FELDSPAR GROUP. 275 



plate has to be revolved for the change between consecutive 

 bands varies 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 angle, 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 snch 

 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 feldsjiars is chemical 

 analysis. These feldspars have been called plagioclase (from 

 the Greek words for oblique audi fracture), as if all were of 

 one species. The term is a convenient cover for ignorance. 



IV. Distinctions prom other Minerals. 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 difficult fusibility ; from nephelite, by form, and also 

 in difficult fusibility, and not gelatinizing with acids, ex- 

 cept in the case of anorthite and labradorite, which fuse 

 w T ith but little more difficulty than nephelite, 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 s Si 2 = 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 evaporation. 



Obs. Occurs in basic eruptive rocks ; also in some meta- 



