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

Abstract

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 to this day. 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 H f°, Gibbs free energy G f° 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 calculation outcomes. All of our data suggest that the formation of Jahnsites is an unlikely process, and This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

Manganese-Iron Phosphate Nodules at the Groken Site, Gale Crater, Mars

“…That database is consistent with the recent work Mn-phosphates [71] and, for the most part, is consistent with the earlier compilation of [72] and yields results similar to those in other recent works [45,73]. Where thermochemical data for a phosphate mineral were not available, they were estimated using the ThermAp method [74,75], which has been shown to yield adequately accurate values for many Mn-bearing minerals [71]. Because we estimated phases other than apatites, we used the estimation parameters from [71].…”

“…The trend of points in Figure 9A of constant low MnO with decreasing SiO 2 is consistent with varying proportions of calcium sulfates in the targets, see Figure 9B and [71]. The Mn-rich and Ca-sulfate components are independent, as shown in Figure 9B-all but a few points there are either Mn-rich and Ca-poor or Mn-poor and Ca-rich.…”

“…It does contain detectable quartz, a Fe-bearing carbonate mineral (siderite or ankerite or both), and a mineral with astrong diffraction at 11.15 • 2θ, or 9.22 Å (Figure 11A). It had been suggested that this last diffraction came from a Mn-rich phosphate mineral [86], but it is now interpreted as representing a mixed talc-serpentine phyllosilicate [21,36,71]. The Groken XRD pattern shows no indication of any Mn-or P-bearing crystalline phases, which suggested that the nodules' Mn and P were in the amorphous component.…”

“…Where thermochemical data for a phosphate mineral were not available, they were estimated using the ThermAp method [74,75], which has been shown to yield adequately accurate values for many Mn-bearing minerals [71]. Because we estimated phases other than apatites, we used the estimation parameters from [71]. Please refer to the Supplementary Materials for specific thermochemical data and calculation methods applied here.…”

“…The calculated solubility products for laueite and strunzite are essentially identical (they differ only in degree of hydration, Table 2). These phases included in the database ThermoddemV1.10_Groken.tdat (Supplementary Materials 2), augmented from that used by [71].…”