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

Rotemite, Ca 4 Cr 2 (OH) 12 Cl 2 ·4H 2 O, 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 3.94 Sr 0.01 ) ∑3.95 (Cr 3+ 1.47 Al 0.52 Fe 3+ 0.05 ) ∑2.04 (OH) 12 [Cl 1.68 (OH) 0.32 (SO 4 ) 0.01 ] ∑2.01 ·4H 2 O. The calculated density, based on the empirical formula and unit cell parameters obtained from single-crystal X-ray diffraction data, is 2.18 g/cm 3 . Single-crystal X-ray diffraction measurements have revealed that rotemite represents a 6-layered polytype with trigonal symmetry ( R 3 ¯ c ) and unit cell parameters: a = 5.7944(2) Å, c = 46.69(4) Å, V = 1357.7(10) Å 3 . The final structural model converged to R 1 = 0.0866. The structure consists of hydrocalumite-type layers [Ca 4 (Cr,Al) 2 (OH) 12 (H 2 O) 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 O H 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 O H bonds within the hydrocalumite layers, respectively. The origin of rotemite and other Cr 3+ -bearing minerals is discussed, along with the possible role of the [Cr(OH) 6 ] 3− anion in their formation. • A new mineral, rotemite, Ca₄Cr₂(OH)₁₂Cl₂·4H₂O, is the first known Cr-analogue of hydrocalumite. • A multimethodological approach confirmed the substitution of Cr 3+ for Al 3+ in hydrocalumite-type layers. • The characteristic Raman band of LDH compounds at ∼525 cm −1 is analyzed in detail. • Polarized Raman spectra showed that OH stretching vibrations depends on orientation, indicating an ordered H₂O and OH aligment. • The origin of rotemite is discussed, emphasizing the possible role of Cr(OH)₆ 3− anions in its formation.