Structural Variations in Carbonates

Abstract

There has been considerable recent interest in the stabilities of carbonate minerals within the Earth. Calcite is the dominant C-bearing phase in the Earth’s crust, and acts as a buffer for the long-term cycling of CO2 between the atmosphere, oceans and the solid Earth (Berner 1994; Bickle 1996). It has been shown that aragonite and dolomite (Kraft et al. 1991) or calcite (Biellman et al. 1992) are stable at pressures of 30 GPa and temperatures of the order of 2000 K. The controls on the stabilities of the carbonates are, of course, defined by the way that their crystal structures respond to varying temperature, pressure and composition. Much of the literature in this area is already reviewed in the excellent chapters of Volume 11 of Reviews in Mineralogy ( Carbonates: Mineralogy and Chemistry , edited by R.J. Reeder, 1983). Here, therefore, I focus on aspects of carbonate crystal chemistry that have been of particular interest since the publication of that volume, and that have seen significant advances in understanding. Below, I discuss the recent work into the nature of polymorphism in the rhombohedral carbonates, in particular the high-temperature disordering transition in calcite and the high-pressure low-temperature metastable modifications of the calcite structure. Transitions in the aragonite-related orthorhombic carbonates at very high pressures are then surveyed, followed by a brief review of recent work on the breakdown of dolomite at high pressures and temperatures. The penultimate topic dealt with here is that of the compression characteristics of the rhombohedral carbonates, and I end with an indication of the breadth of work ongoing into new mineral structures that employ the carbonate ion as a structural building block. I refer the reader to the reviews of Reeder (1983) and Speer (1983) for an introduction to the underlying crystal chemistry of the rhombohedral …