Thermal Behavior of the Calcium - Sulfate - H 2 O System

Introduction: Sulfate species were verified by Pathfinder in 1997 and more recently by the Mars Exploration Rovers (MER). The best evidence for sulfates on Mars came from the Opportunity rover in Meridiani Planum, where data supporting the existence of Ca and Mg-sulfates were collected from alpha-proton X-ray spectroscopy and thermal emission spectroscopy [1,2]. Hyperspectral VNIR OMEGA images also revealed the presence of monohydrated and polyhydrated sulfates in canyon walls and mounds near Valles Marineris [3] and gypsum in northern polar regions [4]. Recent CRISM data provided much greater resolution of these alternating mono/polyhydrated layers in Valles Marineris as well as in Juventae Chasma [5]. There is, however, considerable debate as to whether or not these represent separate conditions of deposition or subsequent hydration/dehydration reactions. In order to infer conditions of deposition and subsequent water cycles from the presence of specific phases, it is necessary to understand the kinetics of hydration and dehydration of these hydrated minerals. For example, gypsum has shown a sluggish response to desiccation in previous studies [6]. Kinetic information should allow us to determine whether specific phases were deposited in their present form or are products of subsequent dehydration/rehydration reactions. The dehydrated forms of gypsum (bassanite and anhydrite) have not been definitively identified on the surface of Mars. However, the spectral resolution of CRISM and OMEGA is not optimized to detect these dehydrated phases, which suggests that their presence cannot be ruled out. Experimental results are presented here that provide a more thorough picture of dehydration rates in the calcium-sulfate-H2O system. Our results suggest that gypsum desiccation will not occur over a diurnal or seasonal cycle under current martian conditions. Methods: X-ray diffraction (XRD) allows crystallographic information to be used to explore phase transitions and stability fields of selected sulfates. The dehydration behavior of gypsum (CaSO4•2H2O) to bassanite (CaSO4•0.5H2O) and anhydrite (CaSO4) was investigated over a pH2O range of 250 to 2250 Pa using an Anton-Paar TTK 450 heating stage on a Bruker D8 diffractometer with a VANTEC-1 position-sensitive detector (Cu radiation). An environmental cell was placed over the stage to control the partial pressure of H2O (pH2O) using an InstruQuest V-Gen relative humidity generator. Precise control of temperature and relative humidity allows the pH2O to be varied in-situ, leading to greater precision in analysis of the thermal behavior of this sulfate system. Time-dependent experiments were performed at various temperatures (70°C, 80°C, 85°C, and 90°C) and relative humidities (20% to 50%). XRD data were measured from 10-55° 2θ to encompass the strongest peaks, with data collection times of 30 minutes and 60 minutes between pattern collections. Measurements were also performed using the detector in fixed-position mode (no detector movement) to measure very short-term changes (10s-120s) as a function of temperature over a small angular range. Measurement parameters for these experiments were chosen between 28o and 34o 2θ because clear transitions between gypsum, bassanite, and anhydrite can be observed in this range. d-spacing measurements used the gypsum 041, bassanite 400, and anhydrite 200 peaks (Figure 1). These peaks were chosen as they were all in close proximity to one another to allow fixed detector scanning and had high peakto-background ratios.