Exsolution of Dolomite from Calcite

Abstract

Calcites from a variety of marbles have been investigated by single-crystal X-ray techniques. Most of the specimens were relatively high-grade metamorphic carbonates, from amphibolite and granulite facies rocks. The typically clouded calcite of these rocks commonly contains dolomite, revealed by the X-ray photographs but not always observed under the microscope. This finely disseminated dolomite is in the same crystallographic orientation as the host calcite and is almost certainly an exsolution product. An oriented intergrowth of dolomite in calcite was produced in the laboratory. The original material was an echinoid test made up of spongy but single-crystal calcite with approximately 10 per cent $MgCO_3$ in metastable solid solution. This amount of Mg is stable in calcite only at temperatures well above 600° C; therefore exsolution of some of the Mg as dolomite took place when the sample was heated under $CO_2$ pressure at 500° C. The host magnesian calcite retained its identity as a single crystal, an...