The 'dolomite problem’ that has puzzled scientists for centuries may finally be solved Research

Dolomite

Despite centuries of attempts to deposit dolomite in the laboratory, the mineral’s geological abundance has remained stubbornly mysterious. But now an international collaboration of researchers believes they have discovered the key to dolomite formation in the laboratory – cycles of enrichment.

Discovered 200 years ago by the French naturalist Diotd de Tolomeu, dolomite (CaMg(CO)3)2) is a calcium-magnesium carbonate mineral. Crystals can usually be grown from solutions that are more saturated than is normally possible in the laboratory – supersaturated solutions. However, although thermodynamically stable and geographically widespread, dolomite stubbornly refuses to grow from supersaturated solutions in the laboratory to near-ambient conditions. The so-called dolomite problem represents a fundamental mystery in crystal growth theory Wenhao SunA materials scientist at the University of Michigan, USA.

Using atomistic simulations, Sun and his team have developed a mechanism for dolomite formation and outlined a solution to the dolomite problem. They first used density functional theory (DFT) to compare the nuclear barrier of dolomite formation with that of other carbonate minerals. They found that it was similar in size to calcite and aragonite, indicating that dolomite precipitation was not nucleation limited, but growth was limited. In other words, dolomite didn’t fail to form under laboratory conditions because of difficulty getting started, but rather something later in the process.

Dolomite crystal rim

Dolomite crystals are highly ordered, exhibiting layers of calcium and magnesium separated by layers of carbonate ions. By examining the growth edge of dolomite crystals, Sun had what he described as the first eureka moment. 'The growth margin is ordered as Mg/Ca/Mg/Ca/etc. There is no way that the ions from the solution come in that exact continuous order! Entropy causes disorders.’ The research team calculated that the formation of ordered dolomite layers on an irregular 'protodolomite’ substrate is energetically unfavorable and self-limiting after a few layers.

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Sun explains that the second eureka moment came when his graduate student and first author Junsoo Kim highlighted Francis Rose’s video in a lab meeting at the Massachusetts Institute of Technology. Ross showed that by pulsing the voltage during the electrodeposition of copper, flat planar growth was obtained instead of fractal dendrites with high chemical potentials. It appeared to the team that by cycling from supersaturation to undersaturation, the disordered regions of protodolomite would dissolve faster than the ordered, and an ordered dolomite surface would gradually emerge. 'I remember jumping out of my seat and clapping at this point during the board meeting. I think I may have scared my students. It was definitely a classic eureka moment!’ Describes the sun.

Yuki Kimura

To confirm their hypothesis experimentally, Sun approached Yuki Kimura, an expert in crystal-growth interferometry at Hokkaido University in Japan. He suggested using in situ liquid cell transmission electron microscopy with a pulsed electron beam to lower the pH and induce dissolution, a solution that Sun says is 'very elegant’. The team placed a 3μm seed of crystalline dolomite in a small fluid cell filled with a supersaturated solution of calcium-magnesium carbonate. After 3840 oscillations of the electron beam over 128 minutes, they directly observed the growth of aggregate dolomite with a size of about 200nm. This corresponds to growing about 300 layers of dolomite – five more than have ever been grown in the lab before.

’It’s a bit counterintuitive to dissolve your sample to promote better crystal growth,’ explains Sun. „But the dolomite story teaches us that defects in materials are high-energy, and dissolution removes these high-energy defect regions first.’

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Some scientists are skeptical that these findings apply to nature. 'Although they were able to synthesize ordered dolomite in the experimental phase of their study, the cycle of saturation state of the precipitating solution would have been very difficult for the origin of many (most) massive dolomite units in geologic history. ’ he says Jay Gregg, Emeritus Professor of Geography at Oklahoma State University. But Surya disagrees. He notes that 'the geological record of where modern dolomite forms in nature is phenomenologically consistent with the concept of dissolution/redeposition cycles’. '[Joonsoo] The dolomite formation sites were all found to be associated with cycles of flooding and subsequent evaporation – which fits perfectly with our supersaturation fluctuation idea.’

Jennifer Roberts, a geologist at the University of Kansas, US, believes this experimental evidence can translate to natural systems. 'The theoretical work is sound, and they make systematic connections with complex ecosystems,’ he says. 'Challenging the prevailing view of the need for continuous supersaturation is particularly compelling as it translates to modern systems where we observe seasonal or episodic supersaturation/undersaturation.’

Sun hopes their work will spur efforts to test their ideas. 'What we have presented in our work is a mechanism, and the first experimental attempt to validate the mechanism. It is certainly not a final and definitive test.’ He remains open to other possible creation methods. 'Our theory … is not necessarily the only mechanism by which dolomite is formed, but it is a possible and sensible pathway.’

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