37 
4 
FELDSPAR. 
There are several varieties of feldspar, all of which consist 
°f silicates of alumina and one or more of the alkali group— 
Potash, soda, lime, and baryta. The hardness of the various 
varieties range from 5 to 7, with an average of 6 on Mohs’ scale. 
With the exception of orthoclase and the rare variety, hyalophane, 
which are monoclinic, the feldspars all belong to the triclinic 
system of crystallization. 
Orthoclase and microcline are the two varieties of chief 
ln dustrial importance. They are identical in composition and 
Physical properties, belonging, however, to different crystal 
systems. The chemical composition of the pure mineral is 
as follows;— 
Silica (Si0 2 ) 64-7% 
Alumina (A1 2 0 3 ) 18-4% 
Potash (K 2 0) 16-9% 
100-0 
Part of the potash is sometimes replaced by soda. 
Alhite is used to some extent, either alone or mixed with 
orthoclase. It is a soda feldspar, having the following composition, 
when pure:— 
Silica (Si0 2 ) 68-6% 
Alumina (A1 2 0 3 ) 19-6% 
Soda (Na 2 0) 11-8% 
100-0% 
Part of the soda is usually replaced by potash and lime. 
While these feldspars are common constituents of many 
igneous rocks they usually occur in such small grains, and inter 
mixed to such a degree with other minerals, that their utilization 
is not economically possible. In some localities they occur 
i fl comparatively large masses in coarsely crystallized peg 
matites along with quartz, tourmaline, and mica, from which, 
m the course of mining, they may be fairly easily separated by