Freshwater carbonate cements in nonmarine sediments form in a diverse assemblage of settings, including fluvial, lacustrine, pedologic, spring, and spelean environments, giving rise to a plethora of textures and structures. Many deposits exhibit both phreatic and vadose zone textures. Cement mineralogy and composition vary considerably from deposit to deposit, as well as within individual deposits, depending on water chemistry and environmental setting. Similarly, cement habits range from highly acicular to nearly equant. The wide variety of textures, mineralogies, and compositions exhibited by both cement and associated sediments suggests that freshwater carbonate precipitation may involve complex processes. A survey of our present knowledge indicates that such cements a e most commonly composed of low magnesian calcite as crystals with rhombohedral terminations. Among those features which appear to be unique to freshwater carbonate cements are crystals displaying trigonal prisms, rhombohedrons ornamented with parallel sharp spikes, and crystals with thorn-shaped vacuoles. Although variation in crystal habit may be influenced by either magnesium or total cation or anion concentrations in the precipitating fluid, the concentration of solutes does not appear to be the sole controlling factor. Growth rate, influenced by a variety of parameters, such as P_CO2, may be the most important factor in predicting crystal habits in freshwater carbonate cements.

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