Scientists solve the 200-year dolomite problem

For over two centuries, geologists have puzzled over the "Dolomite Problem": the mineral dolomite (CaMg(CO₃)₂) is abundant in ancient sedimentary rock formations but nearly absent in modern environments. Moreover, scientists had been unable to synthesize dolomite in the laboratory under conditions mimicking natural environments near ambient temperatures.

In November 2023, a collaborative research team from the University of Michigan and Hokkaido University resolved key aspects of this long-standing mystery. Their findings were published in the journal Science.

Atomic-scale defects and the role of dissolution

The researchers discovered that the primary barrier to dolomite growth is the formation of atomic-scale defects on the crystal surface. During crystallization from a solution containing calcium and magnesium ions, these ions must occupy alternating, ordered positions in the crystal lattice. However, ions often attach in disordered arrangements, creating surface strains that inhibit further layer-by-layer growth.

In ancient natural environments, these defects were periodically removed through cycles of dissolution. When the surrounding solution became undersaturated-due to events such as rainfall or tidal fluctuations-the disordered, high-energy regions of the crystal surface dissolved preferentially. This "cleaning" process left behind a more ordered template, enabling the next layer of properly structured dolomite to form.

Computational simulations and experimental verification

To test this mechanism, the team used advanced atomistic simulations based on density functional theory and cluster expansion methods. These models revealed the energy barriers for ion attachment and detachment at the mineral-water interface. By simulating repeated cycles of supersaturation and undersaturation, the researchers demonstrated that dolomite could grow under low-temperature, near-ambient conditions if defects were periodically removed through mild dissolution.

The theoretical predictions were experimentally verified using aberration-corrected transmission electron microscopy (TEM) with a liquid cell. Researchers at Hokkaido University, led by Yuki Kimura, employed a pulsing electron beam to manipulate the local chemistry of a supersaturated calcium-magnesium carbonate solution. The beam induced controlled cycles of dissolution and re-precipitation.

In the experiment, a dolomite seed crystal grew by approximately 200-300 nanometers after thousands of pulses over about two hours-an unprecedented observation of bulk dolomite growth in the laboratory. This corresponds to the formation of hundreds of ordered atomic layers, far exceeding previous attempts that managed only a few layers.

Implications for geology and materials science

The study explains why dolomite is predominantly found in geological strata that experienced frequent environmental fluctuations, such as coastal areas, sabkhas, or lagoons with alternating wet-dry or tidal cycles. In modern oceans, stable supersaturated conditions without sufficient dissolution cycles prevent significant dolomite formation.

Beyond geology, the findings offer broader insights into crystal growth. Periodic mild dissolution can help eliminate surface defects, providing a strategy for growing high-quality, defect-free crystals. This approach may prove useful in materials science for manufacturing semiconductors, thin films, and other precision crystalline materials where defects limit performance.

Note: This discovery does not claim to explain every instance of dolomite formation in nature-dolomite can also form through replacement of precursor minerals-but it provides a robust mechanistic solution to the long-standing laboratory synthesis paradox and the scarcity of primary dolomite in modern settings.