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The application of foliar sprays of suspensions of relatively insoluble essential element salts is gradually becoming common, chiefly with the introduction of nano-technology approaches in agriculture. However, there is controversy about the effectiveness of such sparingly soluble nutrient sources as foliar fertilizers. In this work, we focussed on analysing the effect of adding Ca-carbonate (calcite, CaCO3) micro- and nano-particles as model sparingly soluble mineral compounds to foliar fertilizer formulations in terms of increasing the rate of foliar absorption. For these purposes, we carried out short-term foliar application experiments by treating leaves of species with variable surface features and wettability rates. The leaf absorption efficacy of foliar formulations containing a surfactant and model soluble nutrient sources, namely Ca-chloride (CaCl2), magnesium sulphate (MgSO4), potassium nitrate (KNO3), or zinc sulphate (ZnSO4), was evaluated alone or after addition of calcite particles. In general, the combination of the Ca-carbonate particles with an essential element salt had a synergistic effect and improved the absorption of Ca and the nutrient element provided. In light of the positive effects of using calcite particles as foliar formulation adjuvants, dolomite nano- and micro-particles were also tested as foliar formulation additives, and the results were also positive in terms of increasing foliar uptake. The observed nutrient element foliar absorption efficacy can be partially explained by geochemical modelling, which enabled us to predict how these formulations will perform at least in chemical terms. Our results show the major potential of adding mineral particles as foliar formulation additives, but the associated mechanisms of action and possible additional benefits to plants should be characterised in future investigations.

The uncommon association of ammonium sulfates identified in the Pastora abandoned mine is the result of a complex mineral evolution. By means of dissolution-(re)crystallization reactions operating during long periods of time, ammonium minerals "adapt"to local spatiotemporal changes in physico-chemical conditions. We found that during such an evolution, seasonal variations in temperature and humidity, the relative solubility of mineral species, and the presence of organic matter play an important role. In addition, our study shows the existence of "mineral niches"and "mineral seasonality,"which can be explained on the basis of the "mineral ecology"concept introduced by Hazen et al. (2015). Our investigation of the formation of hydrated sulfates, particularly of ammonium sulfates, might be of importance for identifying the existence of life in mineral formation environments.

Following defocused ion beam sputtering, large area highly corrugated and faceted nanoripples are formed on calcite (10.4) faces in a self-organized fashion. High resolution atomic force microscopy (AFM) imaging reveals that calcite ripples are defined by facets with highly kinked (11.0) and (2 1 ¯ .12) terminations. In situ AFM imaging during the exposure of such modified calcite surfaces to PbCl aqueous solution reveals that the nanostructured calcite surface promotes the uptake of Pb. In addition, we observed the progressive smoothing of the highly reactive calcite facet terminations and the formation of Pb-bearing precipitates elongated in registry with the underlying nanopattern. By SEM-EDS analysis we quantified a remarkable 500% increase of the Pb uptake rate, up to 0.5 atomic weight % per hour, on the nanorippled calcite in comparison to its freshly cleaved (10.4) surfaces. These results suggest that nanostructurated calcite surfaces can be used for developing future systems for lead sequestration from polluted waters.

The objective of this study was to evaluate the rate of foliar absorption of magnesium (Mg) salts with different deliquescence and efflorescence relative humidity values (DRH and ERH, also known as point of deliquescence (POD) and point of efflorescence (POE), respectively) when supplied to leaves of model plants with different wettability properties. For this purpose, a greenhouse pot experiment was conducted with lettuce (very wettable), broccoli (highly unwettable) and leek (highly unwettable). Foliar sprays contained 0.1% surfactant plus 100 mM Mg supplied as MgCl2·6H2O, Mg(NO3)2·6H2O or MgSO4·7H2O. Leaf Mg concentrations were determined 1 and 7 days after foliar application. Anion concentrations were also measured in lettuce where a significant foliar Mg absorption was detected. Leaf wettability, leaf surface free energy and fertilizer drop deposit appearance onto the foliage were assessed. It is concluded that despite including a surfactant in the spray formulation, leaf wettability plays a major role in foliar Mg absorption.

