THE CRYSTAL STRUCTURE OF GLADIUSITE, (Fe2+,Mg)4Fe3+2 (PO4)(OH)11(H2O)

Abstract

The crystal structure of gladiusite, (Fe2+,Mg)4 Fe3+2 (PO4) (OH)11 (H2O), monoclinic, a 16.950 (2), b 11.650 (1), c 6.2660 (6) A, β 90.000 (4)°, V 1237.3(4) A3, space group P 21/ n, Z = 2, D (calc.) = 3.08 g cm−3, was solved by direct methods for a twinned crystal and refined to an R 1 index of 5.4% based on 1214 observed [ F o > 4σ F ] unique reflections measured with Mo K α X-radiation on a CCD diffractometer. There is one P site tetrahedrally coordinated by four O atoms. There are six distinct octahedrally coordinated M sites containing Fe3+, Fe2+, Mg and Mn2+; the octahedra are coordinated by O atoms, OH and H2O groups. The ( M ϕ6) (ϕ = unspecified anion) octahedra each share two trans edges to form a rutile-like [ M ϕ4] chain along [001]. Pairs of chains, shifted relative to each other by a half-octahedron, link together through common edges to form a ribbon extending along c ; thus, each octahedron shares four edges with other octahedra. These ribbons are connected through common vertices of ( M ϕ6) octahedra into a framework with channels along the c axis. The (PO4) tetrahedra share each of two vertices with three ( M ϕ6) octahedra, and the remaining two vertices are connected to octahedra through hydrogen bonds. There is a system of hydrogen bonds in the crystal structure of gladiusite ( D – A < 3.50 A), some of them bifurcated. The Mossbauer spectrum of gladiusite shows three doublets, two of which may be assigned to Fe2+ in octahedral coordination and one of which may be assigned to Fe3+ in octahedral coordination. The intensity ratios of the various doublets indicate that Fe2+/(Fe2+ + Fe3+) = 0.49.