As mentioned in the previous section, a key goal of our work had been to elucidate the mechanisms of silicification. The key to distinguishing between the two main possibilities of entombment and covalent bond formation required the determination of the exact ranges of Si-O-Si and Si-O-C bands in the IR spectra. This in itself was a challenge that took nearly two years to finally sort out, because the Si-O-Si and Si-O-C IR bands are both found in the 1,000-1,100 cm-1 region and overlap significantly according to the literature (Hino and Sato, 1971; Bellamy, 1975; Benning et al., 2004). In an attempt to resolve the issue, we conducted a series of deuteration experiments inspired by the research of Hino and Sato (Hino and Sato, 1971). Our deuteration experiments did result in some shifts in the IR bands, but only confounded the matter in the long run (Kolb and Liesch, 2006).
Clarification of the matter ultimately came about from a set of experiments involving the gradual hydrolysis of known organo-silicates, such as poly diethox-ysiloxane and poly dimethoxysiloxane, and an IR study of the resulting partially hydrolyzed products. As hydrolysis proceeded, the ethoxy or methoxy groups were replaced by hydroxyl groups, with the concomitant decrease of the Si-O-C band in the IR spectra (Liesch and Kolb, 2007a). These experiments allowed us to determine the ranges of the Si-O-Si (1,125-1,000 cm-1) and Si-O-C bands (1,225-1,150 cm-1) (Kolb and Liesch, 2007a). This information helped clarify the ambiguity of the given literature values (Hino and Sato, 1971; Bellamy, 1975; Benning et al., 2004), and ultimately allowed us to differentiate between the mechanisms of entombment and covalent Si-O-C bond formation. Our research focused on gathering and interpreting the spectral data. The key features gained from the study of the IR spectra are the location of the Si-O-Si band (as dramatic shifts occurred in some cases) as well as the presence of organic peaks and/or the Si-O-C IR band. These trends and features are depicted in Table 1. This table also shows silicates demonstrating the different mechanisms of silicification.
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