species. 



This mineral is sometimes cut as a gem for rings. 



Mellilite in honey-yellow crystals (which includes Hum- 



