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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, UK

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TVRDÝ Jaromír PLÁŠIL Jakub VRTIŠKA Luboš SEJKORA Jiří ŠKODA Radek DOLNÍČEK Zdeněk PETR Martin VESELOVSKÝ František

Rok publikování 2022
Druh Článek v odborném periodiku
Časopis / Zdroj Mineralogical Magazine
Fakulta / Pracoviště MU

Přírodovědecká fakulta

Citace
www http://doi.org/10.1180/mgm.2022.15
Doi http://dx.doi.org/10.1180/mgm.2022.15
Klíčová slova ferroberaunite; new mineral; phosphate; crystal structure; Raman spectroscopy; XPS spectroscopy; Cornwall
Popis 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, United Kingdom. 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 ? = 1.736(2), ß = 1.765(3), ? = 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)?1.00(Fe3+4.88Al0.04)?4.92(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) A, ß = 93.3186(17)°, V = 2066.7(3) A3 and Z = 4. The eight strongest lines in the powder X-ray diffraction pattern are [dmeas/A (Irel. %) (hkl)]: 10.410 (100) (200), 9.606 (14) (002), 7.271 (11) (202 Ż), 5.203 (4) (400), 3.467 (12) (600), 3.325 (6) (602 Ż), 3.201 (6) (006), 2.600 (4) (800). The mineral is isostructural with beraunite, redefined recently as Fe3+6(PO4)4O(OH)4·6H2O.
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