Dolomite is a mineral that forms the sedimentary rock called dolostone, which is widespread in the ancient geological record. The mineral is a calcium magnesium carbonate. Dolostone is composed predominantly of this mineral and is similar to limestone, which is primarily calcium carbonate. Despite its abundance in older rock strata, dolomite is notoriously difficult to form in modern environments or synthesize in a laboratory. This disparity between its presence in the past and its rarity today is known as the “Dolomite Problem.” Scientists explain the vast deposits of dolostone using two primary formation mechanisms: direct precipitation in specific hypersaline settings, and secondary replacement of existing limestone.
The Core Challenge: The Dolomite Problem
The scarcity of dolomite today stems from significant chemical and kinetic barriers that prevent its precipitation at low temperatures. Unlike calcite, which readily forms from supersaturated waters, dolomite requires the simultaneous and orderly arrangement of calcium and magnesium ions within its crystal lattice. This alternating structural arrangement is energetically favorable but kinetically slow to achieve. The most substantial kinetic hurdle is the hydration shell surrounding the magnesium ion in solution. Each magnesium ion is tightly bonded to six water molecules, creating a stable complex. Incorporating the magnesium ion into the growing crystal requires stripping away this hydration shell, a process demanding significant energy input.
This high energy requirement inhibits dolomite growth in natural waters at ambient temperatures. Although seawater is supersaturated with magnesium and calcium, the mineral does not precipitate spontaneously. Consequently, the formation process is extremely slow, often resulting in a less ordered form called protodolomite, rather than the perfectly ordered structure found in ancient rocks.
Direct Formation in Extreme Environments
While large-scale dolomite formation is rare today, direct precipitation occurs in a few localized, extreme modern settings. These environments feature conditions that help overcome the kinetic barriers of the magnesium hydration shell. Hypersaline settings, such as coastal sabkhas and evaporitic lagoons, are the most common examples.
In these environments, intense evaporation increases the water’s salinity far beyond that of normal seawater. This increases the concentration of magnesium relative to calcium, which helps destabilize the hydration shell and pushes the water closer to the saturation point for dolomite. High temperatures in these arid environments also provide energy necessary to remove water molecules from the magnesium ion.
Microbial activity is recognized as a facilitator of direct dolomite precipitation, known as biogenic formation. Certain bacteria, particularly those in microbial mats, can alter the local geochemistry within the sediment. They increase the alkalinity of the pore water or produce organic molecules that promote the dehydration of the magnesium ion, acting as a catalyst for nucleation. This microbially-mediated precipitation results in the formation of small amounts of microcrystalline dolomite.
Replacement Process: Dolomitization
The vast, thick formations of dolostone found throughout the ancient rock record are primarily explained by a secondary mechanism known as dolomitization. This process involves the chemical replacement of pre-existing calcium carbonate rock, such as limestone, by magnesium-rich fluids over geological time. This mechanism requires a constant supply of magnesium ions and a physical mechanism to pump large volumes of fluid through the rock.
Reflux Dolomitization
One widely accepted model for large-scale fluid movement is Reflux Dolomitization. This occurs in arid coastal areas where lagoons or tidal flats experience high evaporation rates, creating dense, hypersaline brines. Because these brines are much denser than normal seawater, they sink through the underlying carbonate sediments, continuously supplying magnesium ions to replace the calcium in the original limestone. This process is capable of pervasively dolomitizing entire carbonate platforms.
Burial Dolomitization
Burial Dolomitization accounts for dolostones formed deep within the subsurface. As sediments are buried, increasing temperatures and pressures mobilize magnesium from surrounding shales or trapped seawater. The elevated temperatures, often 50 to 80 degrees Celsius, help overcome the kinetic barrier to dolomite formation. These conditions drive the reaction, as magnesium-rich brines expelled from compacting sediments move through adjacent limestones causing replacement.