Christopher T. Adcock¹, Elisabeth M. Hausrath¹, Paul M. Forster^2,3, Oliver Tschauner^1,3 and Kirellos J. Sefein¹

¹Department of Geoscience, University of Nevada Las Vegas, 4505 South Maryland Parkway, Las Vegas, Nevada 89154, U.S.A.
²Department of Chemistry, University of Nevada Las Vegas, 4505 South Maryland Parkway, Las Vegas, Nevada 89154, U.S.A.
³HiPSEC, University of Nevada Las Vegas, 4505 South Maryland Parkway, Las Vegas, Nevada 89154, U.S.A.

Merrillite [Ca₉NaMg(PO₄)₇] occurs as a dominant primary Ca-phosphate mineral in martian meteorites and therefore presumably also on Mars. The mineral is an important phase in exploring differences in geologic processes between Earth and Mars, and also has astrobiological implications due to its potential role as a significant source of the bio-essential nutrient phosphate. Merrillite does not occur terrestrially as a discrete mineral phase, making it difficult to obtain for Mars-relevant studies. It can, however, be synthesized from a similar terrestrial mineral, whitlockite (natural or synthetic), through dehydrogenation. Here we present methods for synthesizing relatively large quantities (0.5 g or greater per batch) of coarse crystalline (75 μm+) Mg-whitlockite, Fe-whitlockite, mixed Fe/Mg-whitlockites, and from these synthesized minerals produce Mg-merrillite, ferrous and ferric Fe-merrillite, and ferrous and ferric mixed Fe/Mg-merrillite. Chemistry and atomic structures of synthesized Fe- and mixed Fe/Mg-whitlockite and ferrous and ferric Fe- and mixed Fe/Mg- merrillite resulting from single-crystal X-ray diffraction, infrared spectroscopy, and electron microprobe analyses are presented. We also present a mechanism for maintaining charge balance during the formation of merrillite from whitlockite. Our results shed light on these mineral structures for future martian studies, and provide methods for creating coarse crystalline merrillite for use in Mars-relevant thermodynamic, kinetic, soil/dust simulant, crystallographic, astrobiological, and other studies.

Reference
Adcock CT, Hausrath EM, Forster PM, Tschauner O and Sefein KJ (2014) Synthesis and characterization of the Mars-relevant phosphate minerals Fe- and Mg-whitlockite and merrillite and a possible mechanism that maintains charge balance during whitlockite to merrillite transformation. American Mineralogist 99:1221.
[doi:10.2138/am.2014.4688]
Copyright: The Mineralogical Society of America

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Ema Bobocioiu and Razvan Caracas

Laboratoire de Géologie de Lyon (LGLTPE) CNRS UMR 5276, Université Claude Bernard Lyon 1, Ecole Normale Supérieure de Lyon 46, allée d’Italie, 69364 Lyon, France

We study hydrated magnesium sulfate minerals from first-principles calculations based on density-functional theory. We determine the heat of hydration for MgSO₄·nH₂O, compute the Raman and infrared spectra for several phases and calculate the S isotope partitioning as a function of hydration. We find that epsomite and meridianiite with, respectively, n = 7 andn = 11 water molecules per MgSO₄ unit are particularly stable with respect to other individual or combinations of hydration states. The Raman spectra of all phases present clear SO₄ features that are easily identifiable. We use this to show one can use the vibrational spectroscopic information as an identification tool in a remote environment, like the martian surface. We discuss the character and atomic displacement pattern of all vibration modes and compute the ³⁴S/³²S partitioning; this work shows that hydration favors enrichment in the lighter S isotope ³²S with respect to the heavier ³⁴S, which is accumulated in the less hydrous structures. We show for the first time that the signature of ³⁴S/³²S partitioning could be observed by in situ spectroscopy on the surface of Mars. Moreover this can be related to the diurnal cycle of hydration and dehydration and hence it can improve the modeling of the water circulation on Mars.

Reference
Bobocioiu E and Caracas R (2014) Stability and spectroscopy of Mg sulfate minerals: Role of hydration on sulfur isotope partitioning. American Mineralogist 99:1216.
[doi:10.2138/am.2014.4632]
Copyright: The Mineralogical Society of America

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Valerie M. Tu¹, Elisabeth M. Hausrath¹, Oliver Tschauner^1,2, Valentin Iota² and Gerald W. Egeland³

¹Department of Geoscience, University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Las Vegas, Nevada 89154, U.S.A.
²HiPSEC, University of Nevada Las Vegas, 4505 S. Maryland Parkway, Las Vegas, Nevada 89154, U.S.A.
³Department of Mechanical Engineering, University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Las Vegas, Nevada 89154, U.S.A.

