F.J. Huertas

Chemical, mineralogical and isotope behavior, and phase transformation during the precipitation of calcium carbonate minerals from intermediate ionic solution at 25°C

A set of time-series experiments was performed in which CaCO3 was precipitated (25°C, 1 atm) from aliquots of an NaHCO3 (0.330 M)/CaCl2 (0.0023 M) solution in a closed system to evaluate the effect of phase transformation on calcium carbonate precipitation kinetics and isotope behavior. Monohydrocalcite precipitated initially at a solution saturation state (Ωmhc) slightly greater than one, whereas calcite crystallized later at a significant higher saturation state (Ωcl ≅ 14.4). The precipitation of calcite promoted the dissolution of monohydrocalcite at a rate that exceeded calcite precipitation, producing anomalous behavior in the chemical and isotope composition of the system. The carbon isotope fractionation factors (103 lnα) for monohydrocalcite-HCO3(aq)- and monohydrocalcite-CO2(g) were 0.36 ± 0.01‰ and 8.35 ± 0.01‰, respectively. The oxygen isotope fractionation factor for monohydrocalcite-H2O was 27.8 ± 0.1‰. The carbon isotope fractionation factors for calcite-HCO3 (aq)− and calcite-CO2(g) were 0.94 ± 0.06‰ and 8.93 ± 0.06‰, respectively, whereas the oxygen isotope fractionation factor for calcite-H2O(l) was 28.0 ± 0.2‰. A carbon isotope fractionation factor of 0.58 ± 0.07‰ was determined for the mineral pair calcite-monohydrocalcite, but no fractionation was observed for oxygen isotopes over time steps when both minerals co-precipitated. Fractionation factors for calcite were independent of precipitation rate over the range in rates of 103.96 to 105.63 μmol/m2h. These results extend the upper limit of characterization for the relationship between precipitation rate and isotope partitioning of carbon between calcite, HCO3 (aq)− and CO2(g), and quantitatively document for the first time the independence between precipitation rate and oxygen isotope partitioning in the calcite-H2O(l) system.

Kinetics of montmorillonite dissolution in granitic solutions

Abstract Experiments measuring smectite dissolution rates in granitic solutions were carried out in a semi-batch reactor at 20, 40, and 60°C. The pH conditions of the solutions range from 7.6 to 8.5. Solid samples were confined within a dialysis membrane and introduced in the solution. The solution was renewed every 7 days and the dissolution reaction was investigated by the variation of Si concentration in the solutions. The average rates at pH∼8 were 10−14.13, 10−13.70, and 10−13.46 mol m−2 s−1, at 20, 40, and 60°C, respectively, and the activation energy for the dissolution reaction at pH ∼8 was 30.5±1.3 kJ mol−1. Comparison of the present results with other studies reveals that the montmorillonite dissolution rate depends strongly on the pH of the solution, with a minimum value at pH 8–8.5. At room temperature, the dissolution rate was found to be linearly dependent on proton (acidic conditions) or hydroxyl (basic conditions) activity in solution: Rate =10 −11.39 a H + 0.34 pH Rate =10 −12.31 a OH − 0.34 pH >8.5 The comprehension of the dissolution mechanism can be improved by using surface complexation theory. Correlation between speciation of surface sites and kinetic results indicated that at room temperature the dissolution rate was directly proportional to the surface concentration of >AlOH2+ and >AlO− surface complexes, under acidic or alkaline conditions, respectively. Rate =10 −8.0 {> AlOH 2 + } pH Rate =10 −8.2 {> AlO − } pH >8.5 A multiple variable model is proposed to take into account simultaneously the effect of pH on dissolution rates and on activation energy. The rates estimated using the model are in good agreement with experimental dissolution rates.

