Opaline silica (opal-A) is thermodynamically unstable at surface conditions and readily transitions into opal-CT and at higher temperatures and pressures, quartz. Past work has used the temperature dependency of this phase transition for paleothermometry in opal-bearing sedimentary rocks. Those works determined that bulk concentrations of silica, organic material, and detrital minerals influenced the phase transition temperature. However, previous work only addressed the influence of these impurities on a macroscale and ignore potential microtextural interactions of silica and organic material. In this thesis, I present a set of experiments designed to characterize the effect of organic matter distribution on the opal-A to opal-CT transition in siliceous sedimentary rocks. Silica, humic acid, and mock seawater solution were loaded into Parr hydrothermal vessels as bulk sediments or in defined physical configurations (stratified or intermixed) and were heated at 200°C for up to 14 weeks. The solid products of these experiments were analyzed using X-Ray diffraction (XRD), Raman spectroscopy and Fourier-transform infrared (FTIR) spectroscopy. In organic-free experiments, the broad opal-A diffraction peak at 22° 2θ narrowed over the course of 14 weeks, suggesting increased internal ordering. FT-IR spectra showed changes in the position of the Si-O-Si stretching mode peak at 1060 cm-1 with silica diagenesis. Opal-CT may have been observed to occur sporadically in some stratified experiments and associated controls, but apparently did not occur in intermixed configurations. In stratified experiments, no correlation was observed between opal-A full-width half maximum and proximity to the humic acid layer. Similarly, no trend was observed between FT-IR peak positions and proximity to humic acid. These results suggest that the presence of organic matter does play a role in inhibiting the internal ordering of opal; however, the configuration might not be a primary factor in this transition. The results of this study may be applicable to the Monterey Formation and other hydrocarbon-bearing siliceous sedimentary formations. Our results suggest that the maturation of oil reservoirs in the Monterey Formation may be constrained solely on the silica phases present. / Geology