Nepheline--Alkali Feldspar Equilibria: I. Experimental Data and Thermodynamic Calculations

Nepheline-alkali feldspar equilibria with alkali chloride aqueous solutions have been determined for the temperature range 400 to 700 °C at 1000 bars pressure. Nepheline-alkali feldspar equilibria with alkali chloride melts have been determined for the temperature range 800 to 1100 °C at approximately 6 bars pressure. (1) NaAlSiO4 + KCl(aq) = NaCl(aq) + KAlSiO4 (2) NaAlSiO4 + KCl(melt) = NaCl(melt) + KAlSiO4 (3) NaAlSi3O8(high) + KCl(aq) = NaCl(aq) + KAlSi3O8(San) (4) NaAlSi3O8(low) + KCl(aq) = NaCl(aq) + KAlSi3O8(Mic) (5) NaAlSi3O8(high) + KCl(melt) = NaCl(melt) + KAlSi3O4(San) (6) NaAlSi3O8(low) + KCl(melt) = NaCl(melt) + KAlSi3O8(Mic) From these, two diagrams of phase relationships were derived for the following exchange equilibria: (7) NaAlSiO4 + KAlSi3O8(San) = NaAlSi3O8(high) + KAlSiO4; (8) NaAlSiO4 + KAlSi3O8(Mic) = NaAlSi3O8(low) + KAlSiO4. The effect of pressure on these equilibria has been determined by comparing the experimental data for 1000 and 5000 bars (t = 500 °C) and thermodynamic calculations. It has also been shown that the effect of excess silica in nepheline solid solution on the K—Na distribution between nepheline and alkali feldspar is substantial and opposite to that of temperature. In the high temperature region an increase in silica content in nepheline of 2 wt. per cent eliminates the effect on the redistribution of a temperature increase of 100 °C. These cation exchange data and unit cell data for the crystal phases are used to calculate thermodynamic mixing properties of nepheline solid solution and alkali feldspar solid solution for a wide range of temperature and pressure.