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

Author： Hatert Frédéric Lefèvre Pierre Fransolet André-Mathieu Spirlet Marie-Rose Rebbouh Leila Fontan François Keller Paul

Publisher： E. Schweizerbart'sche Verlagsbuchhandlung

ISSN： 0935-1221

Source： European Journal of Mineralogy, Vol.17, Iss.5, 2005-10, pp. : 749-759

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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 2V angle is 82(1)°, and the calculated 2V 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(Na_0.42Mn²⁺_0.28Ca_0.04□_0.26)_Σ1.00(Fe²⁺_0.71Mn²⁺_0.24Fe³⁺_0.05)_Σ1.00Fe³⁺_1.00(Al^0.82Fe³⁺_0.16Mg_0.02)_Σ1.00[(P_0.99□_0.01)O₄]₃. The single-crystal unit-cell parameters are a = 11.838(1), b = 12.347(1), c = 6.2973(6) Å, β = 114.353(6)°, and V = 838.5(1) Å³, space group P2₁/n. The eight strongest lines in the powder X-ray diffraction pattern [d(in Å)(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)(11 – 2), 2.693(75)(400), 2.677(100)(240). Ferrorosemaryite is the Fe 2+ 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 R₁ = 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).