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. JBrittle. H.=7-75. 
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 Richmond, 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, Chiorophyllite, and Hsmarkite, are names 
that have been given to the altered iolite ; and Gigantolite 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 lamineg, 120°; (8) cleavage emi- 
nent, parallel to the base, affording very thin lamine; 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 Lepidomc- 
lane; 1 to more than 1 in Muscovite, Lepidolite, ete. 
