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Na-carbonate (NaCW) waters are not concentrated in well defined areas, but usually widespread in areas where other water types (e.g., Ca-carbonate) are dominant. NaCW are the product of long-term water-rock interaction with dissolution of Na-silicates in presence of phyllosilicates, silica phases, and calcite. NaCW circulating in calcite-bearing sediments very probably have a Ca-carbonate parent with moderate to low PCO2, which changes its composition assuming increasing Na character as the water-rock interaction proceeds. However, the moderate to low PCO2 values of the potential parent Ca-carbonate waters do not account for the high Na content of many NaCW. The higher PCO2 required may be due to oxidation of organic matter of the sediments and, perhaps, to further addition of CO2 coming from deeper crustal levels, from the mantle, or from other sources. Na-Ca exchange involving a Na-exchanger could be an alternative genetic hypothesis. At present, however, at least for some areas (e.g., Northern Apennines, Italy), this hypothesis is not supported by mineralogical evidence.

Na-carbonate waters of extreme composition: Possible origin and evolution.