Ronald C. Peterson

Crystal molds on Mars: Melting of a possible new mineral species to create Martian chaotic terrain

Images sent back by the Mars Exploration Rover Opportunity from the Meridiani Planum show sulfate-rich rocks containing plate-shaped voids with tapered edges that are interpreted as crystal molds formed after a late-stage evaporite mineral has been removed. Experimental studies of the MgSO4-H2O system at low temperatures reveal that the triclinic phase MgSO4·11H2O exhibits a crystal morphology that matches the shapes of these molds. MgSO4·11H2O melts incongruently above 2 °C to a mixture of 70% epsomite (MgSO4·7H2O) and 30% H2O by volume. When this occurs while crystals are encased in sediment, plate-shaped voids remain. The existence of ice, low surface temperatures, and the high sulfate content of surface rocks and soil on Mars makes MgSO4·11H2O a possible mineral species near the surface at high latitudes or elsewhere in the subsurface. If an evaporite layer contained a significant amount of this phase, incongruent melting would result in a rapid release of a large volume of water and could explain some of the landform features on Mars that are interpreted as outflow channels. MgSO4·11H2O would not survive a sample return mission unless extraordinary precautions were taken.

Jarosite -hydronium jarosite solid- solution series with full iron site occupancy: Mineralogy and crystal chemistry

Abstract Structural changes within the jarosite-hydronium jarosite solid-solution series, (K,H3O)Fe3(SO4)2(OH)6, were investigated by X-ray Rietveld analysis of powder diffraction data collected from synthetic samples. All previous studies of jarosite solid solution consisted of samples that were non-stoichiometric with respect to iron. In this study, stoichiometric samples in the series were synthesized under hydrothermal conditions at 140°C using starting materials of Fe2(SO4)3·5H2O + K2SO4 + H2O in hydrothermal conditions. End-member potassium jarosite was also synthesized under similar conditions from a stoichiometric mixture of FeCl3 + KCl + LiCl + Fe(SO4)3·5H2O + H2O. Crystals were initially zoned with potassium-rich cores and hydronium-rich rims. Samples were homogenized by grinding and re-heating in the reactant solution. One iron deficient sample was synthesized to determine the effect of non-stoichiometry. Substitution of H3O by K changes the unit-cell parameters in a linear fashion; c increases significantly and a decreases to a lesser degree. Unit-cell parameters from stoichiometric samples determined in this study are larger than synthetic samples analyzed in previous studies as a result of full iron occupancy. Potassium substitution in the alkali site (A site) mainly affects the AO2 bond length, which causes the Fe-O2 and Fe-O3 bonds to lengthen and shorten, respectively. As potassium substitutes into the structure, there is an overall increase in the c axis. Iron deficiency leads to a significant decrease in unit-cell volume (large in c, minor in a), which is caused by bond length Fe-O3, which is markedly shorter than stoichiometric samples with similar potassium occupancy. The synthetic samples are compared with natural samples of jarosite and hydronium jarosite collected from mine waste deposits in Rio Tinto, Huelva, Spain. The natural samples have close to full iron occupancy, resulting from high iron content in solution and correlate well to the synthetic samples. Samples were also analyzed using short-wave infrared spectroscopy (SWIR). It was found that there is a subtle difference in spectra between end-members hydronium jarosite and potassium jarosite that can be tracked across the solid-solution series

Crystal chemistry of the natrojarosite-jarosite and natrojarosite-hydronium jarosite solid-solution series: A synthetic study with full Fe site occupancy

Abstract Members of the natrojarosite-hydronium jarosite [(Na,H3O)Fe3(SO4)2(OH)6] and jarosite-natrojarosite [(K,Na)Fe3(SO4)2(OH)6] solid-solution series were synthesized and investigated by Rietveld analysis of X-ray powder diffraction data. The synthesized samples have full Fe occupancy, where in many previous studies there were significant vacancies in the B site. Well-defined trends can be seen in the unit-cell parameters across the solid-solution series in the synthetic samples. The majority of the samples in this study were directly synthesized under hydrothermal conditions at 140 °C. End-member natrojarosite was synthesized using a two-step method, where the initial sample was heated in a 1.0 m H2SO4-0.5Na2SO4 solution at 200 °C for 3 days, yielding a sample with 100% Na occupancy. Many of the samples were initially zoned and required grinding and re-heating in the reactant solution for homogenization. Substitution of H3O and K into natrojarosite changes unit-cell parameters in a linear fashion. The unit-cell parameters presented here are significantly different than the majority of previous studies on synthetic samples, as samples in the current study have full Fe occupancy and the Na-K jarosite series has no H3O substitution in the A site. Substitution in the A site mainly affects unit-cell parameter c with little change in a. As Na occupancy increases there is a decrease in A-O2 and A-O3 distances and a consequent increase in Fe-O2 and Fe-O3 distance leading to an overall decrease in unit-cell parameter c in both the Na-H3O and Na-K jarosite series. The synthetic samples are compared to natural samples from mine waste deposits in Rio Tinto (Huelva, Spain), Ely Mine (Vermont), and a mineral collecting locality near Sharbot Lake (Ontario), as well as natural and synthetic samples documented in the literature. Based on unit-cell parameters many of the natural samples appear to have full Fe occupancy and correlate well with the synthetic samples from this study. The infrared spectra of the samples were analyzed, and there is a gradual change in the spectral features across the solid-solution series between end-members. The results from this study will aid in the interpretation of the possible chemical compositions of natural jarosite group minerals in mine waste and on Mars.