Phosphate is an essential nutrient for life on Earth, and therefore if life exists or ever existed on Mars it may have required phosphate. Amorphous Al- and Fe-phosphates rapidly precipitate from acidic solutions and amorphous Al-phosphates likely control phosphate concentrations in some natural waters on Earth. The amorphous fraction of martian soils has also been shown to be enriched in P, and amorphous phosphates are therefore also likely important in the phosphate cycle on Mars. Despite this importance, however, few dissolution rates exist for amorphous Al- and Fe-phosphates. In this study, dissolution rates of amorphous Al- and Fe-phosphates were measured in flow-through reactors from steady state concentrations of Al, Fe, and P. A pH-dependent rate law, log R = log k – npH was determined from the dissolution rates, where R is the dissolution rate, k is the intrinsic rate constant, and n is the reaction order with respect to H+. For amorphous Al-phosphate, log k = −6.539 ± 1.529 and n = 2.391 ± 0.493. For amorphous Fe-phosphate, log k= −13.031 ± 0.558 and n = 1.376 ± 0.221. The amorphous Al-phosphate dissolves stoichiometrically under all experimental conditions measured, and the amorphous Fe-phosphate dissolves non-stoichiometrically, approaching stoichiometric dissolution as pH decreases, due potentially to Fe oxyhydroxides precipitating and armoring grain surfaces. Perhaps due to these effects, amorphous Al-phosphate dissolution rates are approximately three orders of magnitude faster than the amorphous Fe-phosphate dissolution rates measured under these experimental conditions. Amorphous Al-phosphate dissolution rates measured in this study are also faster than published dissolution rates for the crystalline Al-phosphate variscite. The rapid dissolution rates measured in this study therefore suggest that, if these phases are present on Mars, phosphate would be rapidly released into acidic environments.

Reference
Tu VM, Hausrath EM, Tschauner O, Iota V and Egeland GW (2014) Dissolution rates of amorphous Al- and Fe-phosphates and their relevance to phosphate mobility on Mars. American Mineralogist 99:1206.
[doi:10.2138/am.2014.4613]
Copyright: The Mineralogical Society of America

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Pablo Sobron^1,2,3, Janice L. Bishop^1,3, David F. Blake³, Bin Chen³ and Fernando Rull⁴

¹SETI Institute, 189 Bernardo Avenue, Mountain View, California 94043, U.S.A.
²MalaUva Labs, 822 Allen Avenue, St. Louis, Missouri 63104, U.S.A.
³NASA Ames Research Center, Moffett Field, California 94035, U.S.A.
⁴Unidad Asociada UVA-Centro de Astrobiología, Edificio INDITI, Av.Francisco Valles 8, Parque Tecnologico de Boecillo, Parcela 203, Boecillo 47151, Spain

We have characterized complex iron- and sulfate-bearing samples from Rio Tinto (Spain) using X-ray diffraction (XRD), visible-near infrared reflectance (VNIR) spectroscopy, and laser Raman spectroscopy (LRS). Samples were collected for this study from the Peña de Hierro region of Rio Tinto because this site represents a natural acidic environment that is a potential analog for such environments on Mars. We report an evaluation of the capabilities of these three techniques in performing detailed mineralogical characterization of potential Mars-like samples from a natural acidic terrestrial environment. Sulfate minerals found in these samples include gypsum, jarosite, and copiapite, and iron hydroxide bearing minerals found include goethite and ferrihydrite. These sulfate and iron hydroxide/oxyhydroxide minerals were detected by XRD, VNIR, and LRS. Minor quartz was identified in some samples by XRD as well, but was not identified using VNIR spectroscopy. Coordinating the results from these three techniques provides a complete picture of the mineralogical composition of the samples. Field instruments were used for this study to mimic the kinds of analyses that could be performed in the field or on martian rovers.

Reference
Sobron P, Bishop JL, Blake DF, Chen B and Rull F (2014) Natural Fe-bearing oxides and sulfates from the Rio Tinto Mars analog site: Critical assessment of VNIR reflectance spectroscopy, laser Raman spectroscopy, and XRD as mineral identification tools. American Mineralogist 99:1199.
[doi:10.2138/am.2014.4595]
Copyright: The Mineralogical Society of America

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