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Carbonate precipitation is governed by both thermodynamic and kinetic considerations. The concentration and ionic composition of natural solutions influence not only the mineralogy of the precipitate, but also the crystal size and habit of both metastable and stable phases.
Under marine conditions sparry calcite does not precipitate because of poisoning of sideward crystal growth by Mg, which forces Mg-calcite and aragonite to assume a micritic, steep-rhombic, or fibrous habit. Blocky calcite can form only in low Mg/Ca waters, either from primary meteoric solutions, or where Mg has been removed from marine water by dolomite or clays.
In hypersaline environments precipitation is commonly rapid and, together with the high concentrations of foreign ions, it is difficult for dolomite to form because of the precise Ca-Mg ordering required. Instead, aragonite or Mg-calcite crystallizes. Dolomite can form only if the Mg/Ca ratio exceeds 5-10:1, and even then it is aphanitic and poorly ordered. At progressively reduced salinities dolomite is able to nucleate at still lower Mg/Ca ratios, approaching 1:1 in meteoric waters. The absence of foreign ions and slow crystallization, commonly under phreatic conditions, cause both dolomite and calcite to form exquisitely limpid, euhedral rhombs.
Thus, dolomite probably forms most readily by a reduction in salinity, particularly in a schizohaline environment (alternating between hypersaline and near-fresh conditions, as in a floodable sabkha or a phreatic mixing zone). Flushing marine saline waters with fresh water lowers salinity but maintains a high Mg/Ca ratio; crystallization is slower and the interfering effect of foreign ions is reduced.
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