M 



V^ 



184 ALUMINA. 



IDOCRASE. 



Dimetric. In square prisms usually modified. P a=a 

 /^/V 142° 53' ; a : a*=l29° 29', a : e=127° 07 . 

 ^ Cleavage not very distinct parallel with M. Also 

 found massive granular and subeolumnar. 



Color brown ; sometimes passing into green. 

 In some varieties the color is oil-green in the direc- 

 tion of the axis and yellowish-green at right angles 

 with it. Streak uncolored. Subtransparent to nearly opaque. 

 H=6-5. Gr=3-33— 3.4. 



Composition : silica 37*4, alumina 23*5, protoxyd of iron 

 4'0, lime 29*7, magnesia and protoxyd of manganese 5*2. 

 Before the blowpipe fuses with effervescence to a yellow 

 translucent globule. 



Dif. Resembles some brown varieties of garnet, tourma- 

 line and epidote, but besides its difference of crystallization, 

 \t is much more fusible. 



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

 and hence called Vesuvian. It has since been obtained in 

 Piedmont, near Christiania, Norway, in Siberia, also in the 

 Fassa valley. Specimens of a brown color from Eger, Bo- 

 hemia, have been called egeran. Cyprine includes blue 

 crystals from Tellemarken, Norway ; supposed to be colored 

 by copper. 



In the United States, idocrase occurs in fine crystals at 

 Phipsburg and Rumford, Parsonsfield and Poland, Me. ; 

 Newton, N. J. ; Amity, N. Y. f and sparingly at Worcester, 

 Mass. The xanthite of Amity is nothing but idocrase. 



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

 krasis, mixture ; because its crystalline forms have much re- 

 semblance to those of other species. 



Uses. This mineral is of little value except as a minera- 

 logical curiosity. It is sometimes cut as a gem for rings. 



GARNET. 



j Monometric. Common in dodecahedrons! (fig. 1,) also in 

 trapezohedrons, (fig. 2,) and both forms are sometimes vari- 

 ously modified. Cleavage parallel to the faces of the dode« 



What is the crystallization of idocrase? its color, hardness, and lus- 

 ter ? its composition 1 How does it differ from garnet and touimaline ? 

 What is the usual form of garnet ? 



