Single-crystal neutron diffraction and Mössbauer spectroscopic study of hureaulite, (Mn,Fe)$_5$(PO$_4$)$_2$(HPO$_4$)$_2$(H$_2$O)$_4$

The crystal chemistry of hureaulite from the Jocão (Cigana) pegmatite, Conselheiro Pena, Doce Valley, Minas Gerais (Brazil), was investigated by electron microprobe analysis in wavelength-dispersive mode, single-crystal Laue (at 293 K) and monochromatic neutron diffraction (at 2.3 K), and 57Fe-Mössbauer spectroscopy [M(1),M(2),M(3)(Mn2+3.61Fe2+1.21Ca0.11Mg0.03)(sum=4.96)(P(2)PO4)2(H1.04P(1)PO4)2(H2O)3.92, Z = 4, a = 17.603(6), b = 9.087(2), c = 9.404(4) Å, beta = 96.66(4) °, and V = 1494.1(9) Å3 at 293 K, space group C2/c]. The neutron refinements confirm the general structure model previously reported, showing that five independent H sites (with full site occupancy) occur in the hureaulite structure, one of them as member of the unique hydroxyl group (i.e., O(1)-H(1)) and the other four belonging to two independent H2O molecules (i.e., H(2)-O(9)-H(3) and H(4)-O(10)-H(5)). The hydroxyl group is the vertex of one of the two independent P-tetrahedra (i.e., HOPO3), whereas the two H2O molecules are the vertices of the (Mn,Fe)-octahedra. No zeolitic H2O occurs in hureaulite structure. The complex hydrogen bonding scheme in hureaulite is now well defined, with five hydrogen bonds energetically favorable. The elements distribution among the octahedral sites, deduced on the basis of the neutron structure refinements, shows that the highest fraction of Fe populates the M(2) site (~36%), whereas lower fractions are observed at M(1) and M(3) (i.e., ~13-15 and ~11-12%, respectively). The 57Fe Mössbauer spectrum of hureaulite confirms the absence of ferric iron in the sample. The spectrum shows four well separated absorption lines which are better fitted with three doublets. The well resolved doublet (with the largest quadrupole splitting) is here assigned to the M(3) site, in line with the pronounced preference of Mn2+ for the M(3) site as obtained by the neutron structure refinements. The two innermost doublets have almost identical isomer shifts, though with slightly different quadrupole splitting values; the smaller quadrupole splitting value is assigned to the M(2) site, which is expected to have a slightly lower polyhedral distortions than the M(1) site, according to the experimental findings based on the neutron refinements