¹Volker Kahlenberg, ²Doris E. Braun, ¹Maria Orlova
¹Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria
²Institute of Pharmacy, Pharmaceutical Technology, Innrain 52c, A-6020 Innsbruck, Austria

The low-temperature (LT) dependent behavior of a synthetic alunogen sample with composition Al2(SO4)3·16.61H2O has been studied in the overall temperature range from −100 to 23 °C by DSC measurements, in situ powder and single-crystal X-ray diffraction as well as Raman spectroscopy. Cooling/heating experiments using the different techniques prove that alunogen undergoes a reversible, sluggish phase transition somewhere between −30 and −50 °C from the triclinic room-temperature (RT) form to a previously unknown LT-polymorph. A significant hysteresis for the transition was observed with all three methods and the transition temperatures were found to depend on the employed cooling/heating rates. The crystal structure of the LT-modification has been studied at −100 °C using single crystals, which have been grown from an aqueous solution. Basic crystallographic data are as follows: monoclinic symmetry, space group type P21, a = 7.4125(3), b = 26.8337(16), c = 6.0775(3) Å, β = 97.312(4)°, V = 1199.01(10) Å3, and Z = 2. Structure analysis revealed that LT-alunogen corresponds to a non-stoichiometric hydrate with 16.61 water moieties pfu. Notably, the first-order transition results in a single-crystal-to-single-crystal transformation. In the asymmetric unit there are 2 Al-atoms, 3 [SO4]-tetrahedra, and 17 crystallographically independent sites for water molecules, whose hydrogen positions could be all located by difference-Fourier calculations. According to site-population refinements only one water position (Ow5) shows a partial occupancy. A comfortable way to rationalize the crystal structure of the LT-modification of alunogen is based on a subdivision of the whole structure into two different slabs parallel to (010). The first type of slab (type A) is about 9 Å thick and located at y ≈ 0 and y ≈ ½, respectively. It contains the Al(H2O)6-octahedra as well as the sulfate groups centered by S1 and S2. Type B at y ≈ ¼ and y ≈ ¾ comprises the remaining tetrahedra about S3 and a total of five additional “zeolitic” water sites (Ow1–Ow5), which are not a part of a coordination polyhedron. Within slab-type A alternating chains of (unconnected) octahedra and tetrahedra can be identified, which are running parallel to [100]. In addition to electrostatic interactions between the Al(H2O)63+- and the (SO4)2−-units, hydrogen bonds are also essential for the stability of these slabs. A detailed comparison between both modifications including a derivation from a hypothetical aristotype based on group-theoretical concepts is presented. Since alunogen has been postulated to occur in martian soils the new findings may help in the identification of the LT-form by X-ray diffraction using the Curiosity Rover’s ChemMin instrument or by Raman spectroscopy.

Reference
Kahlenberg V, Braun DE, Orlova M (2015) Investigations on alunogen under Mars-relevant temperature conditions: An example for a single-crystal-to-single-crystal phase Transition. American Mineralogist 100, 2548-2558
Link to Article [doi: 10.2138/am-2015-5342]

Copyright: The Mineralogical Society of America

¹Wei, J., ¹Wang, A., ²Lambert, J.L., ³Wettergreen, D., ⁴Cabrol, N., ⁴Warren-Rhodes, K., ⁵Zacny, K.
¹Department of Earth and Planetary Sciences, McDonnell Center for the Space Sciences, Washington University in St. Louis, 1 Brookings Drive, St. Louis, MO, United States
²Jet Propulsion Laboratory, 4800 Oak Grove Drive, CA, United States
³Robotics Institute, Carnegie Mellon University USA, 5000 Forbes Avenue, Pittsburgh, PA, United States
⁴SETI Institute, Carl Sagan Center, NASA Ames Research Center, Moffett Field, CA, United States
⁵HoneyBee Robotics and Spacecraft Mechanisms Corporation, 398 West Washington Blvd, Suite 200, Pasadena, CA, United States

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Reference
Wei J, Wang A, Lambert JL, Wettergreen D, Cabrol N, Warren-Rhodes K, Zacny K (2015) Autonomous soil analysis by the Mars Micro-beam Raman Spectrometer (MMRS) on-board a rover in the Atacama Desert: A terrestrial test for planetary Exploration. Journal of Raman Spectroscopy 46, 810-821
Link to Article [DOI: 10.1002/jrs.4656]

