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.
