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

¹ Laboratoire de Minéralogie, Département de Géologie, Université de Liège, Bâtiment B.18, B-4000 Liège, Belgium
² Département de Physique, Bâtiment B.5, Université de Liège, B-4000 Liège, Belgium
³ Laboratoire de Minéralogie, LMTG-UMR 5563, Observatoire Midi Pyrénées, 14 Avenue E. Belin, F-31400 Toulouse, France
⁴ Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany

^* e-mail: fhatert{at}ulg.ac.be

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 {01} cleavage. The estimated Mohs hardness is 4. The calculated density is 3.62 g/cm³. Ferrorosemaryite is biaxial negative, with = 1.730(5), ß = 1.758(7), and = 1.775(5) ( = 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_0.040.26)_1.00(Fe²⁺_0.71Mn²⁺_0.24Fe³⁺_0.05)_1.00Fe³⁺_1.00(Al_0.82Fe³⁺_0.16Mg_0.02)_1.00[(P_0.990.01)O₄]₃. The single-crystal unit-cell parameters are a = 11.838(1), b = 12.347(1), c = 6.2973(6) Å, ß = 114.353(6)°, and V = 838.5(1) ų, space group P2_1/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), 2.693(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 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).

Key-words: ferrorosemaryite, new phosphate mineral, wyllieite group, crystal structure, Rubindi pegmatite, Rwanda.

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