APPLICATION OF FOURIER-TRANSFORM INFRARED-SPECTROSCOPY TO SILICA DIAGENESIS - THE OPAL-A TO OPAL-CT TRANSFORMATION

An important goal in silica diagenesis research is to understand the k inetics of opal transformation from noncrystalline opal-A to the disor dered silica polymorph opal-CT, Because the conventional technique for monitoring the transformation, powder X-ray diffraction (XRD), is app licable only to phases with long-range order, we used Fourier transfor m infrared spectroscopy (FTIR) to monitor the transformation. We appli ed this technique, combined with XRD and TEM, to experimental run prod ucts and natural opals from the Mon terey Formation and from siliceous deposits in the western Pacific Ocean. Using a ratio of two infrared absorption intensities (omega = I-472cm-1-/I-500cm-1), the relative pr oportions 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 ti me when a sufficient stratigraphic profile is available. There are thr ee 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 o pal A dissolution; and (3) opal-CT ordering, during which tridymite st acking is eliminated in favor of cristobalite stacking.