Raman Spectroscopy and Single-Crystal High-Temperature Investigations of Bentorite, Ca6Cr2(SO4)3(OH)12.26H2O

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 di raction. 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)S2[(SO4)2.750(CO3)0.499]S3.249(OH)11.502 ~25.75H2O. The Raman spectra of bentorite from two di erent localities exhibit the presence of the main stretching and bending vibrations related to the sulfate group at 983 cm􀀀1 ( 1), 1109 cm􀀀1 ( 3), 442 cm􀀀1 ( 2), and 601 cm􀀀1 ( 4). Moreover, the presence of bands assigned to the symmetric Cr(OH)6 3􀀀 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.