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Interpreting Dolomite
In Sedimentary Rocks
The Problem of Dolomite
Limestone and dolomite are often found together, or closely associated, but their relationships are not always clear. " Pure" dolomite resembles micrite, being a fine-grained carbonate rock with conchoidal fracture, although it can be slightly granular too. But dolomite is a secondary rock produced by the chemical alteration of any preexisting carbonate rock, so in fact dolomites can be as diverse as limestone.
Origin of Dolomite
The origin of dolomite is one of the major unsolved problems in geology. Debate has raged for decades over whether dolomite is of "primary" origin (i.e. precipitated directly from sea water), or "secondary" origin (post-depositional changes of calcitic carbonates), and if secondary by what mechanism dolomitization occurs. No universal solution to this problem yet exists. Research into the problem requires a combination of biological and geochemical thermodynamics. The arguments are complex, confusing, and technical.
The consensus from most researchers is that virtually all dolomite is of secondary origin, which means all dolomite began as a calcitic rock (although this position is beginning to shift back the other way).
There is a lot of field evidence that many of the dolomitic rocks we find began as limestones of some kind. There is also a lot of evidence of the formation of dolomite from limestones through pressure solution. Pressure solution occurs when a carbonate rock begins to dissolve under pressure, either of deep burial, or tectonic squeezing. But there is also evidence that some modern limestones are undergoing dolomitization today, ususally in tidal flat environments. It is unlikely that there is only one mechanism of dolomitization, so at the moment we are left with some uncertainty and confusion.
How To Deal With Dolomitic Rocks
It is important that you recognize and identify dolomitic rocks when you encounter them. There are three problems you must deal with.
First, carbonate minerals form a solid solution series where the Mg++ and other ions freely substitute in the crystal structure of the minerals composing the rock. A carbonate rock can range from pure calcite to pure dolomite and every combination in between. By eye it is difficult to determine the degree of dolomitization of a rock, but it is roughly estimated by the vigor of the reaction with dilute HCl acid.
Second, not all dolomitic rocks are equally or uniformly dolomitic throughout. You may have to test different parts of the rock with dilute HCl acid while examining them under a microscope.
Third, even if you have determined a rock to be dolomitic you still want to know what its calcitic parent was. Sometimes this takes more careful observation than normal since dolomitization recrystallizes the calcite, partially destroying the original textures. Observe them closely.
In all these cases careful observation, comparisons of observations with others, and working with an expert may be necessary to learn these skills. One way or another, when you have found a dolomite, don't just call it a plain dolomite. Try to determine the parent, and name it this way, "dolomite after micrite", "dolomite after fossil limestone", or "partial dolomite after calcarenite."
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