Meridianiite: A new mineral species observed on Earth and predicted to exist on Mars

Abstract Meridianiite, MgSO4·11H2O, was recently discovered on the surface of a frozen pond in central British Columbia, Canada. Meridianiite is stable below 2 °C. Above 2 °C, it melts incongruently to a slurry of epsomite (MgSO4·7H2O) and water. Magnesium sulfate minerals are thought to exist in the soils at Gusev crater and elsewhere on the Martian surface. These minerals can form by precipitation from a saturated solution or through evaporation of a brine solution at or below the surface. Meridianiite, instead of epsomite, is the expected magnesium sulfate phase in equilibrium with saturated brines below 2 °C on or below the Martian surface. It is expected to be the magnesium sulfate mineral in equilibrium with ice in the Martian polar ice caps. Meridianiite, if exposed to low humidity conditions in equatorial regions of Mars, would ultimately dehydrate to a fine dust of kieserite (MgSO4·H2O) that could be dispersed by wind across the Martian surface. The name, meridianiite, was chosen to reflect the locality on Mars where the MER rover Opportunity observed crystal molds in sedimentary rock that are thought to be caused by minerals that have since dehydrated or dissolved.

The crystal structure of ammoniojarosite, (NH4)Fe3(SO4)2(OH)6 and the crystal chemistry of the ammoniojarosite–hydronium jarosite solid-solution series

Abstract The atomic structure of ammoniojarosite,[(NH4)Fe3(SO4)2(OH)6], a = 7.3177(3) Å, c = 17.534(1) Å, space group R3̅m, Z = 3,has been solved using single-crystal X-ray diffraction (XRD) to wR 3.64% and R 1.4%. The atomic coordinates of the hydrogen atoms of the NH4 group were located and it was found that the ammonium group has two different orientations with equal probability. Hydronium commonly substitutes into jarosite group mineral structures and samples in the ammoniojarosite -hydronium jarosite solid-solution series were synthesized and analysed using powder XRD and Rietveld refinement. Changes in unit-cell dimensions and bond lengths are noted across the solidsolution series. The end-member ammoniojarosite synthesized in this study has no hydronium substitution in the A site and the unit-cell dimensions determined have a smaller a dimension and larger c dimension than previous studies. Two natural ammoniojarosite samples were analysed and shown to have similar unit-cell dimensions to the synthetic samples. Short-wave infrared and Fourier transform infrared spectra were collected for samples from the NH4-H3O jarosite solid-solution series and the differences between the end-members were significant. Both are useful tools for determining NH4 content in jarosite group minerals.

Cranswickite MgSO4·4H2O, a new mineral from Calingasta, Argentina

Abstract Cranswickite is a newly recognized mineral of composition MgSO4·4H2O from Calingasta, San Juan Province, Argentina (IMA2010-016). Cranswickite is monoclinic, space group C2/c, a = 11.9236(3), b = 5.1736(1), c = 12.1958(3) Å, β = 117.548(2)°, V= 667.0(1) Å3, Z = 4, dobs = 1.917 g/cm3, and dcalc = 1.918 g/cm3. The mineral occurs0 as a soft white vein filling in a metasedimentary rock. The atomic structure has been determined by direct methods and refined by Rietveld analysis of powder diffraction data. The atomic structure consists of chains of corner-sharing magnesium-containing octahedra and sulfate tetrahedra similar to the structure of pentahydrite. All the water molecules directly coordinate magnesium in the structure. The five strongest lines in the powder X-ray diffraction data are [dobs in angstroms (I) (hkl)]: 5.259 (100) (200), 3.927 (46) (112̄), 3.168 (45) (113̄), 4.603 (29) (111̄), 2.570 (23) (311). Infrared and Raman spectra are very similar to the spectra measured from starkeyite. The chemical composition of cranswickite is the same as starkeyite MgSO4·4H2O, but starkeyite has an atomic structure where two sulfate tetrahedra and two Mg(H2O)6 octahedra share corners to form a four-membered ring and not a chain as in cranswickite. The new mineral is named in honor of Lachlan M.D. Cranswick (1968-2010), an Australian crystallographer who helped to developed and maintain the Collaborative Computational Project No. 14 in Powder and Small Molecule Single Crystal Diffraction (CCP14).

