QUANTIFICATION OF AMORPHOUS FE-OXYHYDROXIDES AND PHYLLOSILICATES: APPLICATIONS FOR THE STUDY OF FE-BEARING PHASES IN EAST AFRICAN LAKES

Fe biogeochemistry in alkaline lake basins involves complex biological, physio-chemical, and redox processes that impact the mineral record of lake sediments. Detrital Fe(III)-oxyhydroxides (e.g. ferrihydrite) are often converted to Fe(II)-phases (e.g. pyrite, siderite), in lake basins through reduction in organic-rich, oxygen-depleted sediment. Various phases may subsequently form depending on complex redox reactions in diagenetic pore waters, including greigite, goethite, hematite, and others.

To better understand the diagenetic history of Fe-bearing phases in East African lake basins we are conducting abiotic water-rock interaction experiments to isolate variables identified from natural systems. This approach is hampered by the difficulty in quantitative X-ray diffraction characterization of amorphous Fe-bearing phases (i.e. ferrihydrite) and crystallographically heterogeneous clay minerals.

Here we present a method to quantify the proportion of amorphous Fe-oxyhydroxides and phyllosilicates through XRD. We used Clay Minerals Society Source Clay NAu-1 (nontronite, Fe-rich smectite), purified to the <1.0µm fraction, and we synthesized 2-line ferrihydrite following Ohmariand and Mateijevic (1992) and Ferreiro et al. (1995). We created five mixtures of varying proportions, and analyzed randomly oriented samples of each by XRD. To reduce Cu K_a-induced fluorescence in these Fe-rich materials, we used a graphite crystal monochromator in the diffracted beam optical path. We deconvolved the diffractograms and quantified nontronite abundance according to the 003 peak (28.38˚2θ) and ferrihydrite by the ratio of the height of the amorphous roll at 35.15˚2θ to a ferrihydrite-free baseline in a nontronite matrix. The relationship between the known abundances and the experimental XRD results is very strong, R²=0.98.

This approach will allow us to conduct experiments testing several hypotheses related to Fe-redox in lacustrine sediment, including the impact of selective reduction, Fe(II) interactions with sulfide and carbonate, and diagenetic products of pyrite oxidation. These processes are poorly understood, but are important contributors to the mineral record in lacustrine sediments, affecting our understanding of paleolimnology and paleomagnetic records.