Crystal growth, Science cover!

• Research Background
  Dolomite CaMg(CO3)2 is a thermodynamically stable carbonate mineral, accounting for approximately 30% of the carbonate minerals in crustal sediments. Despite its high geochemical content, sustained efforts by the scientific community for nearly two centuries have failed to grow dolomite in the laboratory under near-ambient conditions. A famous long-term experiment showed that in a dilute solution at 25°C, dolomite could not precipitate after 32 years despite being supersaturated 1000 times. The apparent contradiction between the abundant deposition of dolomite in nature and the inability of dolomite to grow from supersaturated solutions near ambient conditions is a long-standing mystery known as the "dolomite problem".
  

  
  

  
  

  Research Problem

  Through atomistic simulations, this study found that dolomite initially precipitates on a cationically ordered surface and that high surface strains inhibit further crystal growth. However, mild undersaturation preferentially dissolves these disordered regions, thereby increasing order upon reprecipitation. According to the simulation predictions of this study, frequent cycles of the solution between supersaturation and undersaturation can accelerate the growth rate of dolomite by up to seven orders of magnitude. This study verified the theory proposed in this study using in situ liquid-phase transmission electron microscopy, directly observing the massive growth of dolomite after pulse dissolution. This mechanism explains why modern dolomite occurs primarily in natural environments with fluctuating pH or salinity. It reveals that the growth and maturation of defect-free crystals can be promoted by intentionally mild dissolution periods.
  

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• Figure 1 | Dolomite crystal structure and growth surface

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  Key points:

  1. This study believes that the reason why dolomite cannot be precipitated is that its growth is inhibited. The atomic structure of the dolomite step edges provides important clues to understand the growth phenomenon of dolomite. Like calcite CaCO3, dolomite is a rhombohedral carbonate, except that dolomite has alternating layers of Ca2+ and Mg2+ perpendicular to the [0001] direction (Fig. 1). The main growth surface of orthorhombic carbonates is the closely packed (101-4) surface, while the main step edges for growth and dissolution are along the symmetrically equivalent [481-] and [4-41] directions.
  
  

  2. In ordered dolomite, Ca2+ and Mg2+ occur alternately along these directions. [481-] and [4-41] step edges (Fig. 1B), which means that Ca2+ and Mg2+ ions need to be deposited from solution onto the growing step edges in perfect alternating order. This ordered ion-by-ion attachment is a highly improbable zero-entropy process. If the entropy of disorder exceeds the enthalpy of order, then the initially deposited step edges will exhibit calcium/magnesium disorder. Experiments have shown that calcium/magnesium disorder does exist in the initially precipitated dolomite structure, and atomic force microscopy (AFM) experiments have measured a calcium-rich surface with a stoichiometry of approximately Ca1.7Mg0.3(CO3)2.

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• Figure 2 | Using kinetic Monte Carlo to simulate the step edge growth and ordering of dolomite through dissolution-reprecipitation under constant supersaturation conditions

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  Key points:

  1. This study first simulated the step edge growth of dolomite at constant supersaturation and 25°C, with ion concentrations of [Ca2+], [Mg2+], [CO32-]. This study plots the surface formation energy of dolomite under corresponding Ca/Mg composition and cation ordering conditions (Figure 2A). This study defines the ordering as: +1 means completely ordered, -1 means completely disordered (all cations all at the wrong site), 0 means completely disordered. For each possible surface ordering, this study demonstrates the lowest energy state at a given calcium/magnesium composition and cation ordering (Figure 2A). The orange trace in Figure 2A represents the time evolution of surface ordering during the dissolution-precipitation process calculated by KMC simulations.

  2. This study presents various snapshots of the simulation in Figure 2B. In simulations, dolomite step edge growth initially produces a cationically disordered surface, consistent with previous experiments, with an initial composition of Ca1.5Mg0.5(CO3)2, similar to previous cryogenic AFM experiments. By calculating the gauge ensemble and Helmholtz free energy of disordered surfaces, this study confirms that the entropy of cationic disordering exceeds the enthalpy of ordering during the early stages of dolomite growth.

