Abstract

Angarfite, ideally NaFe3+5(PO4)4(OH)4·4H2O, is a new mineral from the Angarf-Sud pegmatite, Tazenakht, Ouarzazate Province, Souss-Massa-Draâ region, Morocco. The mineral occurs on etched surfaces of triphylite in association with lipscombite–barbosalite, jahnsite-(NaFeMg) and bederite. It is interpreted as having resulted from the reaction of Na-bearing hydrothermal solutions with primary triphylite. The crystals are orange-brown to red-brown needles and prisms, elongate on [001], with poorly formed chisel-like terminations. The mineral is transparent and has a pale brown streak, a vitreous luster, a Mohs hardness of approximately 2½, a splintery fracture and one poorly developed cleavage on {010}. It is brittle, but thin needles are slightly flexible. The measured and calculated densities are 2.76(3) and 2.771 g/cm3, respectively. It is optically biaxial (+),α 1.688(1), β 1.696(1), γ 1.708(2) (white light); 2Vmeas = 80(3)°; 2Vcalc = 79°; strong r > v dispersion; optical orientation: X = b, Y = c, Z = a; pleochroism: X is tan, Y, medium red-brown, and Z, dark red-brown, with X < Y < Z. The average results of five electron-microprobe analyses gave Na2O 2.69, MgO 4.76, Mn2O3 1.79, Fe2O3 37.36, P2O5 34.68, H2O 14.63 (from structure refinement), total 95.91 wt%. The empirical formula, based on 24 O atoms, is Na0.71(Fe3+3.83Mg0.97Mn3+0.19)Σ 4.99(P1.00O4)4(OH)2.71(H2O)1.29·4H2O. Angarfite is orthorhombic, C2221, a 12.7997(3), b 17.9081(4), c 8.2112(6) Å, V 1882.16(15) Å3 and Z = 4. The eight strongest lines in the X-ray powder-diffraction pattern [dobs in Å(I)(hkl)] are: 10.463(43)(110), 9.016(100)(020), 6.459(42)(111), 3.731(27)(022), 3.355(51)(241), 3.026(29)(042), 1.926(33)(462, 263), 1.463(36)(822, 663, 4102, 2103). The crystal structure (R1 = 3.02% for 2074 reflections, Fo > 4σF) contains zig-zag chains of edge-sharing Fe3+O6 octahedra along c, which are linked into sheets parallel to {010} by sharing corners with octahedra in adjacent chains and by sharing corners with peripheral PO4 tetrahedra. Insular octahedra between the sheets share two sets of cis corners with tetrahedra in adjacent sheets, thereby linking the sheets into a framework. Channels in the framework parallel to c contain a partially occupied Na site and a disordered H2O site. Identical sheets of octahedra and tetrahedra are found in the structures of bakhchisaraitsevite and mejillonesite.

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