Experimental study of the hydrothermal formation of kaolinite

Kaolinite was hydrothermally precipitated starting from amorphous aluminosilicates, with Si/Al ratio from 1.8 to 0.76, at temperatures of 150, 175, 200, 225, and 250°C, at time periods varying from 6 h to 60 days. The solutions were analysed for pH, Si, Al, and K, and their saturation state was calculated by using the computer program SOLMINEQ.88. The solids were studied by surface area, XRD and DTA–TG analyses. Kaolinite was the only crystalline phase found in the products of the runs and its formation and crystallinity depended on time, temperature, and Si/Al ratio of the starting material. The kaolinite yield increased as starting material was aged for longer times. Products synthesized at higher temperature contain more kaolinite which was more crystalline than in the experiments carried out at lower temperature. The gel with Si/Al=0.99 produced the most disordered kaolinite and better crystallinity was obtained from products richest in Al or Si. The process of kaolinite formation was the result of two stages. During the first one aggregates of domains having a kaolinite-like structure were formed. The second stage corresponded to the formation of hexagonally outlined platy crystals. The formation rates indicated that the first stage was approximately one order of magnitude faster than the second stage, with average activation energies of 82±5 kJ mol−1 and 71±5 kJ mol−1, respectively. Si/Al ratio of the starting material exerted only a slight influence on the activation energies. The precipitation rates obtained for the second stage were consistent with literature data and may be considered precipitation rates of kaolinite in hydrothermal environments.

Bentonites from Cabo de Gata, Almería, Spain: a mineralogical and geochemical overview

Abstract The Neogene volcanic region of Cabo de Gata, Almería, SE Spain, is dotted with many outcrops of bentonite, some of them of significant economic interest. The bentonites have their origin in the hydrothermal alteration of pyroclastic rocks (15-7 Ma). The deposits are usually associated with fractures. The major mineral is a dioctahedral Fe- and Mg-smectite (89-75%) and this is accompanied by minor amounts of feldspars, quartz, amphiboles, pyroxenes, biotite, zeolites, disordered tridymite, calcite, etc. This paper describes the geological background, the general characteristics of the bentonites and major aspects of their formation, e.g. type of low-temperature hydrothermal solutions, mass balance, chemical evolutions of the smectites and geochemistry of trace elements. Finally, the characteristics of three of the most important deposits are described.

Smectite formation in rhyolitic obsidian as inferred by microscopic (SEM-TEM-AEM) investigation

Abstract Experimental alteration of a rhyolitic obsidian by solutions containing variable concentrations of Mg was carried out at 100, 150 and 200°C, for 30, 60 and 90 days, to investigate the mechanism of smectite formation. The smectite exhibits two distinct morphologies: (1) small flakes (aggregates of a few crystals); and (2) large flakes (massive groups). The small flakes are the earlier alteration products. Both morphological types have distinct chemical compositions: the smallest laminae are dioctahedral and contain more Fe, whereas the particles from the rose-shaped aggregates contain Mg and are trioctahedral. Intermediate compositions have been observed between the extreme compositions. It is suggested that the two morphologies are the result of two distinct genetic processes: (1) the small flakes (Fe-rich smectite) form by solid state rearrangements of the hydrated external layer of glass and/or via the formation of domains within the glass; (2) the rose-shaped aggregates (Mg-rich smectite) form by precipitation from solution.

Morphology of kaolinite crystals synthesized under hydrothermal conditions

Scanning electron microscopy has revealed the presence of spherical, lath and platy kaolinite in gels with Si/Al atomic ratio ranging from 1.84 to 0.76 that are hydrothermally treated under different temperature and time conditions. Hemispherical structures and excavated zones, at different stages of evolution, have been observed on the surface of the gel grains, indicating that spherical particles do not precipitate from the solution but are generated from the gels. The quantity of spherical particles depends on temperature, time and the chemical composition of the starting gel. Products from starting material with Si/Al ≈ 1 yield the highest quantity of these particles. Being metastable, sphere dissolution controls the chemistry of the solution and consequently the morphology of the precipitating crystals thus producing more elongated, curved and irregular outlines when gels with Si/Al ≈ 1 are hydrothermally treated.

