Porcellanite
“Silica-cemented like fine china”
Porcellanite is a hard, dense siliceous sedimentary rock with a texture resembling unglazed porcelain — smooth, dull, and slightly waxy. It forms when opal-CT (a disordered form of cristobalite) cements siliceous sediments, representing an intermediate stage between soft diatomaceous sediment and fully crystalline chert.
opal-CT (cristobalite-tridymite), quartz, clay minerals, diatom/radiolarian remains, calcite (minor)
Monterey Formation (California, USA), North Sea chalk (flint precursor), Japan, Bering Sea sediments, Gulf Coast (USA)
5-6.5
Porcellanite in the Monterey Formation of California is a major petroleum reservoir rock — its fine porosity traps oil, making it responsible for significant oil production along the California coast.
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