Eleonorite, (PO₄)₄O(OH)₄·6H₂O: validation as a mineral species and new data

Eleonorite, ideally (PO₄)₄O(OH)₄·6H₂O, the analogue of beraunite Fe²⁺(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. D_meas = 2.92(1), D_calc = 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), 2V_meas = 75(10)°, 2V_calc = 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 (⁠Al_0.18⁠)_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 = O^2–, 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) ų 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) (20⁠), 4.816 (31) (111, 004), 3.432 (18) (600, 114, 404, 313), 3.197 (18) (510, 51⁠, 006, 31⁠, 602), 3.071 (34) (314, 11⁠).