Crystal structure of leucophosphite, KFe3+2(PO4)2(OH)·2H2O, from the Jocão pegmatite, Minas Gerais, Brazil

The crystal structure of leucophosphite, KFe3+2(PO4)2(OH)·2H2O, was fully characterized using single-crystal X-ray data, collected on a crystal from the Jocão pegmatite, Minas Gerais, Brazil. This crystal belongs to the same fragment accurately analysed by electron microprobe. The refinement was carried out in the P21/n space group and with unit-cell a=9.7734(3) Å, b=9.6606(2) Å, c=9.7429(5) Å, ?=102.447(5)° and V=898.27(6) Å3 for Z=4. The known crystal-structure, based on tetramers of octahedrally coordinated sites connected by the (PO4)-tetrahedra in a complex tridimensional framework with K atoms located in channels aligned along b axis, is confirmed. Difference Fourier map allowed to unambiguously locate the hydrogen positions that were refined at the final steps of the crystal structure anisotropic refinement leading to an R1 index of 2.12%. 162 parameters were refined on the basis of 2406 observed [I>3?(I)] reflections. The H-bonding scheme in the structure of leucophosphite is now fully defined. One hydroxyl group and two H2O groups are exhaustively described. OH group has donor-acceptor distance of 2.96 Å and D-H...A 172?. H2O groups show donor-acceptor distances less than 2.9 Å whereas angles D-H...A are 161-176 ? and 162-176? for OW10 and OW11, respectively. The OW11 atom behaves twice as donor (with H111 and H112) and twice as acceptor (with H91 and H101), results in a distorted tetrahedron with OH9, OW10, O6, and O4. A very good agreement against microprobe chemical analysis was found, also confirmed by bond valence calculation showing expected values for all the atomic sites.