Samples of porcelanite from the Monterey Formation (Miocene) of California were examined by scanning electron microscopy and were found to contain numerous small cavities partially filled with platy crystals. The crystals, interpreted as opal-CT, occur as linings on cavity walls and as lepispheres (microspherical aggregates) within cavities. Based on the distribution and radial structure of the lepispheres and on previous experimental studies of synthetic lepispheres, it is suggested that the crystals precipitated from colloidal silica that was concentrated within cavities formed by localized solution of the lithified porcelanite groundmass. A colloidal origin is also suggested for lepispheres reported from deep sea sediments and for cherts that exhibit pervasive microspherical textures.

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