Description
Porcelanite is a white, sometimes dark blue or gray, low-grade metamorphic rock formed by silicification of the source rock. It forms under elevated pressure and temperature conditions at the edge of opal diagenesis, which originates in deep-sea sediments. The silicon in opal comes from the tiny skeletons of radiolarians, diatoms, sponge needles, or similar marine life with a siliceous skeleton, more rarely from volcanic silica. Porcelain opal is present in low-temperature modifications of cristobalite and tridymite. During diagenesis, it changes from porcelain to quartz, so that the rock is then called quartz chert. The transformation of porcelain into quartz chert is not only dependent on pressure and temperature but also a function of time. Therefore, the oldest known porcelain from the Lower Cretaceous period is found in the North Atlantic Ocean.
