An In-situ High-Temperature Structural Study of Stable and Metastable CaAl2Si2O8 Polymorphs

I. Daniel, P. Gillet, P. F. McMillan and P. Richet
Laboratoire des Sciences de la Terre, Ecole Normale Supérieure de Lyon, URA 726 CNRS, 46 allée d'Italie, 69354 Lyon cedex 07, France
Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287, USA
Département des Géomatériaux, Institut de Physique du Globe de Paris, 4 place Jussieu, 75252 Paris cedex 05, France

Abstract: High-temperature Raman spectroscopy and optical microscopic observations have revealed a new metastable polymorph of CaAl2Si2O8 composition, which brings to four the number of known crystalline phases in this system. Similar to the metastable monoclinic pseudo-orthorhombic and pseudo-hexagonal phases, the new polymorph nucleates prior to anorthite, at around 1545 K, and its pseudo-liquidus temperature is 1700 ± 10 K. It can also be formed from the transformation of the pseudo-hexagonal phase at 1050 K. The actual structure of this new crystalline form is unknown, but its Raman spectrum indicates that it is most likely a 6-membered alumino-silicate framework. We have obtained all three metastable phases as pure single crystals using wire loop heating techniques, and have studied their structures via Raman spectroscopy up to their metastable melting points or transformation temperatures.

Keywords: plagioclases • anorthite polymorphs • metastable nucleation • Raman spectroscopy • high temperature

Mineralogical Magazine; March 1995 v. 59; no. 394; p. 25-33; DOI: 10.1180/minmag.1995.59.394.03
© 1995, The Mineralogical Society
Mineralogical Society (