Crystal-chemical investigation of Fe₃(PO₄)₂ · 8 H₂O vivianite minerals

Abstract

Crystal-chemistry of Fe₃(PO₄)₂ · 8 H₂O vivianite minerals was investigated by means of a multi-methodological approach based on electron microprobe analysis in wavelenght dispersive mode, single-crystal X-ray diffraction and infrared spectroscopy. To this aim, three specimens coming from Commentry (France) (1), Settefrati (Frosinone, Italy) (2) and Ibex Mine (Colorado, USA) (3), have been subject to EMPA analysis, indicating chemical composition (Fe_2.94Mg_0.06Mn_0.01)_Σ3.02(P_1.99O₈) · 8 H₂O (1), (Fe_2.96Mg_0.01Mn_0.04)_Σ3.01((P_1.96Si_0.03Al_0.02)_Σ2.01O₈) · 8 H₂O (2) and (Fe_2.65Zn_0.27Mn_0.06Na_0.03Mg_0.02)_Σ3.03(P_1.99O₈) · 8 H₂O (3); successively they have been subject to structural refinement by means of single-crystal X-ray diffraction within monoclinic space group C2/m, with the following unit cell constants: a = 10.100(3), b = 13.494(3), c = 4.709(3) Å; β = 104.24(4)° and V = 622.1(7) ų (1); a = 10.116(3); b = 13.473(3), c = 4.7064(7) Å; β = 104.33(4)° and V = 621.5(2) ų (2); a = 10.083(2), b = 13.468(2), c = 4.718(1) Å, β = 104.47(2)° and V = 620.3(2) ų (3). Iron cations occupy two special positions, making up single and dimerical octahedral groups, joined by (PO₄) tetrahedra groups, in order to form infinite layers down b, making up, via consistent O—H … O hydrogen bonds, the three-dimensional framework. The present vivianite samples were subject to geochemical and crystal chemical comparisons, the latter also by means of bond valence analysis with literature available phases. Last, IR spectra were performed for the three phases, in order to provide information on bending and stretching modes of the minerals, as well as to investigate H bonds by means of Libowitzki formula application.