A relationship exists between the mineralogy and the environment of deposition of Recent carbonate sediments; deep-sea carbonate sediments contain abundant low-magnesian calcite, whereas shallow-water sediments are high in high-magnesian calcite and aragonite. The mineralogy of carbonate sediments in a deep-sea environment is partly controlled by sedimentation and partly by dissolution of high-magnesian calcite and aragonite. In deep-sea sediments (1000–2000-m depth) of the Red Sea which were studied, the carbonate sediments are composed of high-magnesian calcite and low-magnesian calcite with aragonite absent or present only in subordinate amounts; in deep-sea sediments of the Indian Ocean (3676-m depth) only low-magnesian calcite is found. In carbonate sediments deposited rapidly by turbidity currents on the Bermuda Apron to depths of 4515 m, aragonite and high-magnesian calcite have been preserved in the sandsized fraction, but have been removed by dissolution in intercalated carbonate silt and ooze. Mineralogical changes under deep-sea conditions are a function of water depth and grain size. Under deep sea conditions, low-magnesian calcite is the most stable, and high-magnesian calcite appears to be more stable than aragonite.