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Abstract

Dolomite is a rhombohedral mineral, CaMg(CO3)2; dolostone is the appropriate term for a rock composed of that mineral. Dolomite is best identified through staining, and by its rhombic, often zoned, untwinned habit.

Dolomite is a complex and relatively poorly understood mineral. Thermodynamically, dolomite should be a stable, widespread precipitate from seawater, but kinetic factors (hydration of Mg2+ ions in seawater, the high ionic strength of seawater, the relative efficiency of aragonite and high-Mg calcite precipitation, inhibition effects of SO42- ions) mitigate against its formation. Modern dolomite therefore is relatively scarce. In addition, ordered dolomite is slow-growing, and thus is difficult to synthesize in the laboratory under earth-surface conditions.

True dolomite (stoichiometric, ordered dolomite; top diagram, facing page) is well ordered, with one cation layer entirely composed of Mg2+ and the next entirely composed of Ca2+. If perfectly formed, that also ensures a 50:50 (stoichiometric) balance between Ca2+ and Mg2+ in the dolomite structure. Most modern dolomites, however, are poorly ordered and Ca-rich (termed “protodolomite” by some workers). Those crystals are relatively unstable and “ripen” or eventually neomorphose to more stable, ordered dolomite crystals.

Many models have been proposed for dolomitization (see excellent summaries in Morrow, 1982b; Land, 1985; Tucker, 1990; and Purser et al., 1994). All center around three basic factors: a source of Mg (generally seawater), a way to move large volumes of that water through the sediment package, and a way to reduce the kinetic inhibitions to dolomite precipitation. Sabkha and brine reflux models call upon evaporative concentration of seawater (with removal of sulfate through bacterial reduction or inorganic sulfate precipitation); marine-fresh water mixing zone and Coorong models rely on dilution of seawater; the burial model uses elevated temperatures, modified pore water compositions, and, in some cases, thermochemical sulfate reduction to reduce inhibitions on dolomite precipitation. Organogenic dolomitization relies on intense bacterial sulfate reduction and methanogenesis in organic-rich sediments in a wide range of settings (Mazzullo, 2000). Modern dolomite has been found in small volumes in many settings, ranging from hypersaline sabkhas to normal salinity tidal flats, and subsaline lagoonal environments. Modern dolomite is predominantly a replacement product; subsurface dolomites are found as either replacements or as primary pore-filling precipitates. Some authors have speculated that dolomites of other ages (especially the Precambrian) were primary precipitates, but that hypothesis has not been confirmed.

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