STRUCTURAL VARIATIONS INDUCED BY THERMAL-TREATMENT IN LEAD FELDSPAR (PBAL2SI2O8)

Lead feldspar single crystals were annealed at T = 1050 and 1000 degre es C, starting from a disordered metastable configuration (PbFH, Q(od) = 0) and from an ordered configuration (PbFL, Q(od) = 0.89). Single-c rystal data collection and refinement in space group 12/c show that th e degree of Al-Si order increases to Q(od) = 0.42 after annealing the disordered PbFH at 1050 degrees C and decreases to Q(od) = 0.70 after annealing the ordered PbFL sample at 1000 degrees C. This suggests tha t the equilibrium Q(od) is between 0.70 and 0.42 for temperatures betw een 1000 and 1050 degrees C, where anorthite or strontium feldspar are almost completely ordered. A residual in the difference-Fourier map b ecause of positional disorder was observed near the Pb site in all the refined crystals. The average y/b(Pb) coordinate changes with increas ing Al-Si disorder, as Pb approaches the glide plane. A significant de crease in the intensity of b-type reflections was consequently observe d. A spontaneous strain, with the main axis almost parallel to the a a xis, is associated with Al-Si ordering. Pb polyhedral deformation rela ted with Q(od) accounts for the observed strain. A calibrating equatio n, Q(od) = [(8.427(2) - a) / 0.048(3)](1/2), has been calculated and a pplied to the unit-cell parameters obtained from subsequent thermal tr eatments and from Bruno and Facchinelli (1972) to define the evolution of the Q(od) vs. the treatment temperature. The thermal behavior of t he 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 ord er-disorder process.