Feldspar and Nepheline Syenite Fillers With a Purpose
Nepheline syenite is a rock composed of sodium and potassium feldspars and nepheline. Nepheline is nominally a sodium aluminosilicate, but potassium invariably substitutes for a portion of the sodium. The amount of potassium in natural nephelines ranges from 2.5 to 10% (3 to 12% K2O) by weight. Nepheline can form only in a geologic environment deficient in silica.
Commercial nepheline syenite deposits, and the products made from them, are consequently free of crystalline silica impurities. These deposits are exploited for their high mineral brightness and low level of dark mineral impurities. Commercial products are made by crushing, dry magnetic separation and milling. Fine-particle-size grades are produced by air classification. In North America, typical nepheline syenite consists of approximately 25% nepheline, 55% sodium feldspar (albite) and 20% potassium feldspar (microcline)1. The basic properties of nepheline and the feldspar minerals are compared in Table 1. There is no North American production of coatings-grade calcium feldspar, which in any event would be less desirable than the sodium and potassium varieties, due to its higher refractive index and specific gravity.