Lead feldspar single crystals were annealed at T = 1050 and 1000 degrees C, starting from a disordered metastable configuration (PbF H , Q od = 0) and from an ordered configuration (PbF L , Q od = 0.89). Single-crystal data collection and refinement in space group I2/c show that the degree of Al-Si order increases to Q od = 0.42 after annealing the disordered PbF H at 1050 degrees C and decreases to Q od = 0.70 after annealing the ordered PbF L sample at 1000 degrees C. This suggests that the equilibrium Q od is between 0.70 and 0.42 for temperatures between 1000 and 1050 degrees C, where anorthite or strontium feldspar are almost completely ordered. A residual in the difference-Fourier map because of positional disorder was observed near the Pb site in all the refined crystals. The average y/b Pb coordinate changes with increasing Al-Si disorder, as Pb approaches the glide plane. A significant decrease in the intensity of b-type reflections was consequently observed. A spontaneous strain, with the main axis almost parallel to the a axis, is associated with Al-Si ordering. Pb polyhedral deformation related with Q od accounts for the observed strain. A calibrating equation, Q od = [(8.427(2)-a) / 0.048(3)] (super 1/2) , has been calculated and applied to the unit-cell parameters obtained from subsequent thermal treatments and from Bruno and Facchinelli (1972) to define the evolution of the Q od vs. the treatment temperature. The thermal behavior of the Q od could then be bracketed, suggesting T c between 1150 and 1200 degrees C for the I2/c-C2/m phase transition induced by the Al-Si order-disorder process.

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