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

¹ Dipartimento di Scienze della Terra, Università di Parma, Viale G.P. Usberti 157/A, I-43100, Parma, Italy
² Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

^* E-mail: mario.tribaudino{at}unipr.it

Two feldspars of composition Ca_0.3Sr_0.7Al₂Si₂O₈ (An₃₀SrF₇₀) and Ca_0.6Sr_0.4Al₂Si₂O₈ (An₆₀SrF₄₀) were synthesized from gel and annealed at temperatures between 1150 and 1480°C 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 = 1350°C. 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: feldspars, phase transition, Al-Si order-disorder, spontaneous strain, IR spectroscopy