Ferrirockbridgeite, (⁠Fe0.673+□0.33)2(Fe3+)3(PO4)3(OH)4(H2O), and the oxidation mechanism for rockbridgeite-group minerals

Ferrirockbridgeite, ideally (⁠$Fe0.673+$□_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 $Mn0.312+Fe0.082+Mg0.01Zn0.03Ca0.05Fe4.183+P2.87O17H6.11$⁠. 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) +2e⁻. 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.