An Experimental Study of the Recrystallization of Porcelanite and Its Hearing on the Origin of Some Bedded Cherts

Abstract

In the Monterey Formation of California, chert is at the base, overlain by porcelanite (fine-grained cristobalite), and diatomite toward the top; the biogenically precipitated silica has been progressively recrystallized from diatomite through porcelanite to chert. Hydrothermal experiments on the transformation of porcelanite to quartz show that the percentage of material recrystallized per unit time is constant. Extrapolation of the time required for complete conversion of porcelanite to quartz at 20 degrees C in presence of pure water pore fluid indicates a period of 180 m.y. Chertification occurs in the formation along bedding planes and fractures where apparently alkaline aqueous fluids have been present for long periods of time; earlier work has shown that alkaline solutions speed up the reaction rate.