Aragonitic Fossils in the Lower Carboniferous of Scotland

Abstract

ALTHOUGH many invertebrates form skeletons partly or wholly of aragonite, this normally inverts to calcite within a geologically negligible period of time, a few tens of thousands of years at the most1. Indeed, more than 50 per cent of the aragonite laid down by some of the marine invertebrates studied by Lowenstam2 inverted to calcite within a year. As the geothermal gradient lies completely within the stability field of calcite it has been maintained that aragonite within buried sediments could exist only in peculiar local conditions of high hydrostatic pressure and moderate temperature3. Aragonite in rocks of a respectable age may therefore be presumed rare, and any record of such an occurrence is of interest both because of the problems posed by the preservation of this metastable mineral and the desirability of obtaining unaltered shells for palaeo-ecological studies.