Thermal behavior of natural stellerite: high-temperature X-ray powder diffraction and IR spectroscopy study.

Abstract

The thermal behavior of stellerite from the Savinskoye deposit (Transbaikalia, Russia), Ca7.69Na0.25K0.06(Si56.24Al15.76)O144·53.39H2O, was investigated by in situ high-temperature X-ray powder diffraction (HTXRPD) and ex situ HT infrared (IR) spectroscopic analysis. Four different HTXRPD experimental procedures were used to study the thermal behavior of the powder samples: (1) RT-750°C, (2) RT-220°C -RT, (3) 200-350-RT °C, and (4) 350-700°C. Electron probe microanalysis and single-crystal X-ray diffraction were preliminary used to determine the chemical composition and crystal structure of stellerite. The A → B phase transition (Fmmm → Amma) starts at ∼110°C and is completed at about 140°C (in situ HTXRPD) and 200°C (ex situ HTIR) depending on the experimental conditions. It involves a cell volume decrease of 5.8% (Experiment 1). The thermal expansion of stellerite is more pronounced along the b and c axes, with αa: αb: αc (× 10-5) = 2.50:-25.52:-6.84 at 100°C, 0.44:-21.75:-25.64 at 150°C after the completion of the phase transition, and 3.06:-1.86:-16.94 at 500°C. The reverse B → A transition occurs at temperatures below 100°C during slow cooling (Experiment 2), however, it does not occur upon rapid cooling (Experiment 3). The B → D phase transition above 300°C is not observed (Experiment 4). The temperature barrier of phase transition in the ex situ HTIR spectroscopy experiment is shifted towards high temperatures. The heating above 200°C leads to an increase of 3430cm-1 and a decrease of 3600 and 3260cm-1 bands, which correspond to the stretching vibration of H2O. The heating above 400°C causes complete dehydration of the stellerite.