Page 180 - Petrology of Sedimentary Rocks

solutions in the shallow subsurface (so-called reflux) is coarser, but rather dirty with
lots of inclusions. Dolomite formed by very slow crystallization from dilute solutions,
at Mg/Ca ratios near the stoichiometric I:I, tends to be “limpid”, with perfect crystal
faces and water-clear (Folk & Siedlecka, 1974). The easiest way to obtain dolomite is
to reduce the salinity of a marine or hypersaline water by mixing in some meteoric
water --such as in a schizohaline environment that fluctuates rapidly between super-
high and low salinities, e.g., a hypersaline lagoon flooded by hurricane rains. Dilution
drops salinity radically but maintains a high MgICa ratio, dropping the water composi-
tion solidly into the dolomite field.

In surface waters of very low Mg/Ca ratio, Mg ions are removed from the rocks.
This results in replacement of dolomite by calcite (dedolomitization), and in recrystal-
lization of micrite to microspar or even pseudospar.

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