Long-range AI, Si equilibrium configuration in Sr feldspar treated at high temperatures

摘要

Single crystals of Sr feldspar (SrAl_2Si_20_8, space group: T2/c) were treated isothermally in the temperature range between 1410 and 1640 deg C, in order to determinate the evolution of the Al,Si configurations at equilibrium with thermal treatments. X-ray intensity data collection and structural refinement were performed on the quenched crystals. Significant modifications were observed in the average tetrahedral distances and therefore in Q_(od) values. Two different series of thermal treatments were performed: (1) a crystal of ordered Sr feldspar (Q_(od) = 0.85) was progressively disordered at 1560 deg C (Q_(od) = 0.82), at 1610 deg C (Q_(od) = 0.74) and at 1640 deg C (Q_(od)= 0.69); (2) another crystal with Q_(od) = 0.72 (obtained by annealing a glass at T =1625 deg C), was ordered at T = 1600 deg (Q_(od) = 0.75). As already observed in anorthite quenched from 1530 deg C (Q_(od) = 0.78), also in Sr feldspar, annealing near T_(melt) caused significant disorder in the Al-Si configuration (Q_(od) = 0.69). Observations by transmission electron microscopy show an increase in the average size of the b-type antiphase domains up to 5000 A in the samples quenched from 1640 deg C, in comparison with samples annealed at 1450 deg C ( approx 2000 A). The modifications observed in the Al,Si configurations are due to intradomain disorder.