The pressure dependence of the dehydration of gypsum to bassanite

Abstract

A new experimental determination of the stability relationships for the dehydration of gypsum to the hemihydrate mineral bassanite at elevated temperature and pressure is described. The experimental method used depends on the observation of very small changes in pressure on the onset of reaction due to the potential volume change in the reaction. The technique yields P-T data of very high precision for this dehydration reaction, and the method is likely to be of use for other reactions. The experimental P-T results have been compared with those calculated from existing thermodynamic data for this reaction.

Metamorphic reaction rate controlled by fluid pressure not confining pressure: implications of dehydration experiments with gypsum

“…The dehydration of Volterra gypsum samples in the apparatus, under water pore fluid pressure, produces bassanite (sulphate hemihydrate), a metastable phase in the family of hydrated calcium sulphates (Posjnak 1938;Kelley et al 1941;Zen 1965;Yamamoto and Kennedy 1969;McConnell et al 1987;Mirwald 2008). The stable phase, anhydrite, has not been recorded in our experiments: the water volumes expelled are too small, and no anhydrite was found by X-ray diffraction in dehydrated specimens.…”

Raman intensities near gypsum-bassanite transition in natural gypsum

“…Furthermore, extrapolation of the P-T curve to atmospheric pressure resulted in a transition temperature higher than the observed value (370 K), which the authors attributed to some degree of imperfections in the resultant hemihydrite product. 16 IR spectroscopic studies indicated no change in the hydrogen bonding scheme for the bassanite phase. 5 Hence the sulphate ions would have undergone comparatively larger structural rearrangements during the transition.…”

“…15 Earlier studies on the pressure dependence of the gypsum to bassanite transition, in a synthetic powdered sample, reported a change in entropy of 12.22 cal mol 1 K 1 (1 cal D 4.184 J , at 385 K and 1 kbar pressure. 16 This transition entropy is much larger compared with k B ln 2, where k B is the Boltzmann constant, a typical value for an order-disorder transition. Furthermore, extrapolation of the P-T curve to atmospheric pressure resulted in a transition temperature higher than the observed value (370 K), which the authors attributed to some degree of imperfections in the resultant hemihydrite product.…”

The geology and genesis of the iron skarns of the Turgai belt, northwestern Kazakhstan

“…greenockite) in the late phases of alteration may also be due to local ingress of large volumes of meteoric fluid into the system at temperatures below 150°C (McConnell et al, 1987).…”