Mineralogy of silica phases in deep-sea cherts and porcelanites

Abstract

Cherts and porcelanites of Late Mesozoic to Cainozoic age, recovered at many sites in all major ocean basins by the Deep Sea Drilling Project, contain both quartz and disordered cristobalite as the principal silica phases. The cristobalite is identified as opal-CT and occurs as spherical microcrystalline aggregates of bladed crystals (= lepispheres ) that evidently formed by a solution step. This material represents a metastable intermediate stage in the conversion of amorphous biogenous and hydrothermal silica to quartz. Opal-CT also occurs in siliceous formations on land, both sedimentary and volcanic, but only in those of post-Jurassic ages. Many older cherts now composed entirely of quartz may also have formed from such an intermediate phase but all traces of such a precursor have been removed by recrystallization.