Ferrirockbridgeite, (Fe (sub 0.67) (super 3+) (sub 0.33) ) (sub 2) (Fe (super 3+) ) (sub 3) (PO (sub 4) ) (sub 3) (OH) (sub 4) (H (sub 2) O), and the oxidation mechanism for rockbridgeite-group minerals

Ferrirockbridgeite, ideally (Fe (sub 0.67) (super 3+) (sub 0.33) ) (sub 2) (Fe (super 3+) ) (sub 3) (PO (sub 4) ) (sub 3) (OH) (sub 4) (H (sub 2) O), 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 Mossbauer spectroscopy for FeO and thermogravimetric analysis (TGA) for H (sub 2) O gives the empirical formula Mn (sub 0.31) (super 2+) Fe (sub 0.08) (super 2+) Mg (sub 0.01) Zn (sub 0.03) Ca (sub 0.05) Fe (sub 4.18) (super 3+) P (sub 2.87) O (sub 17) H (sub 6.11) . Ferrirockbridgeite is orthorhombic, space group Bbmm with a = 13.853(1), b = 16.928(1), c = 5.1917(5) Aa and Z = 4. Optically, ferrirockbridgeite is biaxial (-), with alpha = 1.875(5), beta = 1.890(calc), gamma = 1.900(5) (measured in white light) and 2V (meas) is 78(1) degrees 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 Fe (super 2+) from the M2 site according to the reaction [3Fe (super 2+) ] --> [2Fe (super 3+) + ] + Fe (super 2+) (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.