^1,2,3Jangmi Han, ¹Adrian J. Brearley,³Lindsay P. Keller
¹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA
²USRA Lunar and Planetary Institute, Houston, Texas, USA
³NASA Johnson Space Center, Houston, Texas, USA

Two hibonite-spinel inclusions (CAIs 03 and 08) in the ALHA77307 CO3.0 chondrite have been characterized in detail using the focused ion beam sample preparation technique combined with transmission electron microscopy. These hibonite-spinel inclusions are irregularly shaped and porous objects and consist of randomly oriented hibonite laths enclosed by aggregates of spinel with fine-grained perovskite inclusions finally surrounded by a partial rim of diopside. Melilite is an extremely rare phase in this type of CAI and occurs only in one inclusion (CAI 03) as interstitial grains between hibonite laths and on the exterior of the inclusion. The overall petrologic and mineralogical observations suggest that the hibonite-spinel inclusions represent high-temperature condensates from a cooling nebular gas. The textural relationships indicate that hibonite is the first phase to condense, followed by perovskite, spinel, and diopside. Texturally, melilite condensation appears to have occurred after spinel, suggesting that the condensation conditions were far from equilibrium. The crystallographic orientation relationships between hibonite and spinel provide evidence of epitaxial nucleation and growth of spinel on hibonite surfaces, which may have lowered the activation energy for spinel nucleation compared with that of melilite and consequently inhibited melilite condensation. Hibonite contains abundant stacking defects along the (001) plane consisting of different ratios of the spinel and Ca-containing blocks within the ideal hexagonal hibonite structure. This modification of the stacking sequence is likely the result of accommodation of excess Al in the gas into hibonite due to incomplete condensation of corundum from a cooling gas under disequilibrium conditions. We therefore conclude that these two hibonite-spinel inclusions in ALHA77307 formed by high-temperature condensation under disequilibrium conditions.

Reference
Han J, Brearley AJ, Keller LP (2015) Microstructural evidence for a disequilibrium condensation origin for hibonite-spinel inclusions in the ALHA77307 CO3.0 chondrite. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12563]

Published by arrangement with John Wiley & Sons

¹Brennecka, G. A., ¹Budde, G.,¹Kleine, T.
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster, Germany

A handful of events, such as the condensation of refractory inclusions and the formation of chondrules, represent important stages in the formation and evolution of the early solar system and thus are critical to understanding its development. Compared to the refractory inclusions, chondrules appear to have a protracted period of formation that spans millions of years. As such, understanding chondrule formation requires a catalog of reliable ages, free from as many assumptions as possible. The Pb-Pb chronometer has this potential; however, because common individual chondrules have extremely low uranium contents, obtaining U-corrected Pb-Pb ages of individual chondrules is unrealistic in the vast majority of cases at this time. Thus, in order to obtain the most accurate 238U/235U ratio possible for chondrules, we separated and pooled thousands of individual chondrules from the Allende meteorite. In this work, we demonstrate that no discernible differences exist in the 238U/235U compositions between chondrule groups when separated by size and magnetic susceptibility, suggesting that no systematic U-isotope variation exists between groups of chondrules. Consequently, chondrules are likely to have a common 238U/235U ratio for any given meteorite. A weighted average of the six groups of chondrule separates from Allende results in a 238U/235U ratio of 137.786 ± 0.004 (±0.016 including propagated uncertainty on the U standard [Richter et al. 2010]). Although it is still possible that individual chondrules have significant U isotope variation within a given meteorite, this value represents our best estimate of the 238U/235U ratio for Allende chondrules and should be used for absolute dating of these objects, unless such chondrules can be measured individually.

Reference
Brennecka GA, Budde G, Kleine T (2015) Uranium isotopic composition and absolute ages of Allende chondrules. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12567]
Published by arrangement with John Wiley & Sons

¹Birlan, M et al. (>10)*
¹Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE), Observatoire de Paris, CNRS UMR8028, 77 avenue Denfert-Rochereau, 75014 Paris Cedex, France
*Find the extensive, full author and affiliation list on the publishers website

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Reference
Birlan M et al. (2015) Characterization of (357439) 2004 BL86 on its close approach to Earth in 2015. Astronomy & Astrophysics 581, A3
Link to Article [http://dx.doi.org/10.1051/0004-6361/201526460]

¹Harry Y. McSween Jr.
¹Department of Earth and Planetary Sciences and Planetary Geoscience Institute, University of Tennessee, Knoxville, Tennessee 37996-1410, U.S.A.

