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= Silica diagenesis = Silica diagenesis describes two dissolution/precipitation reactions of hydrated silicon dioxide based minerals. First, amorphous typically biogenic silica (opal-A) is transformed into cristobalite-tridymite lepispheres (opal-CT). Second, opal-CT is converted into microcrystalline quartz. We find direct evidence of these processes in scanning electron microscope (SEM) images showing partially dissolved biogenic opal-A next to precipitated opal-CT. Further evidence is based on the strength required to break silicon–oxygen bonds and kinetic data.

Kinetics
The kinetics of silica diagenesis have been analyzed with two approaches.The first approach describes the opal-A/CT and opal-CT/Q transformation as a whole and attempts to reproduce them under controlled conditions in the laboratory. Transformation ratios recorded in the laboratory are used to describe the transformation rate, for example by Arrhenius equation. This equation describes a chemical reaction dependent on temperature using two parameters: (1) activation energy and (2) the pre-exponential factor. Laboratory measurements determine an activation energy of 17.00 kcal/mol and a frequency factor of 6.30·109 a−1 for the opal-A/CT transformation as well as activation energies ranging from 14.30 to 27.49 kcal/mol and frequency factors from 4.09·109 to 4.24·1011 a−1 for the opal-CT/Q transformation. These parameters can be used to simulate silica diagenesis. The significant range of these parameters does however highlight the influence of factors other than time and temperature, for example surface area or pH of the pore fluids.

The second approach examines the chemical reactions during opal-A/CT and opal-CT/Q transformation separately. This involves studying opal-A and opal-CT dissolution, opal-CT and quartz precipitation, as well as a series of chemical reactions that silica experiences in solution. Several studies have shown that opal-A and opal-CT dissolution is influenced by time, temperature, pressure, pH and surface area. Generally, silica dissolution strongly depends on time and temperature as the dissolution rates increase exponentially with these two parameters. Pressure, on the other side, hardly affects silica solubility. Silica solubility increases with pH in basic solutions and exceptionally large surface areas, as observed on biogenic silica, can strongly increase silica solubility.

Silica precipitation is also controlled by numerous factors; including time, temperature, pH and the content of organic matter. Time and temperature appear to have a strong effect on silica precipitation. Precipitation rates of opal-CT may also depend on the pH of the solution. The higher the pH of the solution is, the higher is the precipitation rate of opal-CT. Since organic acids can reduce the pH of pore fluids, organic matter content may also influence silica precipitation rates.