FACTORS AFFECTING THE PRECIPITATION OF QUARTZ UNDER HYDROTHERMAL CONDITIONS

The nature and rates of quartz cementation have been experimentally characterized using natural samples of Pocono Sandstone and sized clasts of synthetic quartz. The experiments were conducted at 300° to 600°C and 150 MPa confining pressure for durations of up to four weeks. Samples charges were typically composed of a layer of amorphous silica powder that served as the source of silica, a brine solution (25 wt% NaCl), and solid AlCl3 powder. A series of experiments with FeO added to the powder or mixed with the clasts of synthetic quartz was also done. The samples were weld-sealed in Au-tubes and run in cold-seal reaction vessels. Because the maximum temperature gradient across the length of the sample charge is <5°C, the driving force for silica transport and cementation in the quartz aggregates is the concentration gradient driven by the much higher solubility of the amorphous silica. After the experiments, the samples were vacuum impregnated with epoxy containing a blue dye and sawn in half along the long axis of the sample charge. The nature and amount of silica transport and cementation in the samples was determined by a combination of Cathodoluminescence (CL), Light Microscopy (LM), and Scanning Electron Microscopy (SEM). Photomosaics of the samples were collected and the amount of cement, porosity, and average grain sizes were determined by point-counting. The cement was easily recognized from the quartz grains by the difference in luminescence, and in experiments with added FeO, the distribution of the opaque iron oxides was easily observed in LM.

The results indicate that the presence of amorphous silica can lead to rapid silica transportation and cementation in quartz aggregates even in the absence of a temperature or pressure gradient, which are commonly cited as driving cementation in nature. The amount of cementation is a function of time, temperature, and ionic strength of the brine. Although most of the cement was derived from the amorphous silica, evidence for local dissolution-precipitation was observed. Contrary to some studies, which suggest that iron inhibits quartz cementation, the addition of iron oxides consistently increased the amount of cementation under similar experimental conditions, although the exact mechanism for this observation is still under investigation.