High-temperature thermal expansion and decomposition of garnets
Thiéblot, Laurent; Roux, Jacques; Richet, Pascal
published: Jan 26, 1998
manuscript accepted: Jul 10, 1997
manuscript received: Mar 18, 1996
ArtNo. ESP147051001002, Price: 29.00 €
Abstract The high-temperature unit-cell volumes of three garnets have been determined through powder X-ray diffraction experiments with synchrotron radiation. The measurements have been made up to 1490, 1630 and 1370 K for a synthetic grossular (Ca3Al2Si3O12), a natural pyrope (Mg3Al2Si3O12) and an almandine close to endmember (Fe3Al2Si3O12) composition, respectively. At high temperatures thermal expansion coefficients increase slightly in the order grossular, pyrope and almandine, being about 2.7, 3.0 and 3.3 10-5 K-1 at 1400 K, respectively. Above 1500 K grossular decomposes rapidly to the equilibrium assemblage anorthite + gehlenite + pseudowollastonite. At similar temperatures, pyrope decomposes to corundum plus enstatite, not to the stable subsolidus assemblage. For almandine, decomposition begins at about 1300 K and is triggered by oxidation of iron, with the formation of hematite, sillimanite and quartz or cristobalite.