JAROSITE STABILITY AND ITS IMPLICATIONS FOR MARTIAN MINERALOGICAL STUDIES

The ferric hydroxysulfate jarosite has gained significant attention as of late due to its apparent discovery in alleged Martian evaporite deposits. According to available jarosite stability data, the presence of this mineral most likely indicates diagenesis in an acidic aqueous environment. Obviously, the immediate implication of this data would be to assume that at one time the Martian surface and shallow subsurface experienced, at the very least, limited exposure to liquid water.

However, current jarosite stability figures are incomplete; nearly all information is limited to low pressure/temperature analyses, and the work that has been done lacks heat capacity, entropy, and enthalpy data (Stoffregen, 1993). Although acquiring these values may not be within the scope of this research, we are attempting to narrowly define the stability of jarosite and the compositional phases that may exist concurrently at high temperatures and pressures.

To this end, we began a series of experiments at a range of pressures, temperatures, and durations in order to delimit the conditions at which jarosite may form. Initial tests began at atmospheric pressure and ranged from 100ºC to 800ºC at 50ºC intervals for samples in open nickel-alloy trays over a duration of 24 hours. Further tests will be run in cold-seal reaction vessels at pressures ranging from 0.5 to 2.0 kbar at the above mentioned temperatures and at durations ranging from 12 to 24 hours.

All samples will be analyzed using XRD, XRF, and SEM techniques. Preliminary XRD results of samples run at atmospheric pressures indicate a compositional gradient proportional to temperature representing a change in phase from jarosite to either hematite or goethite. Interestingly, the intensity of peaks from 0ºC to 200ºC decrease until 300ºC, at which point a lack of intense peaks characterizes the data. From 400º - 800ºC, the intensity of the hematite peaks increase until it is the dominant phase.

Hopefully, results of these experiments will help to determine if jarosite, like various other sulfates, can form at metamorphic conditions or in melts. Perhaps the Martian jarosite signature is due to the exposure of a metamorphic or igneous formation, rather than as an evaporite or secondary encrustation. Ultimately, we expect that these results will add to the Martian jarosite dialogue.