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 12/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)](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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