Experimental Study of Quartz Overgrowths and Synthetic Quartzites

Synthetic overgrowths have been produced on both undulatory and non-undulatory detrital quartz grains using conventional hydrothermal techniques. Runs were conducted at 1, 2, and 3 kb., and at temperatures between 212° and 658° C. Aqueous pore solutions were 4.3 and 8.3 mole per cent NaCl or 1.7 and 8.3 mole per cent NaOH; silicic acid was added in some experiments as a highly soluble source of silica. The enlargements were produced in quantity only in experiments in which silicic acid had been added and fluid pressure was equal to total pressure. In all cases, enlargements are in optical continuity with their detrital nuclei. The initial stage of enlargement occurs as small rhombohedral projections on relatively strain-free portions of the grains. Individual rhombs do not overlap lineage boundaries in the host grain, but coalesce along the projection of such structural discontinuities; in this manner these discontinuities are propagated into the overgrowth. Unstrained grains exhibit more numerous and more fully developed overgrowths than strained grains. Overgrowths did not form on polycrystalline quartz grains. Synthetic quartzites with metamorphic textures were obtained in experiments in which Pfluid was less than Pₜₒₜₐₗ. Recrystallization, sutured grain boundaries, and undulatory extinction were produced only in runs employing 8.3 mole per cent NaOH solutions. In runs containing NaCl solutions of equal strength, lithifiation presumably is due to a submicroscopic film of silica between adjacent grains. Synthetic quartzites were not produced in experiments using more dilute solutions.