The oxidation mechanism of vivianite as studied by M 0 ssbauer spectroscopy

The crystal structure of vivianite contains Fe2* ions in isolated Fef,* octahedra and in pated fet"* octahedra. Mcissbauer spectral measurements of vivianites mechanically ground in air demonstrate that Fe3*/Fel* increases with rising Fe3* concentration, suggesting that there is a greater probability of oxidizing remaining Fef,* ions than the second Fe2s* ion of an existing n!3*-fe."* pair. A model is proposed whereby the electronic configuration of Fe3*Fel* stabilizes the pair relative to Fel*-Fel*. Our experimental data indicate that between 35% and 5Wo of thi Fe2"*-Fel* pairs are stable with regard to oxidation. The mechanism of oxidation of Fe2* ions in vivianite involves H2O ligands which are converted to OHions. As a result, the vivianite structure collapses due to the elimination of hydrogen bonds, facilitating further oxidation. Extensively oxidized vivianites not only disintegrate into friable powders but also become deep blue due to Fel* --+ Fe}* charge transfer transitions. Introduction Vivianite, Fer(POo)r'8HrO, is well known for its vulnerability to atmospheric oxidation (Watson, l9l8; Moore, l97l), during which the color changes from virtually colorless to a dark blue (a)-pale green (B, 1) pleochroism. Although vivianite has been the focus of numerous crystal-chemical and spectral studies (e.g., Faye et al., 1968; Moore, l97l; Mao, 1976; Mattievich and Danon, 1977), ttre mechanism of oxidation of Fe2* ions in the vivianite structure has not been determined. The crystal structure of vivianite (Mori and Ito, 1950) contains iron in two distinct octahedral sites. In the isolated Feo octahedra,Fe2* ions are coordinated to four HrO ligands in the rhombic plane and to two trans-oxygens of [POo]3tetrahedra. In the Fe' octahedra, two HrO ligands are coordinated to Fe2* in a cls arrangement, and four oxygens of [POo]3tetrahedra supply the remaining ligands of the octahedron (Fig. l). Two Fe" octahedra share a cofilmon O-O edge across which the Fe2* ions are separated by only 2.96A along the D axis. Hydrogen bonding between the HrO ligands holds together sheets consisting of linked Feo octahedra, POo tetrahedra, and I Present address: Research School of Earth Sciences, Australian National University, Canberra, ACT 2600, Australia. 0003-004x/80,/0304-{361$02.00 361 double Fe" octahedra. This weak bonding accounts for the perfect (010) cleavage ofvivianite. The occurrence of paired Fe' octahedra and the short Fe"-Fe" interatomic distance provide a simple explanation for the blue-green pleochroism caused by the polarization dependence of the intense absorption band centered at about 15'000 cm-' (E ll b (c) axis); the color originates from Fe'* + Fe'* charge transfer between adjacent ferrous and ferric ions in edge-shared Fe" octahedra (Faye et al-' 1968; Loefler et al., 1975). Vivianite is regarded as a type example of homonuclear intervalence transitions (Hush, 1967; Robin and Day, 1967; Burns et al', 1980). Implicit in these assignmsnl5, however, is the assumption that some of the Fe'* ions in Fe' sites have been oxidized to Fe3*. The question of whether Fel* ions are also oxidized has not been answered. Using the Mossbauer effect, we have studied the oxidation of vivianite resulting from mechanical grinding in air. The results obtained from the Mdssbauer spectral measurements suggest a mechanism for the oxidation of Fe'* ions in vivianite. Experimental Procedures Eight vivianite specimens donated from mineral collections at Harvard, the Massachusetts Institute of Technology, and the Geological Survey of Canada 362 McCAMMON AND BI]RNS: Fig. l. The structure of vivianite viewed along the D axis (after Mori and lto, 1950). Note that hydrogen bonds between crs H2O ligands of Fes octahedra and H2O ligands of Fen octahedra in adjacent a-c planes account for the perfect (010) cleavage of vivianite. were subjected to X-ray diffraction, electron microprobe, optical absorption, and MOssbauer spectral measurements (McCammon, 1978). Two specfunens showing minimal initial oxidation were selected for further studies described below: (a) #1Cf.794, from Ruth, Nevada (Dana Collection, Harvard University); (b) #14198, from Llallagua, Botvia (Geological Survey of Canada, Ottawa). X-ray difraction analysis confirmed the identity and homogeneity of OXIDATION OF VIVIANITE the vivianites, and electron microprobe analyses revealed that less than 0.5 wt.Vo of impurities such as Mg, Mn, and SiO, were present. For the initial M<issbauer measurements, the samples were first gently pulverized in an inert atmosphere, and then bulked with sucrose under acetone to aid in the randomization of the crystallites. The effective thickness of all samples was kept below 10 mg Fe/cm'to prevent line broadening. The M<issbauer spectra were recorded on a constant acceleration Austin Science Associates spectrometer using a 50 mCi Co" source diffused into a Pd matrix. Calibration is reported relative to the spectrum of metallic Fe foil. Samples of the two vivianites chosen for further measurements were oxidized by mechanical grinding in air for different periods ranging from a few minutes to several hours. Two different experimental procedures were used. Five separate samples of vivianite #14198 were ground for successively longer periods of time and the Mdssbauer spectrum recorded for each sample. Conversely, one sample of vivianite #IOO794 was ground in air for a few minutes, mounted and run in the Mdssbauer spectrometer. It was then reground for a longer period of time and another Mclssbauer spectrum was recorded. This cycle was repeated five times.