MICA. 191 



alumina 32*5, magnesia 10, protoxyd of iron 8*0, protoxyd 

 of manganese 0*1, water (hygrometric) 3*1. Before the 

 blowpipe fuses on the edges with difficulty to a blue glass 

 resembling the mineral. 



Dif. The glassy appearance of iolite is so peculiar that 

 it can be confounded with nothing but blue quartz, from 

 which it is distinguished by its fusing on the edges. It is 

 easily scratched by sapphire. 



Obs. Found at Haddam, Conn., in granite ; also in gneis 

 at Brimfield, Mass. ; al Richmond, N. H., in talcose rock 

 The principal foreign localities are at Bodenmais in Bava- 

 ria ; A rendal, Norway : Capo de Gata, Spain ; Tunaberg, 

 Finland ; also Norway, Greenland and Ceylon. 



The name ioK/e is from the Greek iodes, violet, alluding 

 to its color ; it is also called dichroite, from dis, twice, and 

 chroa, color, owing to its having different colors in two 

 directions. 



Uses. Occasionally employed as an ornamental stone ; 

 when cut it presents different shades of color in different 

 directions. 



Note. — Iolite exposed to the air and moisture undergoes a gradual 

 alteration, becoming a hydrate (absorbing water) and assuming a foli- 

 ated micaceous structure, so as to resemble talc, though more brittle 

 and hardly greasy in feel. Hydrous iolite, chlorophyllite, and esmark- 

 ite, are names that have been given to the altered iolite ; and fahlunite 

 and gigantolite are of the same origin. (See pages 162, 163.) 



mica. — Muscovite, 



Trimetric. In oblique rhombic prisms of about 120° and 

 60° ; but the fundamental form right rhom- 

 bic. Crystals usually with the acute edge jj|^ £ 

 replaced. Cleavage eminent, parallel to P, 

 yielding easily thin elastic laminae of ex- 

 treme tenuity. Usually in thinly foliated masses, plates or 

 scales* Sometimes in radiated groups of aggregated scales 

 or small folia. 



Colors from white through green, yellowish and brownish 

 shades to black. Luster more or less pearly. Transparent 

 or translucent. Tough and elastic. H=2 — 2*5. Gr= 

 2-8—3. 



Composition : silica 46*3, alumina 36-8, potash 9*2, per- 



How is iolite distinguished from quartz and sapphire? Why was it 

 called iolite and dichroite ? Describe mica. What is its composition? 



