Angarfite, NaFe3+5(PO4)4(OH)4{middle dot}4H2O, a New Mineral Species from the Angarf-Sud Pegmatite, Morocco: Description and Crystal Structure

Abstract

Angarfite, ideally NaFe^(3+)_5(PO_4)_4(OH)_(4·)4H_2O, 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/cm^3, respectively. It is optically biaxial (+),α 1.688(1), β 1.696(1), γ 1.708(2) (white light); 2V_(meas) = 80(3)°; 2V_(calc) = 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 Na_2O 2.69, MgO 4.76, Mn_2O_3 1.79, Fe_2O_3 37.36, P_2O_5 34.68, H_2O 14.63 (from structure refinement), total 95.91 wt%. The empirical formula, based on 24 O atoms, is Na_(0.71)(Fe^(3+)_(3.83)Mg_(0.97)Mn^(3+)_(0.19))Σ_(4.99)(P_(1.00)O_4)_4(OH)_(2.71)(H_2O)_(1.29·)4H_2O. Angarfite is orthorhombic, C222_1, ɑ 12.7997(3), b 17.9081(4), c 8.2112(6) Å, V 1882.16(15) Å^3 and Z = 4. The eight strongest lines in the X-ray powder-diffraction pattern [d_(obs) 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 (R_1 = 3.02% for 2074 reflections, F_o > 4σF) contains zig-zag chains of edge-sharing Fe^(3+)O_6 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 PO_4 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 H_2O site. Identical sheets of octahedra and tetrahedra are found in the structures of bakhchisaraitsevite and mejillonesite.