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.
Highlights
Bentorite, with the ideal formula Ca6 Cr2 (SO4 )3 (OH)12 ·26H2 O, was approved as a new mineral species by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA) in 1979
Bentorite occurs only under specific conditions, and it is very sensitive to temperature and humidity changes, e.g., for the synthesis of Cr substituted ettringite in the laboratory, it is essential to use the appropriate alkalinity and reagent concentration in the solution as reported by Wieczorek-Ciurowa et al [2]
We present the results of the first single crystal structure refinement of bentorite with 80% Cr3+ occupation at the octahedral site
Summary
With the ideal formula Ca6 Cr2 (SO4 ) (OH)12 ·26H2 O, was approved as a new mineral species by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA) in 1979. Bentorite occurs only under specific conditions, and it is very sensitive to temperature and humidity changes, e.g., for the synthesis of Cr substituted ettringite in the laboratory, it is essential to use the appropriate alkalinity (pH = 9.5–12.2) and reagent concentration in the solution (between 0.003 and 0.037 mol/dm of Cr3+ ) as reported by Wieczorek-Ciurowa et al [2]. This explains an extremely rare occurrence of bentorite in nature and difficulties in finding crystals with good quality and size for Minerals 2020, 10, 38; doi:10.3390/min10010038 www.mdpi.com/journal/minerals. XRD study reveals the onset of the decomposition of bentorite at 45 ◦ C and the formation of a dehydroxylation product similar to “metaettringite” [7]
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