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An in-situ high-temperature structural study of stable and metastable CaAl2Si2O8 polymorphs

Published online by Cambridge University Press:  05 July 2018

I. Daniel
Affiliation:
Laboratoire des Sciences de la Terre, Ecole Normale Supérieure de Lyon, URA 726 CNRS, 46 allée d'Italie, 69354 Lyon cedex 07, France
P. Gillet
Affiliation:
Laboratoire des Sciences de la Terre, Ecole Normale Supérieure de Lyon, URA 726 CNRS, 46 allée d'Italie, 69354 Lyon cedex 07, France
P. F. McMillan
Affiliation:
Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287, USA
P. Richet
Affiliation:
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.

Type
Mineralogy
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1995

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