Coexisting hydroxyl groups and H2O molecules in minerals : a single-crystal neutron diffraction study of eosphorite, MnAlPO4(OH)2·H2O

The crystal chemistry of eosphorite from Chamachhu (Skardu District, Baltistan, Pakistan) [(Mn2+0.94Fe2+0.06Al0.01)Σ1.01AlPO4(OH1.90F0.10)Σ2·H2O, a = 6.9263(4), b = 10.4356(8), c = 13.5234(10) Å, V = 977.5(1) Å3, space group Cmca, Z = 8], has been reinvestigated by means of electron microprobe analysis in wavelength-dispersive mode and single-crystal neutron diffraction at 20 K. The anisotropic structural refinement has been performed with final agreement index R1 = 0.0381 for 82 refined parameters and 860 unique reflections with Fo > 4σ(Fo). The analysis of the difference-Fourier maps of the nuclear density allowed an unambiguous location of the H sites, the description of the H2O molecule and the OH-group configuration, along with the hydrogen-bonding scheme. We can now describe the structure of eosphorite as built by (Mn,Fe)O4(OH,H2O)2 and AlO2(OH)2(OH,H2O)2 octahedra, which both form chains running along [100]. The two types of chains are connected, via shared corners, to form a set of (100) sheets held together by P-tetrahedra (and hydrogen bonds) to form a three-dimensional framework. This material provides the rare opportunity to investigate the H-bond configuration of coexisting hydroxyl groups and H2O molecules in minerals by single-crystal neutron diffraction.