Geochemistry of phosphorus-I. The replacement of Si4+ by P5+ in rock-forming silicate minerals

Abstract

In 45 rock-forming silicate minerals, after removal of apatite by HNO3 dissolution, the phosphorus replacing silicon isomorphously was determined. On the average between 0.2-220 ppm P were found, amounting to about 1-10 P-ions per 104 SiO4-tetrahedra. The maximum content was found in a garnet with 560 ppm P. This amounts to 28 P-ions per 104 SiO4-tetrahedra. The more condensed a (Si,Al)O4-tetrahedral association, the less silicon is replaced by phosphorus. In general the most phosphorus is contained in neso-, the least in tektosilicates. The stronger covalency of the phosphorus bond is the main cause of this behavior. Another important factor regulating the amount of P-replacement is the equilibrium between the apatite always present and the coexisting silicate minerals. In igneous and metamorphic rocks, on the average 2-10 per cent of the total phosphorus is contributed by the camouflaged replacement of phosphorus in the silicates. Sometimes this can amount to as much as 25-30 per cent. © 1965.