The Coalgate Bentonite. A ferriferous-beidellite deposit from Canterbury, New Zealand

Abstract

AbstractThe Coalgate Bentonite deposit has been formed by the alteration of cryptocrystalline basaltic ash erupted in Upper Miocene to Pliocene times and deposited in a freshwater lake on a land surface of earlier tholeiitic flows. Physical and chemical analysis shows the bentonite to consist essentially of non-swelling (Ca2+ and Mg2+) ferriferous-beidellite with minor ferriferous montmorillonite. The process of montmorillonitization probably involved a crystal chemical structural reorganization without solution or precipitation and occurred in a mildly alkaline environment in which initial reducing conditions become oxidizing.