Page 186 - Petrology of Sedimentary Rocks

then it certainly must be able to form in sea water as well. Blatt et. al. asserted that
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theoretical calculations show that normal sea water should indeed be capable of
producing dolomite, but that dolomite does not normally form because of the ordering
difficulty.

Dolomite in Dilute Meteoric Water. Most freshwater lakes and streams average
between SO-500-p-total dissolved salts, whereas most large rivers contain 50-150
wm. Most of these surface waters have Mg/Ca ratios between about I:I 0 and l:3;
thus, calcite should be the expected phase formed. However, some exceptional lakes
have an excess of Mg over Ca. Mill ler reported magnesi an calcite, aragoni te, and
protodolomite forming in sediments of Lake Balaton, Hungary, with a total salinity
under 500 ppm. But the Mg/Ca ratio in the lake varies from I:I to 3:1 and becomes
even higher in the interstitial waters of the muds where protodolomite is present.

In caves, dolomite and other assorted magnesian carbonates can form if the
MgICa ratio exceeds I:I. Dolomite can form in other freshwater deposits such as
spring tufas.

In caliches calcite is again the normal product, as the Mg/Ca ratio of most soil
waters is low. However, dolomitic caliche can form by attack of rain water upon Mg-
rich source rocks.

We conclude that dolomite can form in surface fresh waters with Mg/Ca ratios as
low as I :I providing crystallization rate is slow enough for ordering to take place. A
more rapid crystallization rate requires a greater excess of Mg/Ca. A high total
salinity is not in the least required; indeed, it tends to hamper crystallization of
dolomite and favor calcite or aragonite instead.

Dolomite in Subsurface Waters. Subsurface waters show an extremely wide range
in composition,from drinkable waters of as low as 100 ppm total salinity to very
concentrated brines as high as six times seawater salinity (White, 1965). Because of a
great loss of Mg relative to Ca in most sub-surface brines, the Mg/Ca ratio tends to be
low, between I:2 and l:4.

Subsurface waters of mainly meteoric derivation, though dilute, have Mg/Ca ppm
ratios that approach a limit very close to I :2 (molar ratios of 0.8 Mg to I .O Ca). This is
interpreted to be the point at which under sub-surface conditions calcite and dolomite
are both stable, consequently precipitation of either phase can occur with slight shifts
in composition. Of course, for large amounts of dolomite to be formed, enough time,
adequate total supply of Mg, and actively moving waters must be available.

In summary, these data on natural waters show that, in a hypersaline environment
with high ion concentration and rapid crystallization, the Mg/Ca ratio must exceed 5:1
or IO:1 for dolomite to form. In normal marine waters (such as deep-sea sediments),
dolomite probably forms at Mg/Ca values over 3: I. In sorne fresh water and low-
salinity subsurface waters, dolomite can form at Mg/Ca ratios as low as I:I because of
a lack of competing ions and a generally slower rate of crystallization. The lower the
salinity, the easier it is for dolomite to order.

On the diagram (fig. I), this leads to a diagonal kinetic boundary line for the
dolomite field. The fact that this boundary is diagonal is critically important. It means
that the easiest way to produce dolomite is by lowering the salinity, reducing the
concentration of competing ions, and in most cases slowing the rate of crystallization.
Dolomite also can form at any given salinity by raising the Mg/Ca ratio, but that is not
so common a process.

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