ry 
FELDSPAR GROUP. roan a 
Composition. 4Ca?Na,Al O,, S1,= Silica 61°9, alumina 24:1, 
lime 5:2, soda 8°8=100. <A portion of the soda is usually 
replaced by potash. B.B. fuses without difficulty ; not de- 
composed by acids. 
Obs. It occurs in granite, syenyte, and various meta- 
morphic rocks, especially those containing much silica; and 
in such case usually associated with orthoclase. Swmnstone is 
in part oligoclase containing disseminated scales of hematite 
giving bright reflection from the interior of the stone. Oc- 
curs in Norway. Moonstone isin part a whitish opalescent 
variety. Oligoclase occurs at Unionville, Pa.; Haddam, 
Conn. ; Mineral Hill, Del. ; @hester, Mass., etc. 
Albite. 
Triclinic. Angle between the cleavage planes 93° 36’, 
and 83° 24’. Figures 1 to 6 represent some of its forms ; 

2 and 3 are twin crystals. The crystals are usually more or 
less thick and tabular. Also massive, with a granular or 
lamellar structure. Color white ; occasionally light tints of 
bluish white, grayish, reddish and greenish. ‘Transparent 
to subtranslucent. 
Composition. Na Al O,,8i1;=Silica 68-6, alumina 19°6, 
soda 11:8—100°0. B.B. fuses to a colorless or white glass, 
imparting an intense yellow to the flame. Not acted upon 
by acids. 
