Lepispheres

Abstract

Lepispheres (from the Greek, meaning “spheres of blades”) are 3-micron to 20-micron-diameter rosettes of authigenic silica (SiO2) formed during low-temperature diagenesis (Figure L5). Coined by Wise and Kelts (1972) as a term to describe a secondary siliceous cement viewed by light and scanning electron microscopy in an Oligocene deep-sea chalk, lepispheres are now regarded as morphologic expressions of the incipient mineral phase in the formation of deep-sea chert (Wise et al., 1992). As such, they form in a variety of deep- to shallow-marine settings where precursor materials of amorphous silica such siliceous microfossil tests and volcanic glass are present. The unstable amorphous silica (opal-A of Jones and Signet, 1971) is converted to lepispheres (opal-CT) by a zero-order dissolution–diffusion–reprecipitation reaction. The metastable lepispheres may eventually convert to the more stable mineral phase, quartz (opal-C). In rock terms, the lepispheres may coalesce to form porcelanite, the intermediate phase in the formation of true quartz chert from a siliceous precursor material.