Mineral precipitation and hydrochemical evolution through evaporitic processes in soda brines (East African Rift Valley)

Soda lakes of the East African Rift Valley are hyperalkaline, hypersaline lakes extremely enriched in Na+, K+, Cl-, CO32-, HCO3-, and SiO2. In this paper, we investigate the chemical evolution in these lakes and the pro-duction of chemical sediments by salt precipitation via evaporation. Water samples from tributary springs and three lakes (Magadi, Nasikie Engida and Natron) have been experimentally studied by in-situ X-ray diffraction during evaporation experiments to characterize the sequence of mineral precipitation. These data are com-plemented by ex-situ diffraction studies, chemical analyses and thermodynamic hydrochemical calculations producing detailed information on the activity of all solution species and the saturation state of all minerals potentially generated by the given composition. Major minerals precipitating from these samples are sodium carbonates/bicarbonates as well as halite. The CO3/HCO3 ratio, controlled by pH, is the main factor defining the Na-carbonates precipitation sequence: in lake brines where CO3/HCO3 > 1, trona precipitates first whereas in hot springs, where CO3/HCO3 MUCH LESS-THAN 1, nahcolite precipitates instead of trona, which forms later via partial disso-lution of nahcolite. Precipitation of nahcolite is possible only at lower pH values (pCO2 higher than-2.7) explaining the distribution of trona and nahcolite in current lakes and the stratigraphic sequences. Later, during evaporation, thermonatrite precipitates, normally at the same time as halite, at a very high pH (>11.2) after significant depletion of HCO3- due to trona precipitation. The precipitation of these soluble minerals increases the pH of the brine and is the main factor contributing to the hyperalkaline and hypersaline character of the lakes. Villiaumite, sylvite, alkaline earth carbonates, fluorapatite and silica are also predicted to precipitate, but most of them have not been observed in evaporation experiments, either because of the small amount of precipitates produced, kinetic effects delaying the nucleation of some phases, or by biologically induced effects in the lake chemistry that are not considered in our calculations. Even in these cases, the chemical composition in the corresponding ions allows for discussion on their accumulation and the eventual precipitation of these phases. The coupling of in-situ and ex-situ experiments and geochemical modelling is key to understanding the hydro-geochemical and hydroclimatic conditions of soda lakes, evaporite settings, and potentially soda oceans of early Earth and other extraterrestrial bodies.European Research Council (ERC) European Commission 340863Spanish Government CGL2016-78971-PJunta de Andalucia P18-FR-5008Spanish Government BES-2017-08110

Three study cases of growth morphology in minerals: Halite, calcite and gypsum

AbstractBeyond fundamental aspects of crystal growth and morphology, the growth of minerals is a challenging subject because in most cases we face a problem with unknown growth conditions. Actually, in the field of geological studies, we have to decipher the growth conditions of a crystal using the information contained in the very crystal. One of these characteristics of crystals that contain information about their growth is their morphology and time evolution. In this article, we introduce the subject of crystal morphology by using three important minerals, calcite, halite and gypsum, as three didactic case studies to illustrate the application of the current knowledge in the field

101 contact twins in gypsum experimentally obtained from calcium carbonate enriched solutions: mineralogical implications for natural gypsum deposits

Gypsum twins are frequently observed in nature, triggered by a wide array of impurities that are present in their depositional environments and that may exert a critical role in the selection of different twin laws. Identifying the impurities able to promote the selection of specific twin laws has relevance for geological studies aimed at interpreting the gypsum depositional environments in ancient and modern deposits. Here, the effect of calcium carbonate (CaCO3) on gypsum (CaSO4 2H2O) growth morphology has been investigated by performing temperature-controlled laboratory experiments with and without the addition of carbonate ions. The precipitation of twinned gypsum crystals has been achieved experimentally (101 contact twin law) by adding carbonate to the solution, and the involvement of rapidcreekite (Ca2SO4CO3 4H2O) in selecting the 101 gypsum contact twin law was supported, suggesting an epitaxial mechanism.Moreover, the occurrence of 101 gypsum contact twins in nature has been suggested by comparing the natural gypsum twin morphologies observed in evaporitic environments with those obtained in experiments. Finally, both orientations of the primary fluid inclusions (of the negative crystal shape) with respect to the twin plane and the main elongation of sub-crystals that form the twin are proposed as a fast and useful method (especially in geological samples) to distinguish between the 100 and 101 twin laws. The results of this study provide new insights into the mineralogical implications of twinned gypsum crystals and their potential as a tool to better understand natural gypsum deposits.PRIN 2017 of the Italian Ministry for EducationUniversity and Research (MIUR) (grant No. 2017L83S77)Ayudas I+D+i en Universidades y Centros de Investigacio´n Pu´ blicos (grant No. P18- FR-5008)Proyectos I+D+i 2020 of the Spanish Ministerio de Ciencia e Innovacio´n (grant No. PID2020-112986GB-I00

Mineralochemical Mechanism for the Formation of Salt Volcanoes: The Case of Mount Dallol (Afar Triangle, Ethiopia)

