Single crystals of Sr feldspar (SrAl2Si2O8, space group: I2/c) were treated isothermally in the temperature range between 1410 and 1640°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 Qod values.
Two different series of thermal treatments were performed: (1) a crystal of ordered Sr feldspar (Qod = 0.85) was progressively disordered at 1560°C (Qod = 0.82), at 1610°C (Qod = 0.74) and at 1640°C (Qod = 0.69); (2) another crystal with Qod = 0.72 (obtained by annealing a glass at T =1625°C), was ordered at T = 1600° (Qod = 0.75).
As already observed in anorthite quenched from 1530°C (Qod = 0.78), also in Sr feldspar, annealing near Tmelt caused significant disorder in the Al-Si configuration (Qod = 0.69).
Observations by transmission electron microscopy show an increase in the average size of the b-type antiphase domains up to 5000 Å in the samples quenched from 1640°C, in comparison with samples annealed at 1450°C (~ 2000 Å ). The modifications observed in the Al,Si configurations are due to intradomain disorder.