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Calcite rafts
Calcite crystals form on the surface of quiescent bodies of water, even when the bulk water is not supersaturated with respect to calcium carbonate. The crystals grow, attach to one other and appear to be floating rafts of a white, opaque material. The floating materials have been referred to as calcite rafts or “leopard spots.”
==Chemistry==
Calcium carbonate is known to precipitate as calcite crystals in water supersaturated with calcium and carbonate ions. Under quiescent conditions, calcite crystals can form on a water surface when calcium carbonate supersaturation conditions do not exist in the bulk water. Water evaporates from the surface and carbon dioxide degasses from the surface layer to create a thin layer of water with high pH and concentrations of calcium and carbonate ions far above the saturation concentration for calcium carbonate. Calcite crystals precipitate in this highly localized environment and attach to one another to form what appear to be rafts of a white material. 〔Taylor, P.M., Drysdale, R.N. and Carthew, K.D. (2004). “The formation and environmental significance of calcite rafts in tropical tufa-depositing rivers of northern Australia.” ''Sedimentology.'' 51:5 October. 1089. 〕
Scanning electron micrographs of calcite rafts show interconnected calcite crystals formed around holes on the raft surface. The holes may be caused by air bubbles or other foreign matter on the water surface.〔 Micrographs of calcite rafts show lace-like structure. The surface tension of the water keeps the interconnected calcite crystals, which individually have a specific gravity of 2.7, floating on the water surface.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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