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

Abstract

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.