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsearthspacechem. 2c00075.A genetic model is proposed for the formation and evolution of volcano-like structures from materials other than molten silicate rocks. The model is based on Mount Dallol (Afar Triangle, Ethiopia), currently hosting a conspicuous hydrothermal system with hot, hyper-acidic springs, forming a colorful landscape of unique mineral patterns. We reason that Mount Dallol is the last stage of the formation of a salt volcano driven by the destabilization of a thick sequence of hydrated minerals (the Houston Formation) after the emplacement of an igneous intrusion beneath the thick Danakil evaporitic sequence. Our claim is supported by field studies, calculations of the mineral/water volume balance upon mineral dehydration, and by a geothermal model of the Danakil basin predicting a temperature up to 220 °C at the Houston Formation after the intrusion of a basaltic magma without direct contact with the evaporitic sequence. Although insufficient for salt melting, this heating triggers mineral dehydration and hydrolysis, leading to a total volume increase of at least 25%. The released brine is segregated upward into a pressurized chamber, where the excess volume produced the doming of Mount Dallol. Later, the collapse of the dome formed a caldera and the emission of clastic flows. The resulting structures and materials resemble volcanic lava flows in distribution, structure, and texture but are entirely made of salty materials. This novel mechanism of the generation of pressurized brines and their later eruption extends the relevance of volcanologic studies to lower temperature ranges and unanticipated geologic contexts on Earth and possibly also on other planets.European Research Council under the European Union’s seventh Framework Program (FP7/2007− 2013)/ERC grant agreement no. 340863Spanish Ministerio de Educacion y Ciencia for the financial support of the projects CGL2016-78971-P and P18-FR500

On the Quality of Protein Crystals Grown under Diffusion Mass-transport Controlled Regime (I)

It has been previously shown that the diffraction quality of protein crystals strongly depends on mass transport during their growth. In fact, several studies support the idea that the higher the contribution of the diffusion during mass transport, the better the diffraction quality of the crystals. In this work, we have compared the crystal quality of two model (thaumatin and insulin) and two target (HBII and HBII-III) proteins grown by two different methods to reduce/eliminate convective mass transport: crystal growth in agarose gels and crystal growth in solution under microgravity. In both cases, we used identical counterdiffusion crystallization setups and the same data collection protocols. Additionally, critical parameters such as reactor geometry, stock batches of proteins and other chemicals, temperature, and duration of the experiments were carefully monitored. The diffraction datasets have been analyzed using a principal component analysis (PCA) to determine possible trends in quality indicators. The relevant indicators show that, for the purpose of structural crystallography, there are no obvious differences between crystals grown under reduced convective flow in space and convection-free conditions in agarose gel, indicating that the key factor contributing to crystal quality is the reduced convection environment and not how this reduced convection is achieved. This means that the possible detrimental effect on crystal quality due to the incorporation of gel fibers into the protein crystals is insignificant compared to the positive impact of an optimal convection-free environment provided by gels. Moreover, our results confirm that the counterdiffusion technique optimizes protein crystal quality and validates both environments in order to deliver high quality protein crystals, although other considerations, such as protein/gel interactions, must be considered when defining the optimal crystallization setup.This study was supported by projects ESP2005-23831-E and ESP2007-29071-E (Spanish Ministry of Education and Science) and BIO2016-74875-P (JAG) (MINECO), Spain co-funded by the Fondo Europeo de Desarrollo Regional, FEDER funds, European Union

Role of CaCO3° neutral pair in calcium carbonate crystallization

The molecular structure of the units that get incorporated into the nuclei of the crystalline phase and sustain their growth is a fundamental issue in the pathway from a supersaturated solution to the formation of crystals. Using a fluorescent dye we have recorded the variation of the pH value in time along a gel where CaCl2 and NaHCO3 counter-diffuse to crystallize CaCO3. The same pH–space–time distribution maps were also computationally obtained using a chemical speciation code (phreeqc). Using data arising from this model we investigated the space-time evolution of the activity of the single species (ions and ion pairs) involved in the crystallization process. Our combined results suggest that, whatever the pathway from solution to crystals, the neutral pair CaCO3° is a key species in the CaCO3 precipitation system.European Research Council (European Union’s Seventh Framework Programme (FP7/2007-2013) grant agreement no 340863, and Spanish MINECO grants MAT2014-60533-R and CGL2010-16882 cofounded with FEDERPeer reviewe

Crystal growth kinetics and mechanisms

ISBC2019: International School on Biological Crystallization, in Granada (Spain), 26th-31st May 201

The Symmetry of the Alhambra

7th International School on Biological Crystallization (ISBC2019), Granada (Spain), May 26th-31st, 201

The role of mass transport in protein crystallization

Mass transport takes place within the mesoscopic to macroscopic scale range and plays a key role in crystal growth that may affect the result of the crystallization experiment. The influence of mass transport is different depending on the crystallization technique employed, essentially because each technique reaches supersaturation in its own unique way. In the case of batch experiments, there are some complex phenomena that take place at the interface between solutions upon mixing. These transport instabilities may drastically affect the reproducibility of crystallization experiments, and different outcomes may be obtained depending on whether or not the drop is homogenized. In diffusion experiments with aqueous solutions, evaporation leads to fascinating transport phenomena. When a drop starts to evaporate, there is an increase in concentration near the interface between the drop and the air until a nucleation event eventually takes place. Upon growth, the weight of the floating crystal overcomes the surface tension and the crystal falls to the bottom of the drop. The very growth of the crystal then triggers convective flow and inhomogeneities in supersaturation values in the drop owing to buoyancy of the lighter concentration-depleted solution surrounding the crystal. Finally, the counter-diffusion technique works if, and only if, diffusive mass transport is assured. The technique relies on the propagation of a supersaturation wave that moves across the elongated protein chamber and is the result of the coupling of reaction (crystallization) and diffusion. The goal of this review is to convince protein crystal growers that in spite of the small volume of the typical protein crystallization setup, transport plays a key role in the crystal quality, size and phase in both screening and optimization experiments.We also acknowledge the financial support of the Ministerio de Economıía y Competitividad of Spain for the Project Consolider-Ingenio CSD2006-00015 ‘Factoría de Cristalización, as well as Junta de Andalucía for support of the Project RNM 538