Myxococcus xanthus' killed cells as inducers of struvite crystallization. Its possible role in the biomineralization processes

We studied the possibility of struvite formation using killed cells of Myxococcus xanthus. Cells were killed by heat, UV light and sonication. In all cases, we show that struvite crystallization occurs and we propose that the dead cells or cell debris can act by themselves as heterogeneous crystallization nuclei. We also show that the slime, produced by Myxococcus in a large quantities, is not involved in this process.

Textural and structural transformations of kaolinites in aqueous solutions at 200°C

Abstract We have studied the processes affecting disordered kaolinites during hydrothermal treatment at 200°C, pH=4 and increasing time of reaction. The starting materials include untreated (natural), poorly ordered Georgia kaolinite (KGa-2), and this sample after intense grinding. Chemical analyses of equilibrium solutions show a higher dissolution rate in the case of the ground kaolinite. In both cases, Si contents are higher than Al contents, indicating a non-stoichiometric dissolution. The solid products of the reactions with natural kaolinite show scarce differences with the starting sample: The XRD patterns reveal a slight sharpening of the basal reflections and a moderate increase of the Hinckley index. The solid products of the reactions with ground kaolinite show, on the contrary, remarkable textural and structural differences with the starting material: Most of the observed particles are spherical and the granulometric analyses indicate a notable increase of the mean particle size. The XRD patterns show an increase in both intensity and resolution of the non-basal reflections, which reflect the decrease of translation defects at the same time that the 131 reflection indicates an increase of the WC-type defects. The evolution observed on the IR spectra, at increasing run times, also indicate a notable increase in crystallinity. The formation of spherical aggregates of kaolinite crystals and the increase in structural order indicate that the hydrothermal reactions caused the recrystallization of the ground kaolinite. These results suggest a dissolution–precipitation process, which is notably less developed in the case of natural kaolinite.

Effects of pH and phosphate on the production of struvite by Myxococcus xanthus

We studied the influence of pH and the phosphate content of the culture medium on the precipitation of struvite by Myxococcus xanthus, a bacterium that undergoes autolysis at the end of its exponential growth phase in liquid cultures. The best results were obtained with pH values between 7.2 and 8.0 and with a phosphate concentration of 10 mM. Our studies reveal for the first time that the precipitation of struvite always begins at the onset of autolysis and that culture conditions favoring the early occurrence of autolysis also enhance struvite production.

In situ transformation of amorphous gels into spherical aggregates of kaolinite: A HRTEM study

Abstract Initial stages of the gel-to-kaolinite transformation were studied using HRTEM. Spherical aggregates of kaolinite crystals were produced during the synthesis of kaolinite by hydrothermal treatment of Si-Al amorphous gels. Prior to sphere formation, gels transform into pseudospherical domains that have a Si/Al ratio of one and display no SAED pattern. The spherical particles consist of radially arranged sectors of stacks of planar crystallites. Crystals display nonbasal spacings of 4.5, 4.2 and 3.8 Å and a basal spacing of 7.1 Å , the c* axis following the radius of the sphere. Interpretation of the 3D nanostructure of the spheres is difficult. The c* axis exhibits a radial disposition, but the relative orientation of the a* and b* axes in neighbouring crystallites may produce bent layers or incoherent contacts. In addition, curved layers with a d spacing of 7.4 Å may be attributed to halloysite layers collapsed under microscopy conditions. The disappearance of the spheres during the hydrothermal treatment is probably due to preferential dissolution of either highstress areas near bent layers or non-crystalline material filling crystal boundaries. Dissolution leads to sphere disaggregation and allows the component columnar crystals to continue to grow. Observations of the gel matrix suggest that under our experimental conditions kaolinite crystallizes via an in situ transformation of the gel.