✦ LIBER ✦

Thermodynamics of plagioclase III: Spontaneous strain at the(Ioverline 1 - Poverline 1)phase transition in Ca-rich plagioclase

✍ Scribed by Simon A. T. Redfern; Ann Graeme-Barber; Ekhard Salje

Publisher: Springer-Verlag
Year: 1988
Tongue: English
Weight: 587 KB
Volume: 16
Category: Article
ISSN: 0342-1791
DOI: 10.1007/bf00203199

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✦ Synopsis

The lattice parameters of anorthites AngsAb 2 and Anloo have been measured from 22 to 1100 K. The spontaneous strain arising from the I1-P 1 displacive transition in An98 follows second order Landau behaviour. The spontaneous strain (es) couples quadratically to the order parameter (QO) with esocQ~ * -T) and T* = 530 K in An98. This is in contrast to the tricritical behaviour observed in pure anorthite. These observations are consistent with a Landau model for the free energy of Ca-rich plagioclases in which A1/Si order and Na content renormalize the fourth order coefficient.

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The influence of A1/Si disorder in the anorthite tetrahedral framework upon the I1-P1 displacive transition of that framework has been investigted at high-temperatme by powder X-ray diffraction. The temperaturedependence of the order parameter in a heat-treated (disordered) anorthite and a Ca-rich p