Investigations on alunogen under Mars-relevant temperature conditions: An example for a single-crystal-to-single-crystal phase transition
-
Volker Kahlenberg
,
Doris E. Braun
Abstract
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 grouptheoretical 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.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
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- Trace element partitioning into sulfide: How lithophile elements become chalcophile and vice versa
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- Tracking the thermal decomposition of plasma-sprayed hydroxylapatite
- Experimental constraints on bubble formation and growth during magma ascent: A review
- Ankerite grains with dolomite cores: A diffusion chronometer for low- to medium-grade regionally metamorphosed clastic sediments
- Crystal structure, high-pressure, and high-temperature behavior of carbonates in the K2Mg(CO3)2–Na2Mg(CO3)2 join
- Temperature micro-mapping in oscillatory-zoned chlorite: Application to study of a green-schist facies fault zone in the Pyrenean Axial Zone (Spain)
- Micro- and nano-characterization of Zn-clays in nonsulfide supergene ores of southern Peru
- Mineralogical characterization of individual growth structures of Mn-nodules with different Ni+Cu content from the central Pacific Ocean
- The magma plumbing system of the Emeishan large igneous province and its role in basaltic magma differentiation in a continental setting
- Weathering of the Ethiopian volcanic province: A new weathering index to characterize and compare soils
- Recommended mineral-melt partition coefficients for FRTEs (Cu), Ga, and Ge during mantle melting
- Tetrahedral plot diagram: A geometrical solution for quaternary systems
- Investigations on alunogen under Mars-relevant temperature conditions: An example for a single-crystal-to-single-crystal phase transition
- Crystallization of calcium oxalate hydrates by interaction of calcite marble with fungus Aspergillus niger
- Direct and indirect evidence for free oxygen (O2–) in MO-silicate glasses and melts (M = Mg, Ca, Pb)
- Effect of fluid composition on growth rate of monazite in quartzite at 1.0 GPa and 1000 °C
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