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• Figure 3 | The role of supersaturation fluctuation in accelerating the orderly dissolution-reprecipitation process of dolomite

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  Key points:

  1. Generally speaking, laboratory studies of dolomite growth are conducted under constant supersaturation conditions. However, in natural environments, the supersaturation of fluids surrounding dolomite can be dynamic and fluctuating. Although modern dolomite deposits are rare, small amounts are often found in coastal and evaporative environments that experience cycles of precipitation (rain, snow, or hail) and evaporation. Excess fresh water causes unsaturation of the dolomite, thereby dissolving disordered surface regions of higher solubility (Fig. 3A and B). Subsequent evaporation causes dolomite supersaturation, promoting reprecipitation of dolomite at recently vacated surface sites, resulting in a slight increase in the overall ordering of the surface. In addition to salinity fluctuations, carbonate solubility also varies with temperature, pH, and other factors in the biogeochemical environment, which exhibit diurnal to seasonal fluctuations. For example, oxidation of organic matter in ocean pore waters can form carbon dioxide and carbonic acid, while transient low pH can also promote the dissolution of dolomite.

  2. This study repeated KMC simulations with solution saturation cycling between supersaturation (σ = 4.6) and varying undersaturations. The simulation results show that the dolomite ordering process shortens from 107 years at constant supersaturation to 105 years at undersaturation of σ = -1.5, 104 years at σ = -3.4, and 10 years at σ = -9.2 years (Figure 3C).

  
  

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• Figure 4 | In situ liquid cell TEM image of dolomite crystal growth

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  Key points:

  1. To verify the importance of dissolution in the dolomite growth process, this study designed an in-situ liquid cell transmission electron microscope (TEM) experiment in which the electron beam can both trigger dolomite dissolution and record it. The resulting crystal grows. In addition to low ionic salinity, the low saturation of dolomite may also be thermodynamically driven by low pH. This study uses a TEM beam dose rate of approximately 17 electrons/nm2/s to drive radiolysis of water molecules, thereby lowering the pH enough to dissolve dolomite. When the beam is turned off, the solution can reequilibrate to a supersaturated state within milliseconds. In this study, approximately 3 micron dolomite mineral crystals (Figure 4A) were placed in a supersaturated solution (σ = 3.1) flowing at 80°C; they were then cyclically dissolved between the electron beam on for 10 milliseconds and the electron beam off for 2 seconds. This cycle was repeated 3840 times in 128 minutes.

  2. By comparing the contrast of dolomite images before and after pulsed electron beam irradiation, this study directly confirmed the growth of dolomite. Since each dolomite nanoparticle is a single crystal and maintains its crystallographic orientation, the contrast of the TEM image is linearly related to the thickness of the nanocrystal. Comparing the initial boundary of the dolomite crystal (Fig. 4A and B, blue) with the boundary after 3840 dissolution cycles (Fig. 4B, red), it can be seen that the crystal has grown radially from 60 to 170 nm, corresponding to 200 to 200 nm, respectively. 560 dolomite single layers. The electron diffraction pattern of the newly grown region (Figure 4B and E, green) shows diffraction peaks corresponding to ordered dolomite.

  
  

• Summary and outlook

  From a crystal growth perspective, this study reveals the general idea that the growth and maturation of ordered crystals can be kinetically accelerated by deliberately mild dissolution periods. Any defect in a crystal - whether disorder, dislocation, impurity or other defect - will fundamentally be in a non-equilibrium state. Defected areas have higher energy than pure areas, so they dissolve faster and grow more slowly, resulting in a net flow of atoms from defective to pure locations over time. By deliberately introducing a slight period of undersaturation, it is possible to promote the dissolution of defects that would otherwise be very slow under sustained high supersaturation. In addition to low solute concentrations, temperature oscillations, etching, voltages during electrodeposition, redox potentials in natural soils, etc. can also lead to increased undersaturation.

  
  

  Literature links:

  https://www.science.org/doi/10.1126/science.adi3690

  DOI: 10.1126/science.adi3690