Ferrorosemaryite, NaFe2+Fe3+Al(PO4)3, a new phosphate mineral from the Rubindi pegmatite, Rwanda

Abstract

Ferrorosemaryite, ideally □ NaFe2+Fe3+Al(PO4)3, is a new mineral species from the Rubindi pegmatite, Rwanda. It occurs as large idiomorphic grains reaching 3 mm, embedded in scorzalite. Associated phosphate minerals are scorzalite, trolleite, montebrasite, bertossaite, brazilianite, with accessory augelite, triplite and lacroixite. The mineral is transparent and exhibits a dark-green to bronze colour, with a resinous lustre and with a greenish to brownish streak. It is non-fluorescent, brittle, and shows a perfect {010} cleavage and a good {101} cleavage. The estimated Mohs hardness is 4. The calculated density is 3.62 g/cm3. Ferrorosemaryite is biaxial negative, with α = 1.730(5), β = 1.758(7), and γ= 1.775(5) (Λ = 590 nm). Pleochroism is from dark green ( X ) to dark brown ( Z ). The measured 2 V angle is 82(1)°, and the calculated 2 V angle is 75°. A strong dispersion r < v has been observed, but the optical orientation has not been determined. Electron microprobe analyses gave P2O5 46.00, Al2O3 9.12, Fe2O3 21.01, FeO 11.10, MgO 0.19, MnO 7.96, CaO 0.44, Na2O 2.85, K2O 0.01, total 98.68 wt. %. The resulting empirical formula, calculated on the basis of 12 O, is □1.00(Na0.42Mn2+0.28Ca0.04□0.26)Σ1.00(Fe2+0.71Mn2+0.24Fe3+0.05)Σ1.00Fe3+1.00(Al0.82Fe3+0.16Mg0.02)Σ1.00[(P0.99□0.01)O4]3. The single-crystal unit-cell parameters are a = 11.838(1), b = 12.347(1), c = 6.2973(6) A, β = 114.353(6)°, and V = 838.5(1) A3, space group P 21/ n . The eight strongest lines in the powder X-ray diffraction pattern [ d (in A)( I )( hkl )] are: 8.102(30)(110), 6.167(50)(020), 5.382(40)(200), 4.054(45)(220), 3.448(65)(310), 3.011(40)(112), 2.693(75)(400), 2.677(100)(240). Ferrorosemaryite is the Fe2+ analogue of rosemaryite, and belongs to the wyllieite group of minerals. The crystal structure of ferrorosemaryite has been refined, based on single-crystal X-ray diffraction data, to R 1 = 2.43 %. The infrared spectrum is similar to those of alluaudite-type phosphates. The mineral species and name were approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association (no. 2003–063).