Andrew G. Stack

Ferromanganese nodules from the Clarion-Clipperton Zone (CCZ) located in the eastern Pacific Ocean between Hawaii and Mexico are enriched in a variety of critical metals. Understanding the spatial distribution of different metals, their partitioning between mineral phases, and pore-size distributions within the nodules can provide helpful insights into their formation mechanism. Here, we used a suite of techniques sensitive to the chemistry and texture of the nodule, including laser ablation–inductively coupled plasma–time-of-flight–mass spectrometry (LA-ICP-TOF-MS), Raman spectroscopy, scanning electron microscopy, and X-ray microtomography, to characterize key aspects of a single nodule from the Clarion-Clipperton Zone collected in the 1970s. We observed that parts of the nodules exhibited stratified zones enriched in cobalt (Co), nickel (Ni), copper (Cu), and rare earth elements (REEs) and found that Co, Ni, and Cu are spatially correlated with the distribution of the Mn-rich mineral phases. We also observed a stronger association of Ce and Co with other light REEs compared with heavier REEs. Within the porous rim of the nodule, micron-sized REE-enriched zones were detected, which are surrounded by Si- and Al-bearing phases and are not associated with phosphate phases. The results are consistent with alternating hydrogenetic and diagenetic growth conditions across part of the nodule and reveal heterogeneous REE distributions both within Mn-rich minerals and within distinct, likely inclusion-related phases.

Amorphous magnesium carbonate (AMC) is recognized as a precursor material during the formation of crystalline magnesium carbonates, which are important materials, e.g., for next generation cements and CO2 sorbents, but its amorphous-to-crystalline transition is poorly understood. In this study, we combined in situ X-ray and neutron diffraction and total scattering to determine the activation energy of the amorphous-to-crystalline transition for AMC in water and identify the reaction mechanism. The calculated activation energy Ea of 41 ± 13 kJ/mol for the AMC to nesquehonite (MgCO3·3H2O) transformation matches previous studies indicating a dissolution–reprecipitation mechanism. Ea for AMC to hydromagnesite (Mg5(CO3)4(OH)2·4H2O) of 116 ± 19 kJ/mol is considerably higher and combined with our peak area analysis of total scattering data, we conclude that a second-order solid state transition is likely. Remarkably, we observed 2 orders of magnitude longer lifetime of deuterated AMC compared to protiated AMC in liquid.

The effects of individual component salts on MgO hydration and subsequent carbonation have not been systematically investigated despite the increase in utilization of seawater and other brines in the production of MgO-containing next-generation cements, fire retardants, and sorbents for CO2. Here, we present a study of MgO hydration in the presence of individual saltwater cations at a range of concentrations, as well as binary mixtures at seawater concentrations, and subsequent carbonation. We observed that the presence of the cations increased MgO dissolution rates and that the subsequent carbonation extent of the hydrated material remained constant or even increased. Since the presence of these salts during hydration does not show negative effects on subsequent carbonation in our experimental study, the application of seawater and other brines containing high concentrations of these salts to hydrate MgO in industrial processes is likely feasible.

Structural defects and lattice strain are intrinsic to many crystalline materials, yet their roles in controlling chemical reaction mechanisms and directing crystallization pathways remain poorly understood. Here, we revealed the three-dimensional evolution of strain and dislocation defects at the nanoscale during the growth of heterogeneously nucleated barite (BaSO₄) and calcite (CaCO₃) crystals by using coherent X-ray scattering, electron microscopy, and molecular simulations. Unlike barite, which formed with minimal internal strain, calcite developed dislocation defects and exhibited spatially varying strain that increased during growth. During growth in Sr-rich solutions, calcite likely incorporates Sr²⁺ into the defects, which further modulates the local lattice structure and increases both the compressive and tensile strain. These findings suggest that calcite crystallization was likely dominated by attachment of precursor phases, which gave rise to defect-enriched domain structures not predicted by classical growth models. By linking defect formation to ion incorporation and growth dynamics, this work provides fundamental insight into how lattice-level strain heterogeneity governs the chemical reactivity of ionic crystals.

Water distribution at the surface is a primary driver in the formation of polycrystalline solid structures during mineral precipitation.

Mechanistic insights into rubidium ion adsorption at the quartz (101) surface from quantum chemical metadynamics suggest ion adsorption strength and extent can be modulated through silanol coverage, surface vacancies, and charge.