Ferroberaunite, Fe2+Fe3+5(PO4)4(OH)5·6H2O, a mixed-valence iron member of the beraunite series, from the Gravel Hill Mine, Perranzabuloe, Cornwall, England

• Published in: Mineralogical magazine 2022-06, Vol.86 (3), p.363-372
• Main Authors: Tvrdy, Jaromir, Plasil, Jakub, Vrtiska, Lubos, Sejkora, Jiri, Skoda, Radek, Dolnicek, Zdenek, Petr, Martin, Veselovsky, Frantisek
• Format: Article
• Language: English
• Subjects: chemical composition; coexisting minerals; Cornwall England; crystal structure; Crystals; electron microscopy data; England; Europe; ferroberaunite; Flotation; formula; Gravel; Gravel Hill Mine; Great Britain; Iron compounds; Iron ores; lattice parameters; limonite; metal ores; Mineralogy; Minerals; Morphology; Museums; new minerals; nonsilicates; optical properties; oxides; Perranzabuloe England; Petrology; phosphates; Raman spectra; SEM data; space groups; spectra; unit cell; United Kingdom; Western Europe; X-ray diffraction; X-ray diffraction data; X-ray photoelectron spectra
• ISSN: 0026-461X; 1471-8022
• DOI: 10.1180/mgm.2022.15

The new mineral ferroberaunite, Fe2+Fe3+5(PO4)4(OH)5·6H2O (IMA2021-36; symbol: Fbru), occurs in cavities of 'limonite' iron ore from the Gravel Hill mine, Perranzabuloe, Cornwall, England. Its flattened prismatic crystals up to 400 µm long are dark green to olive green, transparent to translucent, with a vitreous lustre, pearly on cleavages. The Mohs hardness is ∼3-4. The density measured by the flotation method is 2.94(2) g·cm-3, the calculated density is 2.907 g·cm-3. Ferroberaunite is biaxial (-), with α = 1.736(2), β = 1.765(3), γ = 1.786(5) at 589 nm, 2Vmeas. = 68(3)°, 2Vcalc. = 79°; dispersion of optical axes is strong, r > v; orientation is Y = b; X ≈ a, Z ≈ c. Pleochroism is strong: X = bluish-green >> Z = green > Y = yellow. Electron-microprobe analyses gave the empirical formula (Fe2+0.75Ca0.01Mn0.02Fe3+0.22)Σ1.00(Fe3+4.88Al0.04)Σ4.92 (PO4)4O0.11(OH)4.76·6H2O. Ferroberaunite is monoclinic, C2/c, with a = 20.8708(3), b = 5.1590(8), c = 19.2263(3) Å, β = 93.3186(17)°, V = 2066.7(3) Å3 and Z = 4. The eight strongest lines in the powder X-ray diffraction pattern are [dmeas, Å (Irel., %) (hkl)]: 10.410 (100) (200), 9.606 (14) (002), 7.271 (11) (20{2}), 5.203 (4) (400), 3.467 (12) (600), 3.325 (6) (60{2}), 3.201 (6) (006), 2.600 (4) (800). The mineral is isostructural with beraunite, redefined recently as Fe3+6(PO4)4O(OH)4·6H2O.