Why is the mineral dolomite, CaMg(CO), abundant in ancient sedimentary rocks but scarce in modern rock-forming environments? What are the natural processes that lead to the characteristic Mg-Ca order in the dolomite structure? These important geological questions have captivated the interest of geoscientists for over a century, but the answers still remain elusive. Only recently, by the development of experimental work and the generation of large geochemical data sets, we began to suspect that both the formation and Mg-Ca ordering of dolomite could be the result of progressive dissolution-(re)crystallization reactions extended over large geological periods of time. Here, we present an initial estimate of dolomite cation-ordering times derived from previously reported data of Phanerozoic dolomites. This result provides new insight into the process of dolomite formation in nature, and it clearly suggests that can be possible to precisely determine the kinetics of the Mg-Ca ordering of natural dolomites.

Norsethite, BaMg(CO3)2, is a mineral with a structure analogous to dolomite, CaMg(CO3)2, which can be readily synthesized at room temperature. Therefore, norsethite and other carbonates with a norsethite structure have been used to investigate the possible formation mechanisms of dolomite under ambient conditions. In this work, the crystal structures of dolomite, norsethite, and norsethite-like structures (i.e., PbMg(CO3)2, SrMg(CO3)2, and the newly proposed structure of RaMg(CO3)2) have been studied at an atomic scale by means of calculations based on density functional theory (DFT). The energies of these carbonates and precursors calculated have been compared to obtain the relative energies of these double carbonates. We found that norsethite and RaMg(CO3)2 have the lowest relative energies, indicating that these phases must be relatively easy to synthesize under ambient conditions. Differently, PbMg(CO3)2, SrMg(CO3)2, and dolomite, have much higher relative energies, indicating that their synthesis is more difficult. Our calculations are consistent with the results of the synthesis experiments reported to date. Furthermore, our theoretical results open the door to future experimental works on the synthesis of double carbonates aimed at obtaining RaMg(CO3)2 and SrMg(CO3)2.

Celestite (SrSO4) single crystals immersed in highly concentrated Na2CO3 aqueous solutions (0.5 mol/l) are progressively pseudomorphised by aggregates of strontianite (SrCO3) crystals at ambient conditions. This replacement occurs by the coupling of dissolution and crystallisation reactions, eventually resulting in the formation of strontianite pseudomorphs, which reproduce the external shape of the initial celestite crystals. Experiments conducted for different reaction times indicate that the kinetics of the replacement reaction is mainly controlled by both the dissolution of the celestite substrate and the evolution of the porosity of the strontianite replacing layer. Such a porous layer allows the replacing solution to permeate towards the replacement front. During the initial stages of pseudomorph formation, the strontianite replacing layer is highly porous and the replacement front advances rapidly. As porosity decreases after a few days of reaction, the advancement of the strontianite replacement front slows down and an almost constant rate of about 0.3 µm/h is reached. Scanning electron microscope images and X-ray diffraction data have also shown that the growth of replacing strontianite aggregates occurs with a high degree of orientation with respect to the celestite substrate, i.e. the replacement is topotaxial.

Because of the widespread presence of foreign substrates in natural settings, mineral precipitation usually occurs via heterogeneous nucleation. This process is controlled by the interplay between the fluid supersaturation and interfacial energies present between the fluid, nucleus, and substrate. Among a number of physicochemical parameters, the surface wetting properties have been shown to be a key parameter controlling heterogeneous nucleation. The present study aims at elucidating the pathway and kinetics of CaCO3 heterogeneous nucleation on a set of phlogopite micas with and without fluorine/hydroxyl substitutions, yielding substrates with contrasting hydrophilicity. Our results show that, irrespective of surface wetting properties, amorphous calcium carbonate (ACC) is formed during the early stages. The surface wetting properties have a strong effect on the crystallization kinetics: ACC precipitates persist longer on the hydrophilic (hydroxylated) surface than on the less hydrophilic (fluorinated) one. We show that this stabilization could have a thermodynamic origin because of the lower interfacial free energy between the hydrated amorphous precursor and the hydrophilic substrate. These results are highly relevant for biomineralization studies, where differences in wetting properties of organic moieties present in calcifying organisms could be used to accelerate or decelerate the crystallization of the initially formed amorphous precursor phase.

