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We have studied the thermal expansion of 15 synthetic and two natural F-Cl apatite solid solutions through the calculation of unit-cell dimensions at elevated temperatures based on X-ray powder diffraction data collected from room temperature to ~900 °C at 50 to 100 °C intervals. Coefficients of thermal expansion for the a and c unit-cell axes show sensitivity to composition, with αa increasing by about 50% and αc decreasing by a third from chlorapatite to fluorapatite. Despite the relationships observed for a and c, the thermal expansion coefficient for unit-cell volume shows little sensitivity to composition, which can be explained only by a mutually compensating structural adjustment along the latter axes as temperature rises. Results of this study also imply that the thermodynamically ideal volumes of mixing for F-Cl apatite solid solutions observed at ambient conditions continue to at least 900 °C. [ABSTRACT FROM AUTHOR]