Researchers Resolved the Dolomite Problem

dolomite formations of the Italian Alps.

After 200 years, it was clarified: Unlike other types of limestone, dolomite crystals resist growing under normal conditions. However, the mystery of how extensive dolomite deposits formed has now been unraveled. Researchers have addressed this “dolomite problem,” revealing that simply providing the necessary elements in an oversaturated solution is insufficient for dolomite growth. Dolomite crystals only grow when subjected to intermittent dissolving conditions, eliminating obstructive misplacements in the crystal lattice, as reported by a team in the journal Science.

Whether gemstones, stalactites, or the giant crystals in the Mexican Naica Cave, crystals typically grow when dissolved salts precipitate and form an ordered structure called the crystal lattice. This process requires a solution to be oversaturated and the presence of impurities that can act as nucleation sites for crystallization. Over millennia and millions of years, this process has led to the formation of immense mountains and rock formations, primarily composed of the nearly ubiquitous variations of limestone.

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Structure of the growth edge of a dolomite crystal
Structure of the growth edge of a dolomite crystal: rows of magnesium (orange) and calcium ions (blue) alternate in order. In between are carbonate molecules (dark gray).

The given passage discusses an anomaly in the formation of dolomite, a variant of limestone. Despite dolomite constituting about a third of global limestone deposits and forming entire mountain ranges like the Dolomites, the process of its formation remains unclear. Attempts to cultivate dolomite crystals at room temperature in the laboratory have proven challenging, with growth ceasing after approximately five atomic layers. Even a notable long-term experiment spanning 32 years failed to precipitate dolomite at 25 degrees Celsius from a highly saturated solution.

The question that has perplexed scientists for over 200 years is: why? This apparent contradiction between the abundant natural occurrence of dolomite and the inability to cultivate dolomite crystals under normal conditions is referred to as the ‘Dolomite Problem,’ according to researchers. While numerous hypotheses exist regarding why dolomite does not crystallize at room temperature, none have been definitively substantiated—until now.

The Problem of Order

Increase in size of a tiny dolomite crystal during rapid alternation of deposition and solution under the electron microscope.
Increase in size of a tiny dolomite crystal during rapid alternation of deposition and solution under the electron microscope. Image:  Yuki Kimura, Hokkaido Universität.

In their quest for a solution to the dolomite problem, Kim and his team have scrutinized a particular characteristic of this type of limestone. In the crystal lattice of dolomite, magnesium and calcium ions alternate regularly. This means that these ions must adhere to the growth surface of the crystal in a perfect sequence from the solution, explain the scientists.

However, this does not happen as expected. In an oversaturated solution, calcium and magnesium ions consistently end up in the wrong positions. The consequences of this are demonstrated through an atomically precise simulation of the process conducted by Kim and his team using newly developed software. “Each atomic step would normally take over 5,000 CPU hours on a supercomputer,” they explain. Thanks to a new method, the complex density functional theory (DFT) simulation, encompassing energy levels and atomic interactions, could now be comprehended within seconds.

The Solution Lies in the Solution

The result: When the researchers bred the virtual crystal under constantly supersaturated solution conditions, the same phenomenon occurred in the laboratory. Incorrectly deposited ions obstructed the growth so significantly that it would take ten million years to create an ordered dolomite crystal. However, this changed dramatically when they altered the conditions and exposed the crystal nucleus alternately to supersaturated and undersaturated solutions. This led to phases of deposition and dissolution.

Under fluctuating conditions, the calcium and magnesium ions initially deposit on the crystal surface in a disordered manner. However, the subsequent dissolution phase corrected this disorder. “Every defect in a crystal—whether caused by disordered placement, dislocation, impurities, or otherwise generated—is fundamentally in a state of imbalance,” explain Kim and his colleagues. “Therefore, such faulty regions dissolve more easily.”

First Cultivation in the Laboratory

In concrete terms, this means for dolomite: Each dissolution phase selectively removes ions that have been incorporated into incorrect lattice sites. This eliminates the chemical-physical blockade, allowing the crystal to grow more rapidly in the next crystallization phase. Based on these findings, researchers successfully cultivated an ordered dolomite crystal in the laboratory for the first time. They utilized an electron microscope to observe a tiny seed crystal under varying solution conditions.

Indeed, after approximately 3,800 cycles of this time-lapse alternation, the minuscule dolomite crystal had grown by 60 to 170 nanometers. “This corresponds to 200 to 560 atomic layers,” explained Kim and his team. On average, each of these atomic layers required about ten solution cycles to be sufficiently purified for further growth.

How the Dolomite Mountains Grew

Kim and his team may have finally cracked the centuries-old dolomite problem. Their results explain how the massive dolomite formations in nature could have originated: the waters in which this lime variant crystallized over geological periods likely alternated between oversaturated and solubilizing conditions. This could have occurred, for example, through shifts between hot phases with intense evaporation and phases with diluting rainfall. Periodic changes due to tides are also a possibility.

This mechanism explains why modern dolomite is primarily found in natural environments with pH or salinity fluctuations,” say the researchers.

However, this new insight is not only relevant to geology. “If we understand how dolomite grows in nature, we might learn new strategies to promote the crystal growth of modern technological materials,” says senior author Wenhao Sun from the University of Michigan. Ordered, defect-free semiconductor crystals could possibly be cultivated faster and more easily through such shifts between solubilizing and precipitating conditions.

Featured Image: Kandoo Adventures