The determination of the degree of Mg-Ca order in the dolomite structure is crucial to better understand the process or processes leading to the formation of this mineral in nature. I01.5/I11.0 intensity ratios in the X-ray powder diffractograms are frequently measured to quantify dolomite cation order in dolomites. However, the intensity of diffraction peaks can be affected by factors other than the Mg-Ca distribution in the dolomite structure. The most relevant among these factors are (i) deviations from the ideal dolomite stoichiometry, and (ii) the partial substitution of Mg and Ca atoms by Fe, Mn, and Zn impurities. Using the VESTA software, we have constructed crystal structures and calculated I01.5/I11.0 ratios for dolomites with Mg:Ca ratios ranging from 0.5 to 1.5, and with Fe, Mn, and Zn contents up to 30%. Our results show that both deviations from dolomite ideal stoichiometry and the presence of impurities in its structure lead to a significant decrease in I01.5/I11.0 intensity ratios, an effect which must be considered when cation orders of natural dolomites from different origins are compared.

Growth of otavite (CdCO3) and spherocobaltite (CoCO3) has been promoted on dolomite (10.4) surfaces at room temperature by immersing cleaved dolomite crystals in highly supersaturated solutions with respect to those phases. In situ atomic force microscopy (AFM) observations show that otavite and spherocobaltite overgrow preferentially on dolomite step edges and (10.4) terraces by two-dimensional nucleation. High-resolution friction AFM images reveal the parallelism between the lattices of the overgrowths and the substrate, demonstrating that the growth of otavite and spherocobaltite on dolomite (10.4) surfaces is epitaxial. Similar values of lattice parameters measured on high-resolution friction images of substrates and overgrowths indicate the accommodation of the otavite and spherocobaltite (10.4) surface lattices to the dolomite substrate structure. The frictional response of otavite and spherocobaltite overgrowth surfaces resulted to be significantly higher than that of dolomite. These differences in frictional response can be attributed to the strain generated by the misfits between dolomite substrates and the overgrowths.

In this paper, we present a comparative study of the pseudomorphic carbonatation of gypsum and anhydrite single crystals in Mg-bearing aqueous solutions at room temperature. We have found that carbonatation of gypsum an anhydrite occurs via a similar coupled dissolution–crystallization mechanism. However, whereas pseudomorphization of anhydrite precisely preserves the external form and dimension of the initial crystals, pseudomorphs after gypsum are less perfect and the shape and volume of the original crystals are partially lost. Furthermore, the mineralogical compositions of the pseudomorphs after anhydrite and gypsum are different, because gypsum is replaced by relatively large calcite crystals, while anhydrite pseudomorphs consist of aggregates of calcite and aragonite. Such textural and compositional differences can be clearly related to the length scale of the coupling between dissolution and crystallization at the replacement fronts. In situ atomic force microscopy observations of the early stages of carbonatation have shown that dissolution and crystallization are very tightly coupled on anhydrite rather than on gypsum surfaces. This clearly indicates that during the replacement of anhydrite and gypsum, dissolution and precipitation, respectively, are the rate-limiting steps. On the other hand, the higher supersaturation levels reached during the replacement of anhydrite lead to a competition between calcite and aragonite crystallization. This explains the fact that calcite and aragonite coexist within the replacement layers after anhydrite. Finally, we have found that dissolved magnesium in the reacting solutions is almost equally incorporated into the pseudomorphs, regardless of both the precursor phase and the mineralogical composition of the replacements. Our calculations and chemical analyses have shown that the compositions of the MgxCa1–xCO3 solid solution for which supersaturations reach maximum values correspond quite well with maximum Mg/Ca ratios