Neoformation of Ni phyllosilicate upon Ni uptake on montmorillonite: A kinetics study by powder and polarized extended X-ray absorption fine structure spectroscopy

Wet chemistry kinetics and powder and polarized extended X-ray absorption fine structure (EXAFS and P-EXAFS) spectroscopy were combined to investigate the mechanism of Ni uptake on montmorillonite, at pH 8, high ionic strength (0.2 M Ca(NO₃)₂), initial Ni concentration of 660 μM, and solid concentration of 5.3 g/L. Approximately 20% of Ni sorbed within the first 24 h; thereafter, the Ni uptake rate slowed, and 12% of the initial Ni concentration remained in solution after 206 d of reaction time. Powder EXAFS spectra collected on wet pastes at 1, 14, 90, and 206 d showed the presence of Ni-Ni pairs at ∼3.08 Å in an amount that gradually increased with time. Results were interpreted by the nucleation of a Ni phase having either an α-Ni-hydroxide- or a Ni-phyllosilicate-like local structure. The latter possibility was confirmed by recording P-EXAFS spectra of a highly textured, self-supporting montmorillonite film prepared in the same conditions as the wet samples and equilibrated for 14 d. The orientation distribution of the c*-axes of individual clay particles off the film plane, as measured by quantitative texture analysis, was 32.8° full width at half maximum, and this value was used to correct from texture effect the effective numbers of Ni and Si nearest neighbors determined by P-EXAFS. Ni atoms were found to be surrounded by 2.6 ± 0.5 Ni atoms at 3.08 Å in the in-plane direction and by 4.2 ± 0.5 Si atoms at 3.26 Å in the out-of-plane direction. These structural parameters, but also the orientation and angular dependence of the Ni and Si shells, strongly support the formation of a Ni phyllosilicate having its layers parallel to the montmorillonite layers. The neoformation of a phyllosilicate on metal uptake on montmorillonite, documented herein for the first time, has important geochemical implications because this dioctahedral smectite is overwhelmingly present in the environment. The resulting sequestration of sorbed trace metals in sparingly soluble phyllosilicate structure may durably decrease their migration and bioavailability at the Earth’s surface and near surface.     

The kinetics of nucleation and crystal growth and scaling laws for magmatic crystallization

Magmatic crystallization depends on the kinetics of nucleation and crystal growth. It occurs over a region of finite thickness called the crystallization interval, which moves into uncrystallized magma. We present a dimensional analysis which allows a simple understanding of the crystallization characteristics. We use scales for the rates of nucleation and crystal growth, denoted by I _m and Y _m respectively. The crystallization time-scale _c and length-scale d _c are given by (Y _m ³ /I _m )^–1/4 and (·) _m ^1/2 respectively, where is thermal diffusivity. The thickness of the crystallization interval is proportional to this length-scale. The scale for crystal sizes is given by (Y _m /I _m )^1/4. We use numerical calculations to derive dimensionless relationships between all the parameters of interest: position of the crystallization front versus time, thickness of the crystallization interval versus time, crystal size versus distance to the margin, temperature versus time. We assess the sensitivity of the results to the form of the kinetic functions. The form of the growth function has little influence on the crystallization behaviour, contrary to that of the nucleation function. This shows that nucleation is the critical process. In natural cases, magmatic crystallization proceeds in continously evolving conditions. Local scaling laws apply, with time and size given by =(Y ³/I)^–1/4 and R=(Y/I)^1/4, where Y and I are the rates at which crystal are grown and nucleated locally. is the time to achieve crystallization and R the mean crystal size. We use these laws together with petrological observations to infer the in-situ values of the rates of nucleation and growth. Two crystallization regimes are defined. In the highly transient conditions prevailing at the margins of basaltic intrusions, undercoolings are high and the peak nucleation and growth rates must be close to 1cm^–3· ^–1 and 10^–7cm/s, in good agreement with laboratory measurements. In quasi-equilibrium conditions prevailing in the interior of large intrusions, undercoolings are small. We find ranges of 10^–7 to 10^–3 cm^–3 s^–1 and of 10^–10 to 10^–8cm/s for the local rates of nucleation and growth respectively.     

Toward a quantitative model of metamorphic nucleation and growth

The formation of metamorphic garnet during isobaric heating is simulated on the basis of the classical nucleation and reaction rate theories and Gibbs free energy dissipation in a multi-component model system. The relative influences are studied of interfacial energy, chemical mobility at the surface of garnet clusters, heating rate and pressure on interface-controlled garnet nucleation and growth kinetics. It is found that the interfacial energy controls the departure from equilibrium required to nucleate garnet if attachment and detachment processes at the surface of garnet limit the overall crystallization rate. The interfacial energy for nucleation of garnet in a metapelite of the aureole of the Nelson Batholith, BC, is estimated to range between 0.03 and 0.3?J/m² at a pressure of ca. 3,500?bar. This corresponds to a thermal overstep of the garnet-forming reaction of ca. 30°C. The influence of the heating rate on thermal overstepping is negligible. A significant feedback is predicted between chemical fractionation associated with garnet formation and the kinetics of nucleation and crystal growth of garnet giving rise to its lognormal??shaped crystal size distribution.     

