On the occurrence of jahnsite/whiteite phases on Mars: A thermodynamic study

1Christophe Drouet,2Matteo Loche,2Sébastien Fabre,2Pierre-Yves Meslin
American Mineralogist 107, 1807-1817 Link to Article [http://www.minsocam.org/MSA/AmMin/TOC/2022/Abstracts/AM107P1807.pdf]
1CIRIMAT, Université de Toulouse, CNRS, Toulouse INP, UPS, 4 allée Emile Monso, 31030 Toulouse, France
2Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, UPS, CNRS, CNES, 9 Avenue du Colonel Roche, 31400 Toulouse, France
Copyright: The Mineralogical Society of America

Jahnsites/whiteites are a large family of phosphate hydrate minerals of relevance to terrestrial and
martian mineralogy. It was recently hypothesized as being present in Gale Crater sediments from XRD
analyses performed by the CheMin analyzer aboard the Curiosity rover. However, the conditions of
formation and thermodynamic properties of these compounds are essentially unknown. In this work,
we have optimized the ThermAP predictive thermodynamic approach to the analysis of these phases,
allowing us to estimate for the first time the standard formation enthalpy (ΔHf°), Gibbs free energy
(ΔGf°), and entropy (S°) of 15 jahnsite/whiteite end-member compositions, as well as of related phases
such as segelerite and alluaudites. These estimations were then used to feed speciation/phase diagram
calculation tools to evaluate the relative ease of formation and stability of these hydrated minerals,
including considering present martian conditions. Selected laboratory experiments confirmed calcula-
tion outcomes. All of our data suggest that the formation of jahnsites is an unlikely process, and point
instead to the formation of other simpler phosphate compounds. The stability domain, as calculated
here, also raises serious questions about the possible presence of jahnsites on Mars as in Gale Crater,
which appears rather improbable.

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