FELDSPAR IS ONE OF THE MOST COMMON MINERALS USED IN THE CERAMIC AND GLASS INDUSTRIES. IT IS AN ALKALI-BEARING ALUMINOSILICATE CORRESPONDING TO THE EMPIRICAL FORMULA R20, AJO3, 6SIO2, WHERE R REPRESENTS AN ALKALI METAL. AS NONE OF THE FELDSPAR GROUP OF MINERALS IS FOUND PURE, POTASH FELDSPAR ALWAYS CONTAINS SOME SODA FELDSPAR AND VICE-VERSA WITH VARYING AMOUNTS OF FREE QUARTZ, IRON OXIDE AND TRACES OF LIME AND MAGNESIA RESULTING IN A RANGE OF COLORS.