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

Two feldspars of composition Ca0.3Sr0.7Al2Si2O8 (An(30)SrF(70)) and Ca0.6Sr0.4Al2Si2O8 (An(60)SrF(40)) were synthesized front gel and annealed at temperatures between 1150 and 1480 degrees C for durations between 0.025 (1.5 min) and 1073 h. The An(160)SrF(40) feldspar is triclinic for all annealing temperatures, whereas the An(30)SrF(70) possesses, at room temperature, a monoclinic symmetry, C2/m in the shortest annealing runs and 12/c after slightly longer heating, and becomes triclinic after prolonged annealing. A TTT plot for the triclinic-monclinic transition shows in 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 An(30)SrF(70), annealed at T = 1350 degrees C. The linewidth in hard mode is related to the spontaneous strain of the monoclinic-triclinic transition via Delta Gamma alpha q(od)(2) alpha Q(od)(2) alpha epsilon(2)(ss). The evolution of the order parameter with time can be modelled as epsilon(2)(ss) alpha Q(od)(2) alpha lnt, as predicted in order-disorder processes at intermediate degrees of order. It is shown that even after the longest annealing runs in An(30)SrF(70), equilibrium was not obtained whereas, under the same conditions, equilibrium AI-Si order call be obtained in anorthite and Sr-feldspar.