FELDSPAR GROUP. 



277 



Composition. ^CaJXa,Al 14 Si 5 = 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. Sunstone is 

 in part oligoclase containing disseminated scales of hematite 

 giving bright reflection from the interior of the stone. Oc- 

 curs in Norway. Moonstone is in part a whitish opalescent 

 variety. Oligoclase occurs at Unionville, Pa.; Haddam, 

 Conn."; Mineral Hill, Del. ; Chester, Mass., etc. 



Albite. 



Triclinic. Angle between the cleavage planes 93° 36', 

 and 83° 21'. 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. 2 Al 16 Si 6 = 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. 



