The Dolomite Problem

Dolomites are calcium carbonate rocks composed of more than 50 percent of the mineral dolomite [CaMg(CO₃)₂], and to avoid confusion between the rock and the mineral they are sometimes referred to as dolostones or dolomite rock. ^[1] They are abundant and widely distributed in the geologic record, ranging in age from Precambrian to Holocene, although the greatest volumes of dolomites are Paleozoic and older. ^[1] Dolomites occur in close association with limestones and as interbeds within limestone sequences, and are commonly associated with evaporites. ^[1] Because dolomites recur so frequently in the stratigraphic record, they must have formed under conditions that were relatively common and repeated at many localities across geologic time.

What Makes the Dolomite Problem a Problem

The origin of dolomites remains one of the most thoroughly researched but poorly understood problems in sedimentary geology. ^[1] Many coarsely crystalline dolomites are clearly secondary rocks — they preserve relict limestone textures, proving they formed by diagenetic replacement of older limestones — but many fine-crystalline dolomites lack such textural evidence of replacement and cannot be proven to have originated by diagenetic alteration of limestones. ^[1] It is these fine-crystalline dolomites that created the so-called dolomite problem, which geologists have been unable to satisfactorily resolve. ^[1]

The problem has two linked dimensions. First, there is a chemical kinetics obstacle: scientists have not succeeded in precipitating perfectly ordered dolomite in the laboratory at normal Earth-surface temperatures (~25°C) and pressures (~1 atm). ^[1] Elevated temperatures exceeding 60°C are required to produce stoichiometric dolomite in the laboratory. ^[1] Second, even when lab conditions are made favorable, the product is not genuine dolomite but protodolomite — a dolomite-like material that contains excess CaCO₃ in its structure and is therefore not a true (stoichiometric) dolomite. ^[1] Because low-temperature experiments yield only protodolomite, geochemists cannot determine directly from experimental work what geochemical conditions favor dolomite precipitation in natural environments. ^[1]

Stoichiometric Dolomite Defined

Perfectly ordered (stoichiometric) dolomite has exactly 50 percent of the cation sites filled by Mg and 50 percent filled by Ca. ^[1] This strict alternation of Mg and Ca planes is what distinguishes dolomite from high-magnesian calcite, in which Mg substitutes randomly and disorderly in the calcite crystal lattice. The ordered state is chemically stable but kinetically slow to form — at low temperatures, the system simply cannot organise itself into the ordered structure quickly enough to compete with the formation of disordered carbonates.

Why High Temperature Is Needed

The kinetic difficulty with forming dolomite at low temperatures relates in large part to the hydration of the Mg²⁺ ion — it is strongly bound by water molecules in solution and must be stripped of this water shell before it can enter the solid dolomite crystal lattice. ^[1] At low temperatures, Ca²⁺ ions, which are much less strongly bound by water, are more likely to enter the lattice and form CaCO₃ minerals. ^[1] At elevated temperatures, Mg²⁺ ions are less strongly hydrated and thus more easily desolvated, allowing the naked Mg²⁺ ion to enter the crystal lattice to form dolomite. ^[1] A second kinetic barrier is the highly ordered state of dolomite itself: nucleation and growth of the well-ordered dolomite lattice in a solution saturated in calcium bicarbonate are so slow that, in competition for available Ca²⁺ and CO₃^2– ions, well-ordered dolomite is prevented from forming, and aragonite or cation-disordered high-magnesium calcites form instead. ^[1] In fact, at temperatures exceeding about 100°C, most kinetic inhibitors, such as Mg²⁺ hydration, become ineffective. ^[1]

Modern Dolomite Occurrences

Despite the laboratory problem, geologic evidence confirms that dolomite does form naturally at or near surface temperatures. ^[1] Since the mid-1940s, modern dolomite sediments have been reported from localities including Russia, South Australia, the Persian Gulf, the Bahamas, Bonaire Island off Venezuela, the Florida Keys, the Canary Islands, and the Netherlands Antilles, with radiocarbon ages estimated to range from a few years to about 4,000 years. ^[1] Most of these modern dolomites are not perfectly ordered; their mole percent MgCO₃ ranges from about 30–50 percent but falls mainly between 40–46 percent. ^[1]

The discovery of modern dolomite initially led some workers to argue it was precipitated directly as a primary deposit, while others proposed that these modern dolomites formed by dolomitization — rapid alteration of an initial CaCO₃ precipitate to dolomite. ^[1] Subsequent research has not established the relative importance of precipitation versus replacement in the origin of these penecontemporaneous dolomites, and both mechanisms remain viable. ^[1]

Early-Formed vs. Burial Dolomites

Early-formed dolomites are all those that form at or near the surface in the unconsolidated state, as opposed to diagenetic dolomites that form during burial and uplift by replacement of older, consolidated limestones. ^[1] The distinction matters because the volume of dolomite in the modern environment is small, raising a second dimension of the dolomite problem: can the processes responsible for early-formed dolomites account for the vast dolomite deposits of the geologic past? ^[1] That question remains unresolved — the scale mismatch between modern occurrences and the immense ancient dolomite sequences found in the stratigraphic record is one of the deepest unresolved issues in carbonate sedimentology.

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