High-temperature volumes of K-Na mixing have been investigated for a seven-member low albite-microcline ion-exchange series by conducting X-ray powder diffraction measurements from room temperature to approximately 1000 degrees C using Guinier techniques. Volume expansion is a linear function of temperature for all series members and is due mainly to the lengthening of the a unit-cell axis, although some expansion of b and c occurs for relatively sodic members. The maximum expansion possible for each feldspar is determined both by chemistry and temperature, thus Delta V/Delta T slopes are steepest for the sodic third of the series, then decrease abruptly for increasingly potassic feldspars. Volumes of K-Na mixing for this series are essentially constant with temperature, regardless of the mixing model utilized, and thus are adequately represented by regular solution thermodynamic models.
Title
Volumes of K-Na mixing for low albite-microcline crystalline solutions at elevated temperature: A test of regular solution thermodynamic models
Hovis, G. L. and A. Graeme-Barber. (1997) "Volumes of K-Na mixing for low albite-microcline crystalline solutions at elevated temperature: A test of regular solution thermodynamic models." American Mineralogist 82 (1/2): 158-164.