Grain growth and re-orientation of phyllosilicate minerals during the development of slaty cleavage in the South Kitakami Mountains,northeast Japan

Using X-ray diffraction analysis, the mutual relations among illite crystallinity, degree of preferred orientation of chlorite, grain size change of chlorite and illite during metamorphism and development of slaty cleavage have been investigated for argillaceous rocks in the South Kitakami Mountains, northeast Japan.The metamorphic grade of IC (illite crystallinity index) = 0.29 is a critical one, beyond which the homogenization of chlorite composition, coarsening of chlorite and illite grains and degree of preferred orientation of chlorite are abruptly advanced. Grain coarsening is also promoted by the development of slaty cleavage, especially in the range of coarser grain size.The oriented growth by the effects of both the anisotropy of intrinsic growth rate of mineral grains and that of the environment in which grains grow, is considered to bring about the preferred orientation of chlorite and illite.     

Model of nucleation and growth of crystals in cooling magmas

The nucleation and growth of liquidus phases in cooling magmas at constant rates are modeled taking into account homogeneous nucleation, diffusion-limited growth, and depletion of crystallizing component from melt, and the temperature-dependent diffusivity. The formulation of governing equations shows that four dimensionless parameters, whose physical meanings are the nucleation difficulty, the fusion enthalpy, the ratio of the growth rate to the cooling rate, and the activation energy of diffusion, control the crystallization phenomena. The nucleation behavior with time (or temperature) is determined primarily by the competition between increasing nucleation rate with cooling and the reduced supersaturation with depletion by progressive growth of crystals previously nucleated. The maximum nucleation rate and the number density of crystals increase with decreasing interfacial tension and diffusivity, and with increasing fusion enthalpy and cooling rate. Quantitative expressions of the time or temperature interval for which the nucleation remains appreciable, the peak nucleation rate, the number density of crystals and the mean crystal radius are derived as functions of controlling parameters, and can be used to estimate the cooling rate or other unknown parameters from the number density of crystals of a rock.     

Variations in rates of nucleation and growth of biotite porphyroblasts

Differences in rates of nucleation and diffusion‐limited growth for biotite porphyroblasts in adjacent centimetre‐scale layers of a garnet‐biotite schist from the Picuris Mountains of New Mexico are revealed by variations in crystal size and abundance between two layers with strong compositional similarity. Relationships between fabrics recorded by inclusion patterns in biotite and garnet porphyroblasts are interpreted to reflect garnet growth following biotite growth, without substantial alteration of the biotite sizes. Sizes and locations of biotite crystals, obtained via high‐resolution X‐ray computed tomography, document that of the two adjacent layers, one has a larger mean crystal volume (9.5 × 10^?4v. 2.4 × 10^?4 cm³), fewer biotite crystals per unit volume (232 v. 576 crystals cm^?3), and a higher volume fraction of biotite (23%v. 14%). The two layers have similar mineral assemblages and mineral chemistry. Both layers show evidence for diffusional control of nucleation and growth. Pseudosection analysis suggests that the large‐biotite layer began to crystallize biotite at a temperature ～67 °C greater than the small‐biotite layer. Diffusion rates differed between layers, because of their different temperature ranges of crystallization, but this effect can be quantified. The bulk compositional difference between the layers, manifested in different modal amounts of biotite, has an effect on the biotite sizes that is also quantifiable and insufficient to account for the difference in biotite size. After these other possible causes of variation in crystal sizes have been eliminated, variability in nucleation and diffusion rates remain as the dominant factors responsible for the difference in porphyroblastic textures. Numerical simulations suggest that relative to the small‐biotite layer, the large‐biotite layer experienced elevated diffusion rates because of the higher crystallization temperature, as well as increased nucleation rates in order to achieve the observed size and number density of crystals. The simulations can replicate the observed textures only by invoking unreasonably large values for the thermal dependence of nucleation rates (activation energies), strongly suggesting that the observed textural differences arise from variations between layers in the abundance and energetics of potential nucleation sites.     

