Crystal structure and phase stability of polyhalite at high temperatures

As an accessory mineral in marine evaporites, polyhalite, K₂MgCa₂(SO₄)₄·2H₂O, coexists with halite (NaCl) in salt formations, which are a potential type of geological repositories for permanent storage of high-level nuclear wastes. However, because of the heat generated by radioactive decays in the wastes, polyhalite (as well as other co-existing hydrous minerals such as clay and gypsum) may dehydrate, and the released water will dissolve its neighboring salt, potentially affecting the repository integrity. Thus, studying the thermal behavior of polyhalite is important. In this study, a polyhalite sample containing a small amount of halite was collected from the Salado formation at the WIPP site in Carlsbad, New Mexico. 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 water vapor, anhydrite (CaSO₄) and two langbeinite-type phases, K₂Ca_xMg_2-x(SO₄)₃, with different Ca/Mg ratios. XRD peaks of the minor halite disappeared, presumably due to its dissolution by water vapor. With further increasing temperature, the two langbeinite solid solution phases displayed complex variations in crystallinity, composition and their molar ratio and then were combined into the single-phase triple salt, K₂CaMg(SO₄)₃, at ~919 K. Rietveld analyses of the XRD data allowed determination of structural parameters of polyhalite and its decomposed anhydrite and langbeinite phases as a function of temperature. From the results, the thermal expansion coefficients of these phases have been derived, and the structural mechanisms of their thermal behavior been discussed. In addition, to determine phase stability relations, standard enthalpies of formation of polyhalite from constituent oxides and elements were measured using high-temperature drop-solution calorimetry.