The mineralogy and important trace elements of Recent carbonate sediments and environmentally comparable Pleistocene carbonate rocks have been studied. All of the rocks and most of the sediments employed were obtained in southern Florida. Additional sediment samples came from the Bahama Banks, the South and East China seas, and the Persian Gulf. Most sediments were found to consist of aragonite, high-Mg calcite, and low-Mg calcite. Neither dolomite nor vaterite was encountered. Shallow-water carbonate sediments contain an average of about 70% of unstable forms of CaCO 3 with aragonite predominating and high-Mg calcite dominant over low-Mg calcite. Deep-water carbonate sediments are composed predominantly of low-Mg calcite, and high-Mg calcite dominates aragonite. Compositional differences between shallow- and deep-water carbonates are believed mainly to result from differences in the importance of contributions of skeletal material by certain groups of organisms in the 2 environments. The mineralogy of the Pleistocene carbonate rocks studied showed them to consist of low-Mg calcite and indicated that under nearsurface conditions in nature a stability sequence runs as follows: low-Mg calcite>aragonite>high-Mg calcite. Diagenesis of carbonate sediments is accompanied by an important loss in the level of abundance of the elements Mg, Sr, Ba, and Mn. These elements, including Mg, are lost from the chemical system and do not appear to form new minerals in place, although they may later do so at some other place. Since the chemical system is not closed, the significance of important volume changes which accompany diagenesis can not yet be assessed.