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Open AccessArticle

Raman Spectroscopy and Single-Crystal High-Temperature Investigations of Bentorite, Ca6Cr2(SO4)3(OH)12·26H2O

1
Institute of Earth Sciences, Faculty of Natural Sciences, University of Silesia, Będzińska 60, 41-205 Sosnowiec, Poland
2
Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(1), 38; https://doi.org/10.3390/min10010038
Received: 28 November 2019 / Revised: 25 December 2019 / Accepted: 27 December 2019 / Published: 30 December 2019
(This article belongs to the Section Crystallography and Physical Chemistry of Minerals)
The crystal structure of bentorite, ideally Ca6Cr2(SO4)3(OH)12·26H2O, a Cr3+ analogue of ettringite, is for the first time investigated using X-ray single crystal diffraction. Bentorite crystals of suitable quality were found in the Arad Stone Quarry within the pyrometamorphic rock of the Hatrurim Complex (Mottled Zone). The preliminary semi-quantitative data on the bentorite composition obtained by SEM-EDS show that the average Cr/(Cr + Al) ratio of this sample is >0.8. Bentorite crystallizes in space group P31c, with a = b = 11.1927(5) Å, c =21.7121(10) Å, V = 2355.60(18) Å3, and Z = 2. The crystal structure is refined, including the hydrogen atom positions, to an agreement index R1 = 3.88%. The bentorite crystal chemical formula is Ca6(Cr1.613Al0.387)Σ2[(SO4)2.750(CO3)0.499]Σ3.249(OH)11.502·~25.75H2O. The Raman spectra of bentorite from two different localities exhibit the presence of the main stretching and bending vibrations related to the sulfate group at 983 cm−11), 1109 cm−13), 442 cm−12), and 601 cm−14). Moreover, the presence of bands assigned to the symmetric Cr(OH)63− stretching mode and hydroxyl deformation vibrations of Cr–OH units at ~540 cm−1 and ~757 cm−1, respectively, may be used to distinguish between ettringite and bentorite. In situ high temperature single crystal XRD experiments show that the decomposition of bentorite starts at ca. 45 °C and that a dehydroxylation product similar to metaettringite is formed. View Full-Text
Keywords: bentorite; ettringite group; SC-XRD; Raman spectroscopy; Hatrurim Complex; Israel bentorite; ettringite group; SC-XRD; Raman spectroscopy; Hatrurim Complex; Israel
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Juroszek, R.; Krüger, B.; Galuskina, I.; Krüger, H.; Tribus, M.; Kürsten, C. Raman Spectroscopy and Single-Crystal High-Temperature Investigations of Bentorite, Ca6Cr2(SO4)3(OH)12·26H2O. Minerals 2020, 10, 38.

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