Abstract

The formation conditions of the ferric smectite nontronite are not fully understood. The present study couples experimental and analytical data with field observations in an attempt to constrain the rate and temperature of formation of naturally occurring nontronites from Columbia River Basalt flows. Synthetic Fe-Al-Si gels were incubated at temperatures ranging from 4 to 150°C for 4 weeks. Samples were analyzed using Fe K-edge X-ray fluorescence spectroscopy (XAFS). Spectra of the synthesized nontronites were compared with spectra of natural samples collected from weathered Columbia River Basalt flows. Cation ordering in the synthetic samples increased with incubation temperature, but the synthetic clays did not approach the degree of crystal ordering of the natural nontronite samples. These observations suggest that highly ordered natural nontronites require longer crystallization times than are typically used in laboratory experiments. The natural samples were found filling open cracks near flow surfaces, indicating that the clays formed at temperatures below the boiling point of water. A comparison of experimental and field timescales with other estimates of nontronite growth rates suggests that natural nontronite crystallization in the region must have occurred at ambient, near-surface temperatures over timescales of up to millions of years.

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