Single crystal X-ray intensity data have been collected for a grossularite at 25°, 365° and 675°C and a pyrope at 25°, 350°, 550°, and 750°C. Anisotropic least-squares refinements reveal that the oxygen positional parameters remain approximately constant in grossularite but change in pyrope. Linear thermal expansion coefficients per 1°C [(lT - l250)/(T - 25)/l250] for the mean interatomic distances Ca-O and Al-O in grossularite are 0.8 × 10-5 and 1.3 × 10-5, respectively, while those from Mg-O and Al-O in pyrope are 1.3 × 10-5 and 0.8 × 10-5, respectively. The Si-O interatomic distances do not increase significantly as a function of temperature.
In pyrope, with increasing temperature, the rigid SiO4 tetrahedra rotate to allow the shared octahedral edge to lengthen at a greater rate than the unshared edge, thereby decreasing the octahedral bond angle strain. In grossularite the tetrahedra do not rotate, and the octahedral bond angle strain does not change. Structural changes in aluminum silicate garnets as a function of heating are compared to changes resulting from chemical substitution at the triangular dodecahedral site.