FELDSPAR GROUP. 2%) 
plate has to be revolved for the change between consecutive 
bands varics for different species, it being 54° to 74° for 
anorthite, 10° to 14° for labradorite, 4° to 8° for oligoclase, 
6$° to 8° for albite, and 30° for microcline. ‘The shorter 
diagonal of the crystal bisects this angie, so that the angle 
made with this diagonal is 27° to 37° for anorthite, 5° to 
7 for labradorite, 2° to 4° for oligoclase, 3$° to 4° for albite, 
and 15° for microcline. Obtaining cleavage plates for such 
measurements in the case of slices for microscope investiga- 
tion, is seldom possible, and when not so, the only certain re- 
source for the distinguishing of triclinic feldspars is chemical 
analysis. These feldspars have been called plagioclase (trom 
the Greek words for oblique and fracture), as if all were of 
one species. ‘The term is a convenient cover for ignorance. 
IV. DistINcTIONS FROM OTHER MineErAts. When in 
crystals, the form is sufficient to determine a feldspar ; so also 
the fact of two unequal cleavages inclined to one another at 
84° to 90°, one of them quite perfect. No fibrous, columnar, 
or micaceous varieties are known. They differ from rho- 
donite, by the absence of a manganese reaction ; from spodu- 
mene, by the absence of a lithia reaction as well as cleavage 
angles; from scapolite, by form, the cleavage angles, the 
more diificult fusibility ; from nephelite, by form, and also 
in difficult fusibility, and not gclatinizing with acids, ex- 
cept in the case of anorthite and labradorite, which fuse 
with but little more difficulty than peo and often will 
gelatinize. 
Anorthite.—Indianite. Lime Feldspar. 
Triclinic. Angle between the two cleavage planes 85° 50’ 
and 94° 10’. Crystals tabular. Also massive granular or 
coarse lamellar. Color white, grayish, or reddish. 
Composition. CaAl O; Sip=Silica 43:1, alumina 36°8, lime 
20:1=100. B.B. fuses with much difficulty to a colorless 
glass ; decomposed by hydrochloric acid, the solution gela- 
tinizes on evapor ation. 
Obs. Occurs in basic eruptive rocks; also in some meta- 
