Mineralogy of a natural As-rich hydrous ferric oxide coprecipitate formed by mixing of hydrothermal fluid and seawater: Implications regarding surface complexation and color banding in ferrihydrite deposits

We characterized the most As-rich natural hydrous ferric oxide (HFO) materi al ever reported using powder X-ray diffraction (pXRD), transmission electr on microscopy (TEM), X-ray fluorescence spectroscopy (XRF), light element a nalysis using gas chromatography (GC), visible-infrared (vis-IR) diffuse re flectivity, Fe-57 Mossbauer spectroscopy, and superconducting quantum inter ference device (SQUID) magnetometry. We find that the natural As-HFO materi al is very similar to synthetic coprecipitated As-HFO materials, but is: si gnificantly different from all known natural and synthetic As-free HFO mate rials and ferrihydrite samples. The pXRD patterns show systematic differenc es with patterns for 2-line ferrihydrite, that are interpreted as evidence for significant populations of oxygen-coordinated Fe-As pairs. Observations by TEM, combined with energy dispersive spectroscopy (EDS) microanalysis, show agglomerations of nanophase primary particles and no evidence for othe r Fe- or As-bearing phases. Mossbauer spectroscopy shows octahedrally coord inated Fe3+, with a large fraction (similar to 20%) of the octahedral Fe en vironments that are significantly distorted by the presence of As, compared to the Fe local environments in As-free ferrihydrite and HFO samples. The loss on ignition (LOI) is quantitatively consistent with OH + H2O, measured by GC, which, in turn, is consistent with similar to1 nm diameter primary particles having all their surface cations (Fe3+, As5+, Si4+, C4+) coordina ted on the free surface side by OH- and OH2. The banding into adjacent yell owish and reddish layers that occurs in the As-HFO deposits was studied by performing mineralogical analyses of the separated adjacent layers of two c ouplets of yellowish and reddish material. The yellowish samples were found not to contain secondary crystalline phases las did the reddish samples, i n small amounts) and to be relatively As-rich, C- and Si-poor. The observed anticorrelations between As and Si and between As and inorganic C suggest that natural HFOs, which usually contain significant molar amounts of Si, m ay not be as efficient at surface complexing As land P) as their Si and C-f ree synthetic counterparts, unless formed by co-precipitation with the As ( or P). The yellowish and reddish layers were also clearly resolved by both Mossbauer spectroscopy and magnetometry. Complexation of arsenate onto the HFO core was found to significantly increase the average quadrupole splitti ng (QS) obtained from Mossbauer spectroscopy by an amount that could not be explained by other chemical differences and that is consistent with an sim ilar to1 nm diameter particle size and somewhat smaller HFO core. The Munse ll hue YR index (5-10 YR) was found to be strongly correlated to the averag e QS, thereby establishing that the color differences, corresponding to the measured shifts of the main visible band edge, are due to the local distor tions in the [6]Fe3+ environments that are induced by As complexation, via their influence on the relevant ligand field transitions. SQUID magnetometr y allows the following observations. (1)The superparamagnetic to superferro magnetic transitions occur at 25 K and lower in As-HFO, compared to 55 K in synthetic 2-line ferrihydrite, suggesting a smaller magnetic primary parti cle (or core) size for As-HFO and inter-particle magnetic interaction reduc tion by surface complexed As, Si, and C. (2) The ratio of supermoment magnitude to magnetic particle size (m(2)/n, w here m is the net number of Fe3+ atomic moments per supermoment and n is th e number of Fe3+ cations per particle or HFO core) decreases with increasin g As content in the sequence synthetic-HFO > reddish-As-HFO > yellowish-As- HFO. (3) The magnetic susceptibility magnitudes for As-HFO and synthetic 2- line ferrihydrite differ by a factor of 10 and suggest different supermomen t formation mechanisms (m(2)/n < 1 vs, m(2)/n > 1, respectively) related to differences in intra-particle cationic and anionic disorder and magnetic p article size.