Mineralogy and crystal chemistry of the jarosite-alunite and natrojarosite-natroalunite solid solution series.

摘要

The Mars Exploration Rover Opportunity discovered sulfate-rich sedimentary rock outcrops on the surface of Mars that preserve a detailed history apparently involving liquid water at the surface (Grotzinger et al., 2005; Squyres et al., 2005). Among the minerals identified in the sulfate-rich outcrops was jarosite (Klingelhofer et al., 2004). Jarosite is known to precipitate from low-pH waters in a variety of environments on Earth, and the identification of jarosite provides mineralogical evidence that liquid water was once present on the surface of Mars and it also suggests the water was acidic.;The goals of this project were to synthesize and characterize a set of samples along the solid solution between jarosite (KFe3(SO 4)2(OH)6) and alunite (KAl3(SO 4)2(OH)6). Detailed X-ray diffraction data for the synthetic samples were collected. Initial examinations of the diffraction patterns suggested that the synthesis methods yielded pure, single-phase jarosite compositions. However, more detailed analysis of the samples was conducted, and unit-cell parameters, crystallite sizes, and values for microstrain were calculated using Topas, a Rietveld refinement program. The results suggested that strain produced from the substitution of Fe+3 for Al across the jarosite-alunite join resulted in the coexistence of multiple phases for synthesis products of intermediate compositions. These data suggest that the solid solution between jarosite and alunite is not ideal and that there is a miscibility gap at intermediate compositions.