Natural bentorite—Cr3+ derivate of ettringite: determination of crystal structure

  • Yurii V. Seryotkin
  • Ella V. Sokol
  • Svetlana N. KokhEmail author
  • Victor V. Sharygin
Original Paper


Bentorite, a Cr3+-substituted analogue of ettringite (Ca6Cr2(SO4)3(OH)12·26H2O), has been found as veinlet filling at its holotype locality in the Hatrurim Basin (Negev Desert). Quite abundant massive bentorite with Cr/(Cr + Al) ratios up to 0.95–0.99 coexists with sporadic portlandite, gypsum, afwillite, truscottite, fluorapatite, tobermorite, calcite, and vaterite. It occurs as almost monomineralic clots composed of fibrous grains. The crystal structure of bentorite has been determined and refined for the first time by XRD powder diffraction coupled with high-accuracy synchrotron low-temperature measurements at 100, 250 K and 295 K. Bentorite is isostructural with ettringite, space group P31c. The analysed bentorite sample has a trigonal unit cell with the parameters a = 11.16150(5), c = 21.63017(19) Å, V = 2333.650(19) Å3 at 100 K; a = 11.17790(5), c = 21.7375(2) Å, and V = 2352.12(2) Å3 at 250 K, and a = 11.2110(2), c = 21.7654(7) Å, and V = 2369.10(7) Å3 at 295 K. The bentorite structure includes only (SO4)2−-groups, three anions per formula unit, as in ettringite. Unlike ettringite, bentorite has Cr3+ instead of Al3+ cations, this being the only difference in chemistry. The obtained structure confirms the existence of a continuous series of ettringite \(\normalsize {\text{C}}{{\text{a}}_6}{\text{Al}}_{2}^{{\text{M}}}\left[ {{\text{S}}{{\text{O}}_4}} \right]_{3}^{\text{R}}{({\text{OH}})_{12}} \cdot 26{{\text{H}}_{\text{2}}}{\text{O}}\)–bentorite \({\text{C}}{{\text{a}}_6}{\text{Cr}}_{2}^{{\text{M}}}\left[ {{\text{S}}{{\text{O}}_4}} \right]_{3}^{{\text{R}}}{({\text{OH}})_{12}} \cdot 26{{\text{H}}_{\text{2}}}{\text{O}}\) solid solutions over the whole range 0 ≤ Cr3+/(Al + Cr3+) ≤ 1.


Bentorite Crystal structure Bentorite–ettringite solid solutions Chromium 



The study was supported by the Russian Science Foundation, Grant 17-17-01056. The European Synchrotron Radiation Facility is acknowledged for the allocation of experimental beam time. The authors are indebted to Boris Zakharov and to Wilson Mogodi for technical support during the LT experiments.

Supplementary material

269_2019_1022_MOESM1_ESM.doc (67 kb)
Supplementary material 1 (DOC 67 KB)
269_2019_1022_MOESM2_ESM.doc (44 kb)
Supplementary material 2 (DOC 44 KB)


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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Sobolev Institute of Geology and MineralogySiberian Branch of the Russian Academy of SciencesNovosibirskRussia
  2. 2.Novosibirsk State UniversityNovosibirskRussia

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