Page 185 - Petrology of Sedimentary Rocks

Many factors control the precipitation of dolomite. Among those known to be
important are temperature, solution, composition and concentration, rate of crystalli-
zation, and the presence and concentraton of certain organic compounds. But
apparently the most important relations can be explained relatively simply by a diagram
in which salinity is plotted against Mg/Ca ratio of the depositing solution. (Folk and
Land ‘72 GSA abstra., I975 AAPG).

The key idea is that dolomite is a very difficult mineral to form because of the
precise Ca-Mg ordering required. Ideal dolomite consists of carbonate sheets inter-
layered with alternating sheets consisting entirely of Ca atoms or entirely of Mg atoms.
These ions have such similar properties that it is difficult to segregate them, and
segregation of ions to produce dolomite is accomplished more readily if the crystalliza-
tion rate is very slow, or if solutions are dilute, with few interfering ions. Crystalliza-
tion of dolomite is much more difficult if crystallization is rapid or if there is a high
concentration of competing ions.

With a high concentration of Ca, Mg, and CO3 ions, such as exists under
evaporitic conditions, a highly oversaturated solution can lose energy by forming the
improbable and difficult dolomite structure, or it can lose a smaller, but still
substantial amount of energy by crystallizing as the simpler calcite or aragonite
structures. In most cases the easier CaCO3 mineral will form, and the MgICa ratio in
the remaining solution will rise to extreme values, as is observed in evaporitic
environments. Only when extreme concentrations are reached is Mg finally forced to
precipitate, as a poorly ordered, Ca-rich “protodolomite.”

If the concentration of ions is much lower, such as in most meteoric waters, there
is not nearly so much interference in crystal growth caused by lattice impurities. At
slow rates of crystallization minerals can form that are more nearly in theoretical
equilibrium with the surrounding solution; and in the case of dolomite the careful
ordering of Ca and Mg which is required can be accomplished from solutions having
MgICa ratios near the theoretical value of I : I.

In the diagram here presented the dolomite field is divided from the calcite field
by a diagonal line indicating that, at low salinities (and low crystallization rates),
dolomite can form readily at Mg/Ca ratios as low as I:I, but as salinity and
crystallization rate increase, the Mg/Ca ratio at which dolomite is first able to form
rises until it must surpass 5:1 or IO:1 in hypersaline sabkhas. The line shows a
generalized tendency and is a kinetic, not a thermodynamic, boundary; it is not meant
to be a precise separator, because temperature, presence of various foreign ions, etc.,
can shift the precise position of this boundary.

Hypersaline Environments. Precipitation of gypsum and consequent loss of Ca
raises the Mg/Ca ratio of the brine to values of 5:1 or more (by weight) sometimes as
high as lOO:l. Total salinity can rise from about five times that of normal sea water
(at which point gypsum precipitates) to values as high as IO: I or more. Kinsman (1965)
has shown that hypersaline dolomite beings to crystallize only when the Mg/Ca ratio
exceeds 5:1 to lO:l. Any carbonate crystallized below such high Mg/Ca ratios comes
out as the easier-to-form aragonite or magnesian-calcite.

Dolomite b Normally Saline Bays and Deep Sea. Normal sea water has a Mg/Ca
weiqht ratio of close to 3:l. Considerable controversy exists over whether dolomite
’
can-form in marine water of normal salinity, e. g., in nonrestricted bays or in deep-sea
sediments. If dolomite can form in both hyp&saline environments and in freshwater,

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