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 (An30SrF70) and Ca0.6Sr0.4Al2Si2O8 (An60SrF40) were synthesized from gel and annealed at temperatures between 1150 and 1480{degrees}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{degrees}C. The linewidth in hard mode is related to the spontaneous strain of the monoclinic-triclinic transition via {Delta}{Gamma} {propto} [IMG]f1.gif" ALT="Formula" BORDER="0"> {propto} [IMG]f2.gif" ALT="Formula" BORDER="0"> {propto} [IMG]f3.gif" ALT="Formula" BORDER="0">. The evolution of the order parameter with time can be modelled as [IMG]f3.gif" ALT="Formula" BORDER="0"> {propto} [IMG]f2.gif" ALT="Formula" BORDER="0"> {propto} 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