40 
FELDSPAR. MICA. 
and forms a constituent part of numerous rocks. It 
is not as hard as quartz, but more brittle. It is 
composed of thin laminae or plates, which are most 
commonly four or six sided prisms, which have a 
bright, pearly lustre, which distinguishes it from 
quartz, whose lustre is dull. 
Fig. 2. 
An oblique parallelopipedf the primi- 
tive form of feldspar. 
The colours of feldspar are white, gray, milk- 
white, yellowish or reddish white, sometimes in- 
clining to green. When crystaUized it is translu- 
cent. It may be melted into a glass without adding 
any alkali, because an alkali forms one of its con- 
stituent ingredients. It is found, on analysis, to 
contain silex 63; alumine 17; potash 13; lime 3; 
oxide of iron 1 : it is probably owing to the fact that 
feldspar contains potash that it is so easily decom- 
posed, although nearly as hard as quartz. Those 
feldspars which do not decay on exposure to the 
atmosphere contain but little, if any, potash. When 
decomposed it forms a kind of clay called kaolin^ 
from which china or porcelain-ware is made by 
burning. In its manufacture a little lime is added 
as a /war, which tends to soften it in the fire, and, 
on cooling, it assumes the requisite degree of hard- 
ness. Feldspar forms a constituent part of granite, 
gneiss, and mica slate, and enters more or less into 
the formation of greenstone and most volcanic sub- 
stances, as well as porphyry and sienite. In some 
parts of the world it forms entire mountains ; as in 
Siberia, Scotland, Labrador, &c. 
Mica* sometimes called isinglass, is also compo- 
sed of silex, alumine, potash, and oxide of iron, 
* From the Latin micmsj shining. 