Alpersite (Mg,Cu)SO4·7H2O, a new mineral of the melanterite group, and cuprian pentahydrite: Their occurrence within mine waste

Abstract Alpersite, Mg0.58Cu0.37Zn0.02Mn0.02Fe0.01SO4·7H2O, a new mineral species with direct relevance to reactions in mine waste, occurs in a mineralogically zoned assemblage in sheltered areas at the abandoned Big Mike mine in central Nevada at a relative humidity of 65% and T = 4 °C. Blue alpersite, which is isostructural with melanterite (FeSO4·7H2O), is overlain by a light blue to white layer dominated by pickeringite, alunogen, and epsomite. X-ray diffraction data (MoKα radiation) from a single crystal of alpersite were refined in P21/c, resulting in wR = 0.05 and cell dimensions α = 14.166(4), b = 6.534(2), c = 10.838(3) Å, β = 105.922(6)°, Z = 4. Site-occupancy refinement, constrained to be consistent with the compositional data, showed Mg to occupy the M1 site and Cu the M2 site. The octahedral distortion of M2 is consistent with 72% Cu occupancy when compared with the site-distortion data of substituted melanterite. Cuprian pentahydrite, with the formula (Mg0.49Cu0.41Mn0.08Zn0.02)SO4·5H2O, was collected from an efflorescent rim on a depression that had held water in a large waste-rock area near Miami, Arizona. After dissolution of the efflorescence in de-ionized water, and evaporation of the supernatant liquid, alpersite precipitated and quickly dehydrated to cuprian pentahydrite. These observations are consistent with previous experimental studies of the system MgSO4-CuSO4-H2O. It is suspected that alpersite and cuprian pentahydrite are widespread in mine wastes that contain Cu-bearing sulfides, but in which solubilized Fe2+ is not available for melanterite crystallization because of oxidation to Fe3+ in surface waters of near-neutral pH. Alpersite has likely been overlooked in the past because of the close similarity of its physical properties to those of melanterite and chalcanthite. Alpersite is named after Charles N. Alpers, geochemist with the United States Geological Survey, who has made significant contributions to our understanding of the mineralogical controls of mine-water geochemistry.

Combined neutron powder and X-ray single-crystal diffraction refinement of the atomic structure and hydrogen bonding of goslarite (ZnSO4·7H2O)

Abstract The atomic structure of synthetic, deuterated goslarite (ZnSO4·7D2O), a = 11.8176(6) Å, b = 12.0755(7) Å, c = 6.8270(4) Å, space group P212121, Z = 4, has been refined in a combined neutron powder diffraction and X-ray single-crystal data refinement to wRp 1.92%, Rp 1.45% and R(F2) 12.66% for the neutron powder data contribution and R(F2) 8.72% for the X-ray single-crystal data contribution. Both data sets were necessary to achieve the best overall fit agreement in the Rietveld refinement and reasonable geometry within structural units. The results of this study confirm that the H-bonding scheme for goslarite is the same as that of the other epsomite group minerals. Small but significant variations of the Zn-O bond lengths can be attributed to details of the H bonds to the O atoms of the Zn octahedra. This investigation of the atomic structure and hydrogen bonding of goslarite is groundwork for future studies into phase relationships and the mechanisms of hydration and dehydration in the ZnSO4-H2O system.

The crystal structure and hydrogen bonding of synthetic konyaite, Na2Mg(SO4)2·5H2O

Abstract The crystal structure of synthetic konyaite, Na2Mg(SO4)2·5H2O, a = 5.7690(8), b = 23.951(3), c = 8.0460(11) Å, β = 95.425(2)°, V = 1106.8(3) Å3, space group P21/c, Z = 4, was solved using singlecrystal X-ray diffraction. Hydrogen atom positions were determined and the structure solution was refined to R1 = 3.31% and wR2 = 6.28% for the 2167 measured independent reflections. Three distinct cation sites host the Mg and Na atoms in distorted octahedra and eight-coordinated polyhedra. The coordination polyhedra share edges to form compact sheets oriented perpendicular to b and linked to one another by hydrogen bonds. This results in a {010} tabular habit. A comparison of this structure is made to that of blödite [Na2Mg(SO4)2·4H2O], the dehydration product of konyaite. Konyaite is discussed within the context of the Na2O-MgO-SO4-H2O system. This study is part of ongoing investigations into the dehydration mechanisms and phase stability of this system.

Sodium magnesium sulfate decahydrate, Na2Mg(SO4)2·10H2O, a new sulfate salt

The structure of synthetic disodium magnesium disulfate decahydrate at 180 K consists of alternating layers of water-coordinated [Mg(H(2)O)(6)](2+) octahedra and [Na(2)(SO(4))(2)(H(2)O)(4)](2-) sheets, parallel to [100]. The [Mg(H(2)O)(6)](2+) octahedra are joined to one another by a single hydrogen bond, the other hydrogen bonds being involved in inter-layer linkage. The Mg(2+) cation occupies a crystallographic inversion centre. The sodium-sulfate sheets consist of chains of water-sharing [Na(H(2)O)(6)](+) octahedra along b, which are then connected by sulfate tetrahedra through corner-sharing. The associated hydrogen bonds are the result of water-sulfate interactions within the sheets themselves. This is believed to be the first structure of a mixed monovalent/divalent cation sulfate decahydrate salt.