TUFA

(Geology) The calcareous and siliceous rock deposits of springs, lakes, or ground water. Typically consist of calcium carbonate (CaCO3) deposits created by precipitation from supersaturated waters entering a cold lake from thermal springs. Some forms of shoreline tufa were created by an algal process in which carbon dioxide was extracted by the algae to produce insoluble calcium carbonate; this was then precipitated as aragonite (as opposed to calcite). Several primary forms of tufa have been identified: (1) Lithoid: deposited in superimposed layers, compact and stony; (2) Thinolite: made up of elongated skeletal crystals; (3) Dendritic: the most abundant variety, of branching structure; (4) Cellular: found as coatings; and (5) Coralline: occurring in heads and coatings. Also see Travertine.