A survey of the carbonate mineralogy of cheilostome Bryozoa

The bulk skeletal carbonate composition of several Holocene species of cheilostome Bryozoa was determined by X-ray diffraction and differential straining techniques. Carbonate exoskeletons of the species studied were composed either wholly of calcite, wholly of aragonite, or of mixtures of these two calcium carbonate minerals. The amount of MgCO3 in solid solution with the calcite generally ranged from four to eight percent, with a maximum of 13 percent. The majority of the species studied belonging to the suborder Anasca were calcitic; the exceptions were the species characterized by free-living lunulitiform colonies, which were either wholly aragonitic or of mixed mineralogy. Species belonging to the suborder Ascophora ranged from wholly calcitic, through various calcite-aragonite mixtures, to wholly aragonitic. In Ascophora, having mixed mineralogy, the site of aragonite deposition is typically the outer surface of the frontal wall. The amount of aragonite present appears to be related, in part, to maturity of the colony. The rate at which individuals of the colony mautre may be regulated to some extent by environmental parameters such as water temperature.