Cation ordering in BaAl2Ge2O8-feldspar: implications for the phase transition in anorthite
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1432-2021
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| Keywords: |
Key words Phase transition ; Anorthite ; Ising-Model ; BaAl2Ge2O8
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| Source: |
Springer Online Journal Archives 1860-2000
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| Topics: |
Chemistry and Pharmacology
Geosciences
Physics
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| Notes: |
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.
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| Type of Medium: |
Electronic Resource
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| URL: |