Note:  Do not rely on this information. It is very old.

Orthoclase

Orthoclase (Potash-felspar), typically consisting of 64-6 per cent of silica, 18-5 of alumina, and 16-9 of potash, and crystallising in the Oblique system. It occurs in large crystals in porphyritic rocks, such as the granite of Shap Fell, and is usually opaque, white, cream-colour, or pink. Its hardness is 6, and its specific gravity 2-5; it] is scarcely affected by acids, and is only fusible at the edges with the mouth-blowpipe. It occurs often associated with quartz and hornblende, as an essential constituent of granite, syenite, felsite and gneiss, and readily undergoes kaolinisation or hydration into china-clay.