Page 188 - Petrology of Sedimentary Rocks

characterized by dynamic water movement. In each case, mixing drastically lowers the
salinity, but the Mg/Ca ratio remains high because the amounts of Ca and Mg added by
fresh water are very small compared to the large amounts of these cations present in
sea water. Mg/Ca ratios only can be lowered to typical subsurface values of I:2 or
I:4 by crystallizing a large amount of dolomite or Mg-clays.

The failure to find gypsum in the supratidal flats of the Bahamas suggests that
some of the dolomite crusts may be formed by wet-season rain-water dilution of sea
water that is either of normal salinity or that has not yet evaporated down to the
gypsum stage; thus, the cause of the dolomite is, again, freshwater dilution.
Mg-purging--Incongruent dissolution of Mg calcites can cause dolomitization at
nearly constant salinity by increasing the Mg/Ca ratio of the pore waters (Land and
Epstein, 1970). Only minor amounts of dolomite, restricted to Mg-calcite allochems or
their internal pores, can be produced by this method, however.

Limpid Dolomite: Produce of Dilute Waters. Freshwater dolomite is characteris-
tically limpidmd Land, I9z;Secka, 1972; Land, 1973). Crystals are readily
identifiable with a low-power binocular microscope (or even hand lens or naked eye)
because of their mirror-smooth faces, reflecting sunlight like tiny faceted gems.
Crystals are usually perfectly brillant, transparent, and water-clear, lacking any
inclusions; they look like minute diamonds. Under the petrographic microscope they
appear as perfect euhedra with geometrically exact faces, and again are extremely
clear though some are zoned. At higher magnifications obtained by electron micro-
scope they appear as near-perfect crystals with absolutely plane crystal faces, lacking
any imperfections, inclusions, growth steps, or any other sort of blemish.

Concl usi ons. Dolomite, because of the difficulty of ordering required for
crystallization, can form most easily by slow crystallization. At more rapid crystalliza-
ti on rates, aragonite and magnesian calcite. crystallize because they are simpler
structures. Dolomite is favored similarly when solutions are dilute because few
impurities disrupt the precise ordering of the lattice.

Dolornite can form from any natural solution providing the Mg/Ca ratio is over
approximately I: I, even in lakes or subsurface waters of very low total salinity. An
important way to precipitate dolomite is to dilute sea water or sabkha-evaporitic water
with fresh water. Dilution allows the Mg/Ca ratio to remain very high, but slows the
crystallization rate and reduces the concentration of competing ions. Two ideal sites
where such a mixing mechanism can take place (schizohaline environments) are
floodable sabkhas or inundatable shallow lagoons where salinity undergoes rapid
fluctuation between hypersaline and nearly fresh conditions; another site is the
subsurface zone where sea water or evaporitic waters come into contact with a wedge
or lens of meteoric water and salinity reduction occurs. In both cases Mg is supplied by
saline waters, but precipitation is permitted only by dilution with fresh waters.

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