Composition.— Common feldspars are crys talline compounds of silica, alumina, and one or more of the bases, potash, soda, and lime, and rarely baryta. There are two principal classes: (1) the potash, and potash-soda feld spars, and (2) the soda, soda-lime and lime feldspars.
Pure potash feldspars are orthoclase and microcline: they are alike in chemical composi tion and in physical properties, and for com mercial purposes are practically identical. Their theoretical composition is: silica, 64.7 per cent; alumina, 18.4 per cent; and potash, 16.9 per cent. Potash feldspar usually contains some soda in place of the full theoretical comple ment of potash. If the soda content is greater than the potash content, the feldspar is called anor thoclase.
The theoretical composition of pure soda feldspar (albite) is: silica, 68.7 per cent; alum ina, 19.5 per cent; soda, 11.8 per cent. Soda feldspars of the soda-lime group generally con tain lime replacing a part of the normal soda content, or through the intermixture of lime feldspar.
Two or more of the various feldspars may be present in the same deposit, or even in the same crystal. None of the lime feldspars have a commercial value. The feldspar mined in the United States is of the potash or soda varieties, or a mixture of the two. Commonly,
the potash variety predominates. The average potash content is 13 per cent, but taking the run of the mine, the yield of potash is not more than 10 per cent.
Orthoclase, or potash feldspar, is a potas sium-aluminum polysilicate, K Al SW,. This compound is dimorphous, microcline, having an identical composition. Sodium often in part replaces the potassium, making soda-orthoclase or soda-microcline. When the sodium largely predominates the mineral passes into anortho clase (Na,K) Al Sia0.; when it replaces all the potassium, the mineral is albite, Na Al Si30.. Anorthite is a calcium-aluminum polysilicate, Ca Al,Si3O,. Between it and albite lie various triclinic feldspars, which are regarded as iso morphous mixtures of albite and anorthite.
Bibliography.— Bayley, W. S., (Minerals and Rocks' (New York 1915) ; Dana, J. D., (Manual of Mineralogy' (New York 1912) ; Iddings, J. P., (Rock Minerals' (New York 1911) ; Phillips, A. H., (Mineralogy) (New York 1912) ; Rogers, A. F., 'Introduction to the Study of Minerals' (New York 1912) ; United States Geological Survey 420' (Washington 1910) • United States Mines Bureau 53' (Washington 1913).