Single-crystal neutron diffraction and Mössbauer spectroscopic study of hureaulite, (Mn,Fe)5(PO4)2 (HPO4)2 (H2O)4

Abstract

The crystal chemistry of hureaulite from the Jocao (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 57 Fe-Mossbauer spectroscopy [ M(1),M(2),M(3) (Mn 2+ 3.61 Fe 2+ 1.21 Ca 0.11 Mg 0.03 ) ∑=4.96 ( P(2) (PO 4 ) 2 (H 1.04 P(1) PO 4 ) 2 (H 2 O) 3.92 , Z = 4, a = 17.603(6), b = 9.087(2), c = 9.404(4) A, β = 96.66(4)°, and V = 1494.1(9) A 3 at 293 K, space group C 2 /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 H 2 O 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. , HOPO 3 ), whereas the two H 2 O molecules are the vertices of the (Mn,Fe)-octahedra. No zeolitic H 2 O occurs in hureaulite structure. The complex hydrogen bonding scheme in hureaulite is now well defined, with five hydrogen bonds energetically favorable. The element 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 57 Fe Mossbauer spectrum of hureaulite confirms the absence of ferric iron in the sample. The spectrum shows four well-separated absorption lines which are best 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 Mn 2+ for the M(3) site as shown 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 distortion than the M(1) site, according to the experimental findings based on the neutron refinements.