Rotemite, Ca4Cr2(OH)12Cl2·4 H2O – the trigonal chromium analog of hydrocalumite and Friedel’s salt – a new mineral from the Hatrurim Complex, Israel

Abstract

Rotemite, Ca₄Cr₂(OH)₁₂Cl₂·4H₂O, is a newly discovered mineral and the first Cr-bearing member of the hydrocalumite group within the hydrotalcite supergroup. The new mineral is a trigonal chromium analog of both hydrocalumite and its synthetic counterpart, Friedel's salt, representing a large family of novel compounds of lamellar/layered double hydroxides (LDH). This work presents the first comprehensive study of a Ca, Cr-bearing compound within the LDH family. Rotemite was discovered in pyrometamorphic rocks of the Hatrurim Complex, Israel. It forms tiny hexagonal platelet-shaped crystals that appear pale bluish-violet (daylight) to pinkish-violet (artificial light), depending on the type of illumination. The streak is pale light-purple. Optically, rotemite is negatively uniaxial with ω = 1.565(2), ε = 1.544(2) (λ = 589 nm). It has a hardness of 2.5–3 on the Mohs scale. The crystals exhibit perfect cleavage on {0001}. The electron microprobe analyses indicated the empirical formula of (Ca_3.94Sr_0.01)_∑3.95(Cr³⁺_1.47Al_0.52Fe³⁺_0.05)_∑2.04(OH)₁₂[Cl_1.68(OH)_0.32(SO₄)_0.01]_∑2.01·4H₂O.

The calculated density, based on the empirical formula and unit cell parameters obtained from single-crystal X-ray diffraction data, is 2.18 g/cm³. Single-crystal X-ray diffraction measurements have revealed that rotemite represents a 6-layered polytype with trigonal symmetry ($R\overline{3}c$) and unit cell parameters: a = 5.7944(2) Å, c = 46.69(4) Å, V = 1357.7(10) ų. The final structural model converged to R₁ = 0.0866. The structure consists of hydrocalumite-type layers [Ca₄(Cr,Al)₂(OH)₁₂(H₂O)₄]²⁺ with intercalated Cl⁻ ions. Raman spectra of rotemite are characterized by a prominent band at ∼525 cm⁻¹, typical of LDH compounds. Polarized Raman spectroscopy revealed that signals from OH stretching vibrations at 3441–3442 cm⁻¹ and 3609–3612 cm⁻¹ strongly depend on the orientation of the crystal relative to the polarization of the incident laser beam. This indicates both an ordered arrangement of water molecules and a perpendicular arrangement of OH bonds within the hydrocalumite layers, respectively. The origin of rotemite and other Cr³⁺-bearing minerals is discussed, along with the possible role of the [Cr(OH)₆]³⁻ anion in their formation.