The carbonates are compounds of one or more metallic elements or semi-metals with the carbonate (CO32-) radical. One member of this class, calcium carbonate (CaCO3), is sufficiently common to form large bodies of limestone, a sedimentary rock. The carbonates are brittle minerals, and many of them break easily along crystal directions to develop the characteristic rhombohedral cleavage. A few carbonate minerals are in bright colors, but many of them are white, colorless, or transparent. Chesterman, p. 431
An admittedly Old Earth system the "Dunham classification" relies on empirical observation. The concept of grain support figures heavily in the Dunham classification, which divides carbonate rocks into two broad groups; those whose original components were not bound together during deposition and those whose original components formed in place and consist of intergrowths of skeletal material. The former group is further subdivided as to whether or not the grains are mud-supported or grain supported. The latter group are called boundstones. If the rock consists of less than 10% grains it is called a mudstone. If it is mud supported with greater than 10% grains it is called a wackstone. If the rock is grain supported, it is called a packstone, if the grains have shapes that allow for small amounts of mud to occur in the interstices, and a grainstone if there is no mud between the grains.
- Chesterman, Charles W. The Audubon Society Field Guide to North American Rocks and Minerals, Alfred A. Knopf: New York (1987)