THERMAL DECOMPOSITION OF POLYHALITE: A MINERAL RELEVANT TO SALT REPOSITORY
An important hydrous mineral coexisting with salt is polyhalite, K2MgCa2(SO4)4·2H2O. Like other hydrous phases, polyhalite dehydrates on heating, where the released water will interact with its neighboring salt. In this work, a polyhalite sample containing a small amount of halite was collected from the Salado formation at the WIPP site in Carlsbad, New Mexico. To determine its thermal behavior, in-situ high-temperature synchrotron X-ray diffraction was conducted from room temperature to 1066 K with the sample powders sealed in a silica-glass capillary. At about 506 K, polyhalite started to decompose into anhydrite (CaSO4), triple salt (K2SO4·MgSO4·CaSO4) and water vapor. XRD peaks of the minor halite disappeared, presumably due to its dissolution by water vapor. With further increasing temperature, the triple salt decomposed into two langbeinite-type phases along the K2SO4·2MgSO4 - K2SO4·2CaSO4 join. Rietveld analyses of the synchrotron data allow determination of structural parameters of polyhalite and its decomposed phases as a function of temperature. From these results, the thermal expansion coefficients have been derived, and the mechanisms of polyhalite decomposition been discussed.