Temperature and humidity dependent formation of CaSO4•xH2O (x=0...2) phases

The temperature and humidity dependent formation of different crystalline phases CaSO4 center dot xH(2)O with 0 <= x <= 2 was investigated by in situ X-ray powder diffraction using a Bruker D8 diffractometer equipped with an Anton Paar CHC+ cryo - / humidity chamber. By isothermal measurements over time ranges up to 60 h at temperatures between 76 degrees C and 140 degrees C the dehydration of gypsum (CaSO4 center dot 2H(2)O) to bassanite (CaSO4 center dot 1/2H(2)O) and gamma-anhydrite (gamma-CaSO4) was studied under low relative humidity of < 7% and under consideration of the dependence of the dehydration process on particle size. beta-anhydrite (beta-CaSO4, mineral anhydrite) was not found as dehydration product up to 140 degrees C, but only in small amounts as dehydration product of bassanite after 6 months at 80 degrees C. Rehydration of previously formed bassanite between 30 degrees and 10 degrees C at relative humidity of 95% (without free H2O in the sample) was investigated analogously to the dehydration study. With quantitative phase analysis by the Rietveld method of the isothermal X-ray powder diffraction measurements fraction of conversion versus time curves were derived and the applicability of kinetic models is discussed. From the results it can be concluded that gypsum dehydration/rehydration under climatic conditions present in the Atacama Desert is unlikely to occur over diurnal cycles, however, very small fractions of gypsum dehydrated to bassanite and subsequently rehydrated to gypsum are conceivable over very extended time scales in the top layer of the soil surface. Faster dehydration and slower rehydration for small particle size (< 20 mu m) could favor a minor accumulation of gypsum dehydration products. However, the formation of large amounts of beta-anhydrite (which is found, besides gypsum, as main sulfate constituent of 'gypsum crust' in the Atacama Desert) by gypsum/bassanite dehydration under climatic conditions in the Atacama Desert, is unlikely for the pure CaSO4 center dot xH(2)O system without gypsum/bassanite nucleation-inhibiting additives.