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

DOI: 10.1180/mgm.2020.66 DOI Help

Authors: M. S. Rumsey (Natural History Museum) , M. D. Welch (Natural History Museum) , J. Spratt (Natural History Museum) , A. K. Kleppe (Diamond Light Source)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Mineralogical Magazine

State: Published (Approved)
Published: September 2020

Abstract: The occurrence and characterization 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 mineralization 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 X-ray powder 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 x 0.1 x 5 μm) precluded a single-crystal X-ray study and both density and optical determinations. Grguricite is triclinic with space group P1 ̄. Unit-cell parameters refined from the powder diffraction data are: a = 5.724(2) Å, b = 6.5304(9) Å, c = 14.646(4) Å, α = 81.682(1)°, β = 83.712(2)°, γ = 86.365(2)°, V = 537.8(2) Å3, Z = 2. The five strongest peaks in the powder pattern are [dhkl , I/Imax, (hkl)]: [6.222, 100, (011)], [3.227, 87, (020)], [6.454, 63, (010)], [2.883, 58, (005, 023, 121)], [7.208, 45, (002)]. The mineral is named after Australian geologist Ben Grguric.

Subject Areas: Earth Science

Instruments: I15-Extreme Conditions