MICA GROUP. 265 



dark blue glass ; often deep blue in direction of the axis, and 

 yellowish gray transversely. Streak uncolored. Lustre vit- 

 reous. Transparent to translucent. Brittle. H. = 7-7*5. 

 G. =2-6-2-7. 



Composition. A silicate of aluminum, magnesium and 

 iron, corresponding to Silica 49*4, alumina 33*9, magnesia 

 8*8, iron protoxide 7 -9 = 100. B.B. loses its transparency 

 and with much difficulty fuses. 



Diff. 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 gneiss 

 at Brimfield, Mass.; at Kichmond, N. H., in talcose rock. 

 The principal foreign localities are at Bodenmais in Bavaria; 

 Arendal, Norway ; Capo de Gata, Spain ; Tunaberg, Fin- 

 land ; also Norway, Greenland and Ceylon. 



The name iolite is from the Greek ion, violet, alluding 

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

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

 rections. 



Occasionally employed as an ornamental stone, and is cut 

 so as to present different shades of color in different direc- 

 tions. 



Iolite exposed to the air and moisture undergoes a gradual altera- 

 tion, becoming hydrous, and assuming a foliated micaceous structure, 

 so as to resemble talc, though more brittle and hardly greasy in feel. 

 Hydrous Iolite, Fahlunite, Ghlorophyllite, and Esmarkite, are names 

 that have been given to the altered iolite ; and Oigantolite and a num- 

 ber of other like minerals are of the same origin. (See p. 315.) 



MICA GROUP. 



The minerals of the mica group are alike in having 

 (1) their crystals monoclinic ; (2) the front plane angle of 

 the base, or of the cleavage laminse, 120°; (3) cleavage emi- 

 nent, parallel to the base, affording very thin laminse ; and 

 (4) aluminum and potassium among the essential constituents. 

 The combining or quantivalent ratio for the bases and sili- 

 con is usually 1 to 1 in Biotite, Phlogopite, and Lepidome- 

 lane ; 1 to more than 1 in Muscovite, Lepidolite, etc. 



* Hbtya g Oh/ a I* 



