Journal of Petroleum Geology, Vol.10, No.1, pp. 73-86, 1987

©Copyright 2000 Scientific Press, Ltd.

PETROLOGY AND KINETICS OF GYPSUM -- ANHYDRITE TRANSITIONS

S. M. Billo*

* Geology Dept., King Saud University, Ad-Dir'yah, Riyadh, Saudi Arabia.

Abstract

Despite suggestions by some workers that most, if not all, the calcium sulphate in natural evaporites was originally deposited as gypsum, controversy concerning the origin of anhydrite has arisen, both because bedded anhydrite is common throughout much of the geologic record, and also as a result of the discovery of the recent anhydrite formations in the UAE, Persian/Arabian Gulf, and other areas. In addition, an inability to synthesize anhydrite in the laboratory under geologically reasonable conditions has caused the reliability of previous experimental methods to be doubted. However, new evidence from studies of chemical kinetics indicates that supersaturation and surface energies in the CaSO4--H2O system may regulate primary anhydrite precipitation. Arguments for and against the universality of gypsum as the sole origin of anhydrite are here related to specific facts and relevant observations from the fields of petrology, geology, and geochemistry.