High-temperature thermal expansion and decomposition of garnets

The high-T unit-cell volumes of a synthetic grossular, and natural pyrope and almandine close to end-member compositions have been determined at up to 1490, 1630 and 1370 K by powder XRD using synchrotron radiation. At high T, thermal expansion coefficients increase slightly in the order grossular, pyrope and almandine, with values approximately 2.7, 3.0 and 3.3 X 10 (super -5) /K, respectively. At >><$I> 1500 K, grossular decomposes rapidly to the equilibrium assemblage anorthite + gehlenite + pseudowollastonite; at similar T, pyrope decomposes to corundum + enstatite, not to the stable subsolidus assemblage; for almandine, decomposition begins at approximately 1300 K and is triggered by oxidation of iron, giving hematite, sillimanite and quartz/cristobalite.