Coexisting hydroxyl groups and H2O molecules in minerals: A single-crystal neutron diffraction study of eosphorite, MnAlPO4(OH)2{middle dot}H2O

Abstract

The crystal chemistry of eosphorite from Chamachhu (Skardu District, Baltistan, Pakistan) [(Mn 2+ 0.94 Fe 2+ 0.06 Al 0.01 ) Σ1.01 AlPO 4 (OH 1.90 F 0.10 ) Σ2 ·H 2 O, a = 6.9263(4), b = 10.4356(8), c = 13.5234(10) Å, V = 977.5(1) Å 3 , 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 H 2 O 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)O 4 (OH,H 2 O) 2 and AlO 2 (OH) 2 (OH,H 2 O) 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 H 2 O molecules in minerals by single-crystal neutron diffraction