Abstract

An important goal in silica diagenesis research is to understand the kinetics of opal transformation from noncrystalline opal-A to the disordered silica polymorph opal-CT. Because the conventional technique for monitoring the transformation, powder X-ray diffraction (XRD), is applicable only to phases with long-range order, we used Fourier transform infrared spectroscopy (FTIR) to monitor the transformation. We applied this technique, combined with XRD and TEM, to experimental run products and natural opals from the Monterey Formation and from siliceous deposits in the western Pacific Ocean. Using a ratio of two infrared absorption intensities (omega = I (sub 472 cm (super -1) ) /I (sub 500 cm (super -1) ) ), the relative proportions of opal-A and opal-CT can be determined, The progress of the transformation is marked by changes in slope of omega vs. depth or time when a sufficient stratigraphic profile is available. There are three stages in the opal-A to opal-CT reaction: (1) opal.A dissolution; (2) opal. CT precipitation, whose end point is marked by completion of opal-A dissolution; and (3) opal-CT ordering, during which tridymite stacking is eliminated in favor of cristobalite stacking.

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