Grguricite, CaCr2(CO3)2(OH)4.4H2O, a new alumohydrocalcite analogue

The occurrence and characterisation of a new member of the dundasite group are reported. Grguricite, ideally CaCr2(CO3)2(OH)4·4H2O, is the Cr analogue of alumohydrocalcite, CaAl2(CO3)2(OH)4·4H2O and occurs as lilac crusts of very fine-grained crystalline aggregates in the Pb-Ba-V mineralisation found at the Adeghoual Mine, Mibladen, Morocco (32°46'0"N, 4°37'59"W). The identification was based upon a close match with the powder X-ray diffraction data for alumohydrocalcite, the confirmation of anion components identified by Raman spectroscopy and the cation composition determined by electron-probe microanalysis. The empirical formula based upon 14 oxygen atoms per formula unit is Ca0.84Pb0.03Cr1.65Al0.39Mg0.02(CO3)2(OH)4·4H2O, with carbonate, hydroxyl and water contents set to those of the alumohydrocalcite stoichiometry. The fine-grained nature of the crystals (c. 0.5 µm × 0.1 µm × 5 µm) precluded a single-crystal X-ray study and both density and optical determinations. Grguricite is triclinic with space group P${\bar 1}$. Unit-cell parameters refined from the powder diffraction data are: a = 5.724(2), b = 6.5304(9), c = 14.646(4) Å, α = 81.682(1), β = 83.712(2), γ = 86.365(2)°, V = 537.8(2) Å3 and Z = 2. The five strongest peaks in the powder pattern are [dhkl, Å (I/Imax)(hkl)]: 6.222(100)(011), 3.227(87)(020), 6.454(63)(010), 2.883(58)(005, 023, 121) and 7.208(45)(002). The mineral is named after Australian geologist Ben Grguric.