The presently available data on the thermodynamic properties of water and the solubilities of quartz and calcite suggest the following conclusions: (1) Both calcite and quartz can be deposited when two solutions of different temperatures are mixed at constant pressure. (2) Quartz deposition should predominate over calcite above 300 degrees C but calcite should predominate at temperatures below 150 degrees C when pressure-temperature changes in rising solutions cause the precipitation. (3) Replacement of quartz by calcite and calcite by quartz may be caused by transport of CaCO 3 and SiO 2 along thermal gradients.

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