Morphology of calcium carbonate crystals is controlled mainly by rate of crystallization, and Mg and Na content of the precipitating waters. Together, these factors integrate to provide important indicators of environment. Magnesium selectively poisons sideward growth of calcite; thus CaCO 3 prefers to crystallize as aragonite, or as minute fibers or steep rhombs of magnesian-calcite, whose sidewise growth is generally stopped at widths of a few microns. Thus in Mg-rich environments, such as beaches or marine bottoms, micritic or fibrous aragonite and magnesian calcite cements form. As seawater is buried, Na usually remains high but Mg is selectively lost so that the Mg/Ca ratio drops from 3:1 to about 1:3. Thus, in the absence of Mg-poisoning, coarse sparry calcite cement can form in the subsurface, and crystallizes as irregular polyhedra. In meteoric waters, both Mg and Na are very low. If precipitation is very rapid, calcite micrite may form (caliche). Fresh-water calcite can also occur as euhedral rhombs in very dilute solutions. In the phreatic-meteoric zone, sparry calcite develops. Carbonate ooze initially contains much Mg. Upon lithification, it is proposed that much of the Mg is retained as a sort of "cage" around each polyhedron of calcite, preventing growth beyond a few microns. Fresh-water flushing removes this Mg-cage, and allows recrystallization to coarser microspar.