Cation ordering in BaAl2Ge2O8-feldspar: implications for the phase transition in anorthite |
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Authors: | T Malcherek M A Carpenter H Kroll E K H Salje |
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Institution: | Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK E-mail: thomas@minp.esc.cam.ac.uk, GB Institut für Mineralogie, Westf?lische Wilhelms-Universit?t, Corrensstr. 24, D-48149 Münster, Germany, DE
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Abstract: | BaAl2Ge2O8-Feldspar undergoes an order-disorder phase transition I2/c↔C2/m at T
tr
≈1690 K. The thermodynamics of the Al,Ge cation ordering process is described in terms of the compressible Ising model in
mean field approximation. The mean field potential predicts a first order character of the phase transition. This is compared
to antiferromagnetic ordering in a two-dimensional square Ising model with NN-pair interactions and four-spin interactions
on alternating squares. Calculated order parameters and short range ordering are in good agreement with the corresponding
properties observed in BaAl2Ge2O8-feldspar by means of X-ray diffraction, hard mode infrared spectroscopy and TEM. Using known calorimetric data a similar
model is postulated for Al,Si ordering in anorthite, CaAl2Si2O8, for which the derived potential describes a transition with slightly stronger first order character at T
tr
≈1928 K.
Received: 30 January 1998 / Revised, accepted: 29 August 1998 |
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Keywords: | Phase transition Anorthite Ising-Model BaAl2Ge2O8 |
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