Infrared (IR) spectral changes with time of biogenic and inorganic silica have been examined using in situ IR micro-spectroscopy by using an original hydrothermal diamond cell. Centric diatoms (diameters = 100-350 µm) and silica gels (C-300, Wako Chemicals) were heated at 125-185 ℃ range with a pressure of 3 MPa. Decreases of 950 cm (Si-OH) peak heights could be fitted by a combination of exponential and linear decreases (y = A exp (-kt) - k t + A). The first-order rate constants k [s] for Si-OH decreases of diatoms and silica gels are similar but the activation energy was lower for diatoms (61 kJċmol< 106 kJċmol). The first-order rate constants k [s] for Si-OH decreases of diatoms and silica gels are much faster than reported hydrothermal transformation rates of silica (Opal A to Opal CT and Opal CT to quartz). These results indicate that the exponential Si-OH decreases observed in biogenic and inorganic silica during hydrothermal reactions are considered to correspond to dehydration-condensation reactions in the amorphous states (Si-OH + HO-Si → Si-O-Si). In fact, band area ratios 1220 cm/1120 cm increased exponentially indicating more bridging of Si-O-Si. On the other hand, the linear decreases of Si-OH of silica gels (k [s]) were considered to be due to dissolution of silica. By using the grain size and density of silica gels, the zero-order dissolution rate constants k* [molċmċs] were calculated from k [s]. The obtained dissolution rates k* are larger than reported values for silica glass and quartz. The zero-order dissolution rates k [s] for diatoms are similar to those for silica gels but with a lower activation energy (32 kJċmol< 60 kJċmol). The smaller activation energy values for diatoms than silica gels both for the first and zero-order decrease rates of Si-OH might indicate catalytic effects of organic components bound to biogenic silica for the dehydration-condensation reaction and dissolution. The present in situ hydrothermal IR micro-spectroscopy is useful for characterizing transformation of amorphous materials including inorganic-organic composites.
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