Jouravskite: refined data on the crystal structure, chemical composition and spectroscopic properties

Abstract

Refined data on the crystal structure, chemical composition and properties of jouravskite, ideally Ca3Mn4+(SO4)(CO3)(OH)6·12H2O, have been obtained on a sample from N’Chwaning 3 Mine, Kuruman, Kalahari manganese field, Northern Cape Province, South Africa. The chemical composition determined using a combination of different methods (including ICP-OES, gas chromatography of products of ignition and electron microprobe) is (wt%): CaO 25.88, SrO 0.19, BaO 0.23, B2O3 0.39, Fe2O3 1.01, MnO2 12.00, SiO2 0.06, CO2 6.8, SO3 12.44, H2O 41.8, total 100.80, which corresponds to the empirical formula (Z = 2): (Са2.98Sr0.01Ba0.01)Σ3.00(Mn4+0.89Fe3+0.08Si0.01)Σ0.98{(SO4)1.00(CO3)1.00[B(OH)4]0.07}Σ2.07(OH)5.78·11.94H2O. Tetravalent state of Mn was confirmed by Mn K-edge XANES spectroscopy. The IR spectrum of jouravskite contains characteristic bands of Mn4+(OH)6 octahedra, CO32− and SO42− anions, and H2O molecules. The crystal structure was determined using single-crystal X-ray diffraction data and refined to R = 0.0332. Jouravskite is isostructural with thaumasite. The parameters of the hexagonal (space group P63) unit cell are: a = 11.07129(14) A, c = 10.62650(14) A, V = 1128.02(3) A3 and Z = 3. Investigation of other samples of ettringite-group minerals from N’Chwaning 3 Mine demonstrates wide variations of the contents of manganese, iron and boron, and possible existence of a Mn4+-dominant analogue of sturmanite with the presumed idealized formula Са6Mn4+2(SO4)2[B(OH)4](OH)10O2·nH2O.