Petrologic investigations of martian rocks have been accomplished by mineralogical, geochemical, and textural analyses from Mars rovers (with geologic context provided by orbiters), and by laboratory analyses of martian meteorites. Igneous rocks are primarily lavas and volcaniclastic rocks of basaltic composition, and ultramafic cumulates; alkaline rocks are common in ancient terranes and tholeiitic rocks occur in younger terranes, suggesting global magmatic evolution. Relatively uncommon feldspathic rocks represent the ultimate fractionation products, and granitic rocks are unknown. Sedimentary rocks are of both clastic (mudstone, sandstone, conglomerate, all containing significant igneous detritus) and chemical (evaporitic sulfate and less common carbonate) origin. High-silica sediments formed by hydrothermal activity. Sediments on Mars formed from different protoliths and were weathered under different environmental conditions from terrestrial sediments. Metamorphic rocks have only been inferred from orbital remote-sensing measurements. Metabasalt and serpentinite have mineral assemblages consistent with those predicted from low-pressure phase equilibria and likely formed in geothermal systems. Shock effects are common in martian meteorites, and impact breccias are probably widespread in the planet’s crustal rocks. The martian rock cycle during early periods was similar in many respects to that of Earth. However, without plate tectonics Mars did not experience the thermal metamorphism and flux melting associated with subduction, nor deposition in subsided basins and rapid erosion resulting from tectonic uplift. The rock cycle during more recent time has been truncated by desiccation of the planet’s surface and a lower geothermal gradient in its interior. The petrology of Mars is intriguingly different from Earth, but the tried-and-true methods of petrography and geochemistry are clearly translatable to another world.

Reference
McSween Jr. HY (2015) Petrology on Mars. American Mineralogist 100, 2380-2395
Link to Article [doi: 10.2138/am-2015-5257]

Copyright: The Mineralogical Society of America

¹Maria Eugenia Varela,²Ernst Zinner
¹Instituto de Ciencias Astronómicas de la Tierra y del Espacio (ICATE), San Juan, Argentina
²Laboratory for Space Sciences and the Physics Department, Washington University, St. Louis, Missouri, USA

Glass-bearing inclusions hosted by different mineral phases in SNC meteorites provide important information on the conditions that prevailed during formation of early phases and/or on the composition of the primary trapped liquids/melts of these rocks. Although extensive previous work has been reported on such inclusions, several questions are still unresolved. We performed a chemical and petrographic study of the constituents (glasses and mineral assemblage) of glassy and multiphase inclusions in Shergotty and Chassigny. We focused on obtaining accurate trace element contents of glasses and co-existing minerals and discussing their highly variable REE contents. Our results reveal an unusual geochemistry of trace element contents that appear to be independent of their major element compositions. Chemical equilibrium between phases inside inclusions as well as between glasses and host minerals could not be established. The LREE contents of glasses in glass inclusions can vary by up to two orders of magnitude. The depletion in trace element abundances shown by glasses seem to be inconsistent with these phases being residual melts. The light lithophile element contents of glasses are highly variable with enrichment in incompatible elements (e.g., Be, Sr, Ba, and LREE) indicating some processes involving percolation of fluids. All of these features are incompatible with glass-bearing inclusions in the host minerals acting as closed systems preserving unmodified primary liquids/melts. Glass-bearing inclusions in Shergotty and Chassigny appear to have been altered (as was the rock itself) by different postformational processes (e.g., shock, metamorphism, metasomatic [?] fluids) that affected these meteorites with different degree of intensity. Our results indicate that these inclusions could not preserve a reliable sample of the primary trapped melt.

Reference
Varela ME, Zinner E (2015) Glass-bearing inclusions in Shergotty and Chassigny: Consistent samples of a primary trapped melt? Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12566]
Publsihed by arrangement with John Wiley & Sons

¹Julie E. M. McGeoch,²Malcolm W. McGeoch

¹Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts, USA
²PLEX LLC, Fall River, Massachusetts, USA

It has been proposed that exothermic gas phase polymerization of amino acids can occur in the conditions of a warm dense molecular cloud to form hydrophobic polymer amide (HPA) (McGeoch and McGeoch 2014). In a search for evidence of this presolar chemistry Allende and Murchison meteorites and a volcano control were diamond burr-etched and Folch extracted for potential HPA yielding 85 unique peaks in the meteorite samples via matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI TOF/MS). The amino acids after acid hydrolysis in Allende were below the level of detection but many of the Allende peaks via the more sensitive MALDI/TOF analysis could be fitted to a polymer combination of glycine, alanine, and alpha-hydroxyglycine with high statistical significance. A similar significant fit using these three amino acids could not be applied to the Murchison data indicating more complex polymer chemistry.

