Hydrothermal Transformation of Inorganic and Biogenic Silica as Studied Using in Situ Hydrothermal Infrared Microspectroscopy

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⁻¹ (Si–OH) peak heights could be fitted by a combination of exponential and linear decreases (y = A₁ exp (−k₁t) − k₀ t + A₀). The first-order rate constants k₁ [s⁻¹] for Si–OH decreases of diatoms and silica gels are similar but the activation energy was lower for diatoms (61 kJċmol⁻¹< 106 kJċmol⁻¹). The first-order rate constants k₁ [s⁻¹] 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⁻¹/1120 cm⁻¹ increased exponentially indicating more bridging of Si–O–Si. On the other hand, the linear decreases of Si–OH of silica gels (k₀ [s⁻¹]) 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 k₀* [molċm⁻²ċs⁻¹] were calculated from k₀ [s⁻¹]. The obtained dissolution rates k₀* are larger than reported values for silica glass and quartz. The zero-order dissolution rates k₀ [s⁻¹] for diatoms are similar to those for silica gels but with a lower activation energy (32 kJċmol⁻¹< 60 kJċmol⁻¹). 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.