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RUIFRANCOITE, A NEW Fe³⁺-DOMINANT MONOCLINIC MEMBER OF THE ROSCHERITE GROUP FROM GALILÉIA, MINAS GERAIS, BRAZIL

Daniel Atencio^1,, Nikita V. Chukanov², José M.V. Coutinho³, Luiz A.D. Menezes Filho⁴, Viktor T. Dubinchuk⁵ and Steffen Möckel⁶

¹ Instituto de Geociências, Universidade de São Paulo, Rua do Lago, 562, 05508-080, São Paulo, SP, Brazil
² Institute of Problems of Chemical Physics, Chernogolovka, Moscow Oblast, 142432, Russia
³ Instituto de Geociências, Universidade de São Paulo, Rua do Lago, 562, 05508-080, São Paulo, SP, Brazil
⁴ Rua Esmeralda, 534, Prado, 30410-080, Belo Horizonte, MG, Brazil
⁵ All-Russian Scientific-Research Institute of Mineral Resources (VIMS), Staromonetny per. 31, Moscow 119017, Russia
⁶ Alpha-Geophysik in Sachsen, Neudorfer Str. 18, D–09629 Burkersdorf, Germany

E-mail address:: datencio{at}usp.br

Ruifrancoite, Ca₂(,Mn²⁺)₂(Fe³⁺,Mg,Mn²⁺,Fe²⁺,Al)₄Be₄(PO₄)₆(OH)₄(OH,H₂O)₂·4H₂O, monoclinic, is a newly recognized member of the roscherite group. It occurs as a late vug-filling mineral in albite and muscovite, closely associated with cyrilovite and meurigite in a granite pegmatite at the Sapucaia (Proberil) mine, Sapucaia do Norte, Galiléia County, Minas Gerais, Brazil. Ruifrancoite occurs as needles or laths 1 to 2 µm thick and up to 100 µm long in fan-shaped botryoidal arrangements. The color is reddish brown, the streak is orange-brown, and the luster is vitreous. Ruifrancoite is transparent and non-fluorescent. The measured density is 2.88(1) g/cm³ (by the hydrostatic weighing method), and the calculated density is 2.859 g/cm^3. The mineral is biaxial (+), 1.665(3), β1.665(3), 1.682(3) (white light), 2V(obs.) 0–10°, 2V(calc.) 0°. The orientation is X = b, Y ^ c 16°; dispersion r > , very strong. Pleochroism is (brownish red) > = β(light brownish red). Infrared and Mössbauer spectroscopic data are given. The chemical composition is (wt.%, EDS mode, electron microprobe, Fe²⁺:Fe³⁺ ratio by Mössbauer data): CaO 9.81, MgO 3.23, MnO 8.10, FeO 3.93, Fe₂O₃ 12.51, Al₂O₃ 0.86, BeO (by ICP–AES) 8.41, P₂O₅ 39.46, H₂O (by Penfield method) 12.56, total 98.87. The empirical formula, based on six (PO₄) groups, with calculated BeO of 9.23%, is Ca_1.89( _1.45Mn²⁺_0.55)_2.00 (Fe³⁺_1.69Mg_0.86Mn²⁺_0.68Fe²⁺_0.59Al_0.18)_4.00 Be_4.00(PO₄)₆ [(OH)_4.75(H₂O)_1.25]·3.90H₂O. The ideal end-member formula of ruifrancoite is Ca_{2 2}Fe³⁺₄Be₄(PO₄)₆(OH)₆·4H₂O. The strongest reflections of the powder-diffraction pattern [d in Å(I)(hkl)] are: 9.485(44)(10), 5.943(100)(020), 4.821(65)(310), 3.176(44)(330), 2.784(41)(240), and 2.643(42)(600). The unit-cell parameters refined from powder data are a 15.911(7), b 11.894(7), c 6.625(7) Å, β94.5(1)°, V 1250(1) ų, Z = 2. The space group is C2/c. The mineral is named in honor of Professor Rui Ribeiro Franco (b. 1916), in recognition of his outstanding contributions to Brazilian mineralogy and geology. Former descriptions of "roscherite" at Sapucaia mine, and "Fe³⁺-dominant species" at Pomarolli farm, Linópolis, Divino das Laranjeiras, Minas Gerais, Brazil, should be considered to refer to ruifrancoite.

Keywords: ruifrancoite, new mineral species, phosphate, roscherite group, Sapucaia mine, Galiléia, Minas Gerais, Brazil.