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 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 ΔΓ ∝

\(\mathrm{q}_{\mathrm{od}}^{2}\)
\(\mathrm{Q}_{\mathrm{od}}^{2}\)
\({\epsilon}_{\mathrm{ss}}^{2}\)
. The evolution of the order parameter with time can be modelled as
\({\epsilon}_{\mathrm{ss}}^{2}\)
\(\mathrm{Q}_{\mathrm{od}}^{2}\)
∝ 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.

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