DEGEO - Departamento de Geologia
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Item Eleonorite, Fe3+6 (PO4)4O(OH)4·6H2O : validation as a mineral species and new data.(2017) Chukanov, Nikita V.; Aksenov, Sergey M.; Rastsvetaeva, Ramiza K.; Schäfer, Christof; Pekov, Igor V.; Belakovskiy, Dmitriy I.; Cipriano, Ricardo Augusto Scholz; Oliveira, Luiz Carlos Alves de; Britvin, Sergey N.Eleonorite, ideally Fe3þ 6 (PO4)4O(OH)4·6H2O, the analogue of beraunite Fe2+Fe3þ 5 (PO4)4(OH)5·6H2O with Fe2+ completely substituted by Fe3+, has been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification as a mineral species (IMA 2015-003). The mineral was first described on material from the Eleonore Iron mine, Dünsberg, near Giessen, Hesse, Germany, but during this study further samples were required and a neotype locality is the Rotläufchen mine, Waldgirmes, Wetzlar, Hesse, Germany, where eleonorite is associated with goethite, rockbridgeite, dufrénite, kidwellite, variscite, matulaite, planerite, cacoxenite, strengite and wavellite. Usually eleonorite occurs as red-brown prismatic crystals up to 0.2 mm × 0.5 mm × 3.5 mm in size and in random or radial aggregates up to 5 mm across encrusting cavities in massive ‘limonite’. The mineral is brittle. Its Mohs hardness is 3. Dmeas = 2.92(1), Dcalc = 2.931 g cm–3 . The IR spectrum is given. Eleonorite is optically biaxial (+), α = 1.765(4), β = 1.780(5), γ = 1.812(6), 2Vmeas = 75(10)°, 2Vcalc = 70°. The chemical composition (electron microprobe data, H2O analysed by chromatography of products of ignition at 1200°C, wt.%) is: Al2O3 1.03, Mn2O3 0.82, Fe2O3 51.34, P2O5 31.06, H2O 16.4, total 99.58. All iron was determined as being trivalent from a Mössbauer analysis. The empirical formula (based on 27 O apfu) is (Fe3þ 5:76Al0.18Mn3þ 0:09)Σ6.03(PO4)3.92O(OH)4.34·5.98H2O. The crystal structure (R = 0.0633) is similar to that of beraunite and is based on a heteropolyhedral framework formed by M(1–4)Ø6-octahedra (where M = Fe3+; Ø=O2– , OH– or H2O) and isolated PO4 tetrahedra, with a wide channel occupied by H2O molecules. Eleonorite is monoclinic, space group C2/c, a = 20.679(10), b = 5.148(2), c = 19.223(9) Å, β = 93.574(9)°, V = 2042.5(16) Å3 and Z = 4. The strongest reflections of the powder X-ray diffraction pattern [d,Å (I,%) (hkl)] are 10.41 (100) (200), 9.67 (38) (002), 7.30 (29) (202), 4.816 (31) (111, 004), 3.432 (18) (600, 114, 404, 313), 3.197 (18) (510, 511, 006, 314, 602), 3.071 (34) (314, 115).Item The molecular structure of the phosphate mineral beraunite Fe^2+Fe5^3+(PO4)4(OH)5.4H2O – A vibrational spectroscopic study.(2014) Frost, Ray Leslie; López, Andrés; Cipriano, Ricardo Augusto Scholz; Xi, Yunfei; Lana, Cristiano de CarvalhoThe mineral beraunite from Boca Rica pegmatite in Minas Gerais with theoretical formula Fe^2+ Fe5^ 3+(PO4)4(OH)5.4H2O has been studied using a combination of electron microscopy with EDX and vibrational spectroscopic techniques. Raman spectroscopy identifies an intense band at 990 cm^-1 and 1011 cm^-1. These bands are attributed to the PO4^-3 v1 symmetric stretching mode. The v3 antisymmetric stretching modes are observed by a large number of Raman bands. The Raman bands at 1034, 1051, 1058, 1069 and 1084 together with the Raman bands at 1098, 1116, 1133, 1155 and 1174 cm^-1 are assigned to the v3 antisymmetric stretching vibrations of PO4^-3 and the HOPO3^-2 units. The observation of these multiple Raman bands in the symmetric and antisymmetric stretching region gives credence to the concept that both phosphate and hydrogen phosphate units exist in the structure of beraunite. The series of Raman bands at 567, 582, 601, 644, 661, 673, and 687 cm^-1 are assigned to the PO4^-3 v2 bending modes. The series of Raman bands at 437, 468, 478, 491, 503 cm^-1 are attributed to the PO4^-3 and HOPO3^-2 v4 bending modes. No Raman bands of beraunite which could be attributed to the hydroxyl stretching unit were observed. Infrared bands at 3511 and 3359 cm^-1 are ascribed to the OH stretching vibration of the OH units. Very broad bands at 3022 and 3299 cm^-1 are attributed to the OH stretching vibrations of water. Vibrational spectroscopy offers insights into the molecular structure of the phosphate mineral beraunite.