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Mineralogical Magazine; June 2007; v. 71; no. 3; p. 321-326; DOI: 10.1180/minmag.2007.071.3.321
© 2007 Mineralogical Society of Great Britain and Ireland
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Crystal chemistry of aegirine as an indicator of P-T conditions

L. Secco1,*, A. Guastoni1, F. Nestola1, G. J. Redhammer2 and A. Dal Negro1

1 Dipartimento di Geoscienze, Università di Padova, Corso Garibaldi 37, I-35137, Padova, Italy
2 Department of Materials Engineering and Physics, Division of Mineralogy, University of Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria

* E-mail: luciano.secco{at}unipd.it

One metamorphic and four magmatic aegirines, together with two end-member aegirines synthesized at atmospheric pressure and different temperatures, were investigated by single-crystal X-ray diffraction. The limited compositional differences allow the polyhedral volumes to be almost constant in all the aegirines investigated (VM1 {approx} 11.0 Å3; VM2 {approx} 26.3 Å3; VT {approx} 2.21 Å3). However, differences in polyhedral distortions are responsible for the cell-volume variations, reflected mainly in the change of a and ß cell parameters. Cell volume is only partly related to the composition of these aegirines: with increasing formation temperature, an increase in the unit-cell volume of ~1.2 Å3 is observed, while a significant contraction of the cell volume occurs during high-pressure formation. As the difference in cell volume between the two synthetic aegirines is ascribed to the different conditions of synthesis temperature, the same interpretation could be adopted for the differences observed in natural aegirines.

KEYWORDS: clinopyroxene, X-ray diffraction, single-crystal, aegirine






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