64 



MINERALOGY. 



Fig. 23. 



times feldspar is opalescent 

 that is, resembling opal in lus- 

 tre, and some specimens of 

 this variety are used in jew- 

 elry. Sometimes it is irides- 

 cent, from the glistening of 

 minute crystals of specular 

 iron contained in it. Feld- 

 spar is used largely in the 

 manufacture of porcelain, the 

 clay which comes from the 

 decomposition of the feldspar 

 being called kaolin. Albite, so called from the fact that 

 white is its most common color, differs from feldspar 

 chiefly in containing soda instead of potash. It takes 

 the place of feldspar sometimes in the rocks. It does 

 so in granite, and albite granite differs from feldspar 

 granite in having a lighter color, owing to the whiteness 

 of the albite. Lahradorite\^ another mineral sometimes 

 found in granite. It differs from feldspar and albite in 

 having a large percentage of lime. 



130. Mica. This is another of the three constituents 

 of granite. It has chiefly the same substances in its 

 composition as feldspar, but there are added small 

 amounts of oxyd of iron, fluoric acid, and water. It has 

 many varieties, but that which is so familiar to us under 

 the common but inappropriate name of isinglass con- 

 sists of extremely thin plates, transparent, with a bright 

 lustre, the colors being white, yellow, gray, brown, and 

 blackish green. It is very elastic, being unlike talc in 

 this respect, though resembling it in some others. It is 

 used for doors in lanterns and windows in ships, where 

 glass, from its brittleness, is liable to be broken. As it 

 bears heat well it is used much in stoves. In Siberia, 

 where it is obtained in large quantities, it is used in place 

 of glass. Plates of it three feet square have been brought 

 from that country, and they have been obtained of the 