Reference
McGeoch JEM, McGeoch MW (2015) Polymer amide in the Allende and Murchison meteorites. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12558]

Published by arrangement with John Wiley & Sons

^1,2Penelope J. Wozniakiewicz, ³Hope A. Ishii, ^1,2Anton T. Kearsley, ³John P. Bradley, ¹Mark. C. Price, ¹Mark J. Burchell, ⁴Nick Teslich,¹Mike J. Cole
¹School of Physical Science, Centre for Astrophysics and Planetary Sciences, University of Kent, Canterbury, UK
²Department of Earth Sciences, Impacts & Astromaterials Research Centre (IARC), Natural History Museum, London, UK
³Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mānoa, Honolulu, Hawai’i, USA
⁴Lawrence Livermore National Laboratory, Livermore, California, USA

Comet 81P/Wild 2 samples returned by NASA’s Stardust mission provide an unequalled opportunity to study the contents of, and hence conditions and processes operating on, comets. They can potentially validate contentious interpretations of cometary infrared spectra and in situ mass spectrometry data: specifically the identification of phyllosilicates and carbonates. However, Wild 2 dust was collected via impact into capture media at ~6 km s−1, leading to uncertainty as to whether these minerals were captured intact, and, if subjected to alteration, whether they remain recognizable. We simulated Stardust Al foil capture conditions using a two-stage light-gas gun, and directly compared transmission electron microscope analyses of pre- and postimpact samples to investigate survivability of lizardite and cronstedtite (phyllosilicates) and calcite (carbonate). We find the phyllosilicates do not survive impact as intact crystalline materials but as moderately to highly vesiculated amorphous residues lining resultant impact craters, whose bulk cation to Si ratios remain close to that of the impacting grain. Closer inspection reveals variation in these elements on a submicron scale, where impact-induced melting accompanied by reducing conditions (due to the production of oxygen scavenging molten Al from the target foils) has resulted in the production of native silicon and Fe- and Fe-Si-rich phases. In contrast, large areas of crystalline calcite are preserved within the calcite residue, with smaller regions of vesiculated, Al-bearing calcic glass. Unambiguous identification of calcite impactors on Stardust Al foil is therefore possible, while phyllosilicate impactors may be inferred from vesiculated residues with appropriate bulk cation to Si ratios. Finally, we demonstrate that the characteristic textures and elemental distributions identifying phyllosilicates and carbonates by transmission electron microscopy can also be observed by state-of-the-art scanning electron microscopy providing rapid, nondestructive initial mineral identifications in Stardust residues.

Reference
Wozniakiewicz PJ, Ishii HA, Kearsley AT, Bradley JP, Price MC, Burchell MJ, Teslich N, Cole MJ (2015) The survivability of phyllosilicates and carbonates impacting Stardust Al foils: Facilitating the search for cometary water. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12568]

Published by arrangement with John Wiley & Sons

¹Ebbe N. Bak, ²Svend J. Knak Jensen, ¹Per Nørnberg, ^1,3Kai Finster
¹Department of Bioscience, Aarhus University, Ny Munkegade 116, Building 1540, 8000 Aarhus C, Denmark
²Department of Chemistry, Aarhus University, Langelandsgade 140, Building 1511, 8000 Aarhus C, Denmark
³Stellar Astrophysics Center, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, Building 1520, 8000 Aarhus C, Denmark

The only organic compounds that have been detected in the Martian soil are simple chlorinated compounds released from heated surface material. However, the sources of the organic carbon are in dispute. Wind abraded silicates, which are widespread on the Martian surface, can sequester atmospheric methane which generates methylated silicates and thus could provide a mechanism for accumulation of reduced carbon in the surface soil. In this study we show that thermal volatilization of methylated silicates in the presence of perchlorate leads to the production of chlorinated methane. Thus, methylated silicates could be a source of the organic carbon released as chlorinated methane upon thermal volatilization of Martian soil samples. Further, our experiments show that the ratio of the different chlorinated compounds produced is dependent on the mass ratio of perchlorate to organic carbon in the soil.

Reference
Bak EN, Knak Jensen SJ, Nørnberg P, Finster K (2015) Methylated silicates may explain the release of chlorinated methane from Martian soil. Earth & Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2015.10.044]
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