176 ALUMINA. 



orthoclase. — Common Feldspar.* 



Monoclinic. In modified oblique rhombic £risms. T 

 T=118° 49', P : T=67 J 15' ; T : e=12(T 40 . Usually 

 A in thick prisms, often rectangular, (£g. 2,) 2 



and also in modified tables, (fig. 1.) 

 Cleavage perfect parallel with e, the 

 shorter diagonal ; 'also distinct parallel I i N—-^ 

 to P. f Also massive, with a granular \J \^/\\ 

 structure, or coarse lamellar. Colors light ; NJi— Jl 

 white, gray, and flesh-red common ; also greenish and bluish 

 white and green. Luster vitreous ; sometimes a little pearly 

 on the face of perfect cleavage. Transparent to subtranslu- 

 cent. H=6. Gr=2*39— 2*62. 



Composition: silica 64*20, alumina 18*40, potash 16*95, 

 Fuses only on the edges. With borax forms slowly a trans- 

 parent glass. Not acted upon by the acids. 

 «*** Varieties. Common feldspar includes the common sub- 

 translucent varieties ; adularia, the white or colorless sub- 

 1 transparent specimens. The name is derived from Adula, 

 ^one of the highest peaks of St. Gothard. f "filassy feldspar 

 and ice-spar include transparent vitreous crystals, found in 

 lavas. Some crystals called by these names belong to the 

 species anorthite. Ryacolite and Loxoclase belong here. 



Moonstone is an opalescent variety of adularia, having 

 when polished peculiar pearly reflections. Sunstone is simi- 

 lar ; but contains minute scales of mica. Aventurine feld- 

 spar often owes its iridescence to minute crystals of specular 

 or titanic iron, or limonite. 



Dif. Distinguished from scapolite by its more difficult 



fusibility, and by a slight tendency to a fibrous appearance 



in the cleav-age surface of the latter, especially in massive 



varieties ; ' from spodumene by its blowpipe characters. 



Obs. Feldspar is one of the constituents of granite, 



I gneiss, mica slate, porphyry and basalt, and often occurs in 



\ these rocks in crystals. [ St. Lawrence county, N. Y., affords 



fino crystals ; also Orange county, N. Y. ; Haddam and 



What is the crystallization and appearance of feldspar ? What is ita 

 hardness? what its composition? Mention the principal varieties, with 

 their peculiarities ? In what rocks is feldspar an ingredient 1 



* The following species, from feldspar to nepneline inclusive, form a 

 natural group called the feldspar family. 



