Ferrirockbridgeite, ideally (□0.33)2(Fe3+)3(PO4)3(OH)4(H2O), is a new member of the rockbridgeite group. The type specimen is from the Palermo No. 1 pegmatite in North Groton, Grafton County, New Hampshire, USA. Electron microprobe analysis, coupled with Mössbauer spectroscopy for FeO and thermogravimetric analysis (TGA) for H2O gives the empirical formula . Ferrirockbridgeite is orthorhombic, space group Bbmm with a = 13.853(1), b = 16.928(1), c = 5.1917(5) Å and Z = 4. Optically, ferrirockbridgeite is biaxial (–), with α = 1.875(5), β = 1.890(calc), γ = 1.900(5) (measured in white light) and 2V (meas) is 78(1)° from extinction data. The dispersion is strong, with r > v. The optical orientation is X = c, Y = a, Z = b. The pleochroism is X = yellow brown, Y = olive brown, Z = dark olive green; Z > Y > X. Crystal structure refinements on ferrirockbridgeite and other oxidized rockbridgeite-group species, including type rockbridgeite and type frondelite, show that oxidation is accompanied by loss of Fe2+ from the M2 site according to the reaction [3Fe2+] → [2Fe3+ + □] + Fe2+(removed) +2 e−. A variable portion of the Fe removed from the M2 site becomes trapped at M3 site vacancies. A general formula for oxidized rockbridgeite-group minerals is presented.
Ferrirockbridgeite, (Fe0.673+□0.33)2(Fe3+)3(PO4)3(OH)4(H2O), and the oxidation mechanism for rockbridgeite-group minerals
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Ian Edward Grey, Anthony R. Kampf, Erich Keck, John D. Cashion, Colin M. MacRae, Yesim Gozukara, Finlay L. Shanks; Ferrirockbridgeite, (□0.33)2(Fe3+)3(PO4)3(OH)4(H2O), and the oxidation mechanism for rockbridgeite-group minerals. European Journal of Mineralogy doi: https://doi.org/10.1127/ejm/2019/0031-2846
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