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Cation ordering and phase transitions in feldspars along the join CaAl2Si2O8-SrAl2Si2O8: a TEM, IR and XRD investigation

Published online by Cambridge University Press:  05 July 2018

M. Tribaudino ,
M. Zhang  and
E. K. H. Salje
M. Tribaudino*
Affiliation:
Dipartimentodi Scienze della Terra, Universita` di Parma, Viale G.P. Usberti 157/A, I-43100, Parma, Italy
M. Zhang
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
E. K. H. Salje
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
*
* E-mail: mario.tribaudino@unipr.it
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Abstract

Two feldspars of composition Ca0.3Sr0.7Al2Si2O8 (An30SrF70) and Ca0.6Sr0.4Al2Si2O8 (An60SrF40) were synthesized from gel and annealed at temperatures between 1150 and 1480ºC for durations between 0.025 (1.5 min) and 1073 h. The An60SrF40 feldspar is triclinic for all annealing temperatures, whereas the An30SrF70 possesses, at room temperature, a monoclinic symmetry, C2/m in the shortest annealing runs and I2/c after slightly longer heating, and becomes triclinic after prolonged annealing. A TTT plot for the triclinic–monclinic transition shows an activation energy of 101±12 kcal, less than, but still close to, that observed for Al-Si order in anorthite. TEM investigation shows antiphase domains and Carlsbad twins the size of which increases with annealing. IR spectroscopy was performed on a series of samples of An30SrF70, annealed at T = 1350ºC. The linewidth in hard mode is related to the spontaneous strain of the monoclinic–triclinic transition via . The evolution of the order parameter with time can be modelled as lnt, as predicted in order-disorder processes at intermediate degrees of order. It is shown that even after the longest annealing runs in An30SrF70, equilibrium was not obtained whereas, under the same conditions, equilibrium Al-Si order can be obtained in anorthite and Sr-feldspar.

Keywords

feldsparsphase transitionAl-Si order-disorderspontaneous strainIR spectroscopy.
Type
Research Article
Information
Mineralogical Magazine , Volume 73 , Issue 1 , February 2009 , pp. 119 - 130
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2009

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