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- Title
Raman Spectroscopy and Single-Crystal High-Temperature Investigations of Bentorite, Ca<sub>6</sub>Cr<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>(OH)<sub>12</sub>·26H<sub>2</sub>O.
- Authors
Juroszek, Rafał; Krüger, Biljana; Galuskina, Irina; Krüger, Hannes; Tribus, Martina; Kürsten, Christian
- Abstract
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-1 (ν1), 1109 cm-1 (ν3), 442 cm-1 (ν2), and 601 cm-1 (ν4). 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.
- Subjects
ISRAEL; RAMAN spectroscopy; SINGLE crystals; CHEMICAL formulas; QUARRIES &; quarrying; CRYSTAL structure
- Publication
Minerals (2075-163X), 2020, Vol 10, Issue 1, p1
- ISSN
2075-163X
- Publication type
Article
- DOI
10.3390/min10010038