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 °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 ΔVT 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.

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