High-temperature in situ structural investigation on lead feldspar

Single-crystal X-ray diffraction was performed in situ at T = 20,, 230, 465, and 700 degrees C on a partially ordered lead feldspar (PbAl(2)Si(2)O(8), I2/c, a = 8.402, b = 13.033, c = 14.308 Angstrom, beta = 115.30 degrees, V = 1417.6 Angstrom(3); Q(od) = 0.71), The unit-cell expansion (1.26 x 10(-5) degrees C(-1)) is close to that observed for other feldspars, sanidine in particular, and occurs predominantly along a*. The electron-density at the Pb site evolves with temperature toward a bean-like configuration close to that observed in disordered lend feldspar. The average Pb position approaches the c-glide plane with increasing temperature. Consequently the intensity of the b-type reflections reduces dramatically without evidence of an increase of AI-Si disorder. The evolution of atomic displacement parameters of the Pb atom with temperature supports the view that at room temperature Pb shows considerable positional disorder. Dark-field in situ TEM observations show that b antiphase domains (APD) persist unchanged in shape and size up to T = 690 degrees C, No diffuse component appears in b-type reflections in SAD patterns up to 935 degrees C, showing that the above changes in the Pb configuration do not affect the APD. The results suggest that, at T > 700 degrees C, Pb reaches the glide plane assuming a configuration that may favor the Al-Si disorder.