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

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

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