Application of glauconite in sedimentary geochronology

Abstract

Glauconite is widely distributed in space and time. In China glauconite —bearing strata are of extensive occurrence ranging from Late Cretaceous to Middle Proterozoic in age. X-ray powder analysis, X-ray diffraction analysis and differential thermal analysis revealed that glauconite has a mica —type structure between dioctahedron and trioctahedron. Its chemical composition is: Al₂O₃ 10.67%, SiO₂ 49.23%, MgO 3.24%, Fe₂O₃ 17.40%, FeO 1.8%, and K₂O 7.92%. Its crystal form and thermal properties are very similar to those of mica—like clay minerals. According to isotopic age determinations of glauconite in modern oceans (including the East China Sea, the South China Sea and the Yellow Sea), we consider that in the extreme case glauconite is an authigenic mineral. It is generally formed in sedimentary rocks. It has further proved that glauconite can be used to determine the ages of sedimentary rocks.