Composition and origin of the ‘zebra rock’ from the East Kimberley region of Western Australia

Abstract

The origin of the peculiar banded claystone known as ‘zebra rock’ from the Ord River area of Western Australia has long been shrouded in controversy. A re‐examination of the mineral composition indicates that the principal phases include kaolinite, dickite, quartz, sericite, alunite and hematite, and that the amount of each varies appreciably even within a single hand specimen. The unusual composition and structure of the rock are attributed to authigenesis involving an attack on original sericite and kaolinite by sulphate ions produced by the oxidation of pyrite. Alunite, aluminium and ferrous sulphate, and silicic acid were the initial products; but with subsequent leaching of the sulphate ions, aluminium combined with silicic acid to yield dickite. Simultaneously, the ferrous ions were converted to colloidal ferric oxide which periodically coagulated, resulting in diffusion or Liesegang banding.

Key words:

• alunite
• dickite
• kaolinite
• Liesegang banding
• zebra rock