Moabite, NiFe³⁺(PO₄)O, a new natural oxyphosphate

Abstract. Moabite, NiFe3+(PO4)O, is a new natural oxyphosphate discovered in pyrometamorphic rocks of the Daba-Siwaqa complex, a subdivision of the Hatrurim Formation in central Jordan. The mineral is named for the Kingdom of Moab, an ancient state that existed on the territory of the modern Jordan. Moabite is an accessory phase in the phosphide–phosphate assemblages, where it associates with diopside; anorthite; crocobelonite, CaFe23+(PO4)2O; yakubovichite, CaNi2Fe3+(PO4)3; hematite; negevite, NiP2; murashkoite, FeP; transjordanite, Ni2P; halamishite, Ni5P4; native iron (α-Fe); and an alluaudite-group phosphate whose composition is exactly midway between the two endmembers NaNaCa(Fe3+Mg)(PO4)3 and □NaCa(Fe3+Fe3+)(PO4)3. The mineral forms isometric to short prismatic crystals and euhedral grains up to 30 µm across. Macroscopically, it has a deep-brown colour. In the polished sections in transmitted light, the mineral is translucent red-brown. It has a Mohs hardness rating of 4. Cleavage was not observed. The density, 4.324 g cm−3, was calculated based on the empirical formula and unit-cell parameters obtained from single-crystal refinement. The chemical composition was as follows (electron microprobe, wt %): NiO 29.75, CuO 1.73, MgO 0.45, Fe2O3 36.04, Al2O3 0.19, Cr2O3 0.18, V2O5 0.47, P2O5 31.22, total 100.03. The empirical formula calculated on the basis of 5 oxygen atoms per formula unit (apfu) is (Ni0.90Cu0.05Mg0.03)Σ0.98(Fe3+ 1.01Al0.01Cr0.01)Σ1.03(P0.99V5+ 0.01)Σ1.00O5, corresponding to the ideal NiFe3+(PO4)O. Moabite is orthorhombic; the space group is Pnma (no. 62); and a=7.2161(16), b=6.4064(15), c=7.4706(19) Å, V=345.4(1) Å3 and Z=4. The strongest lines of X-ray powder diffraction pattern are as follows [d in Å (I) (hkl)]: 5.20(63)(101), 3,321(37)(102), 3.251(83)(201), 2.7262(100)(121), 2.5946(37)(202), 2.3542(25)(103) and 2.3044(24)(122). The crystal structure has been solved and refined to R1=0.033 for 389 unique observed reflections. Moabite is the first mineral that crystallizes in the α-Fe2PO5 (α-Fe2OPO4) structure type. It has a direct synthetic analogue, and it is isotypic to antiferromagnetic transition metal oxyphosphates of the general formula A2+B3+OPO4, where A2+= Fe, Ni, Co and Cu and B3+= Fe, V and In.