Composition,nucleation, and growth of iron oxide concretions

Iron oxide concretions are formed from post depositional, paleogroundwater chemical interaction with iron minerals in porous sedimentary rocks. The concretions record a history of iron mobilization and precipitation caused by changes in pH, oxidation conditions, and activity of bacteria. Transport limited growth rates may be used to estimate the duration of fluid flow events. The Jurassic Navajo Sandstone, an important hydrocarbon reservoir and aquifer on the Colorado Plateau, USA, is an ideal stratum to study concretions because it is widely distributed, well exposed and is the host for a variety of iron oxide concretions.Many of the concretions are nearly spherical and some consist of a rind of goethite that nearly completely fills the sandstone porosity and surrounds a central sandstone core. The interior and exterior host-rock sandstones are similar in detrital minerals, but kaolinite and interstratified illite–smectite are less abundant in the interior. Lepidocrocite is present as sand-grain rims in the exterior sandstone, but not present in the interior of the concretions.Widespread sandstone bleaching resulted from dissolution of early diagenetic hematite grain coatings by chemically reducing water that gained access to the sandstone through fault conduits. The iron was transported in solution and precipitated as iron oxide concretions by oxidation and increasing pH. Iron diffusion and advection growth time models place limits on minimum duration of the diagenetic, fluid flow events that formed the concretions. Concretion rinds 2 mm thick and 25 mm in radius would take place in 2000 years from transport by diffusion and advection and in 3600 years if transport was by diffusion only. Solid concretions 10 mm in radius would grow in 3800 years by diffusion or 2800 years with diffusion and advection.Goethite (α-FeO (OH)) and lepidocrocite (γ-FeO (OH)) nucleated on K-feldspar grains, on illite coatings on sand grains, and on pore-filling illite, but not on clean quartz grains. Model results show that regions of detrital K-feldspar in the sandstone that consume H⁺ more rapidly than diffusion to the reaction site determine concretion size, and spacing is related to diffusion and advection rates of supply of reactants Fe²⁺, O₂, and H⁺.     

Simulation of the nucleation and growth of binary solid solutions in aqueous solutions

We present a model which, for the first time, accounts for nucleation, growth and/or resorption of particles of variable composition in aqueous solutions (AS). Devised for describing the precipitation of binary solid solutions, it yields the time evolution of all ion activities in the AS, together with the particle population characteristics: number, size and composition profile of particles as a function of time and of their time of nucleation. We apply this numerical approach to the prototypical case of (Ba,Sr)CO₃ solid solution precipitation. We demonstrate the great sensitivity of the composition profiles and particle sizes to the initial conditions under which the AS is prepared, and thus illustrate the possibility of engineering the particle characteristics into a chosen state. Finally, by comparing the precipitation of two solid solutions (Ba,Sr)CO₃ and (Ba,Sr)SO₄, we evidence the sensitivity of the particle composition profiles to the ratio of the end-member solubility products, which leads to the formation of core-shell particles in the case of (Ba,Sr)SO₄.     

Molecular simulation studies on natural gas hydrates nucleation and growth: A review

How natural gas hydrates nucleate and grow is a crucial scientific question. The research on it will help solve practical problems encountered in hydrate accumulation, development, and utilization of hydrate related technology. Due to its limitations on both spatial and temporal dimensions, experiment cannot fully explain this issue on a micro-scale. With the development of computer technology, molecular simulation has been widely used in the study of hydrate formation because it can observe the...     

Kinetics of gypsum nucleation and crystal growth from Dead Sea brine

The Dead Sea brine is supersaturated with respect to gypsum (Ω = 1.42). Laboratory experiments and evaluation of historical data show that gypsum nucleation and crystal growth kinetics from Dead Sea brine are both slower in comparison with solutions at a similar degree of supersaturation. The slow kinetics of gypsum precipitation in the Dead Sea brine is mainly attributed to the low solubility of gypsum which is due to the high Ca²⁺/SO₄²⁻ molar ratio (115), high salinity (∼280 g/kg) and to Na⁺ inhibition.Experiments with various clay minerals (montmorillonite, kaolinite) indicate that these minerals do not serve as crystallization seeds. In contrast, calcite and aragonite which contain traces of gypsum impurities do prompt precipitation of gypsum but at a considerable slower rate than with pure gypsum. This implies that transportation inflow of clay minerals, calcite and local crystallization of minerals in the Dead Sea does not prompt significant heterogeneous precipitation of gypsum. Based on historical analyses of the Dead Sea, it is shown that over the last decades, as inflows to the lake decreased and its salinity increased, gypsum continuously precipitated from the brine. The increasing salinity and Ca²⁺/SO₄²⁻ ratio, which results from the precipitation of gypsum, lead to even slower kinetics of nucleation and crystal growth, which resulted in an increasing degree of supersaturation with respect to gypsum. Therefore, we predict that as the salinity of the Dead Sea brine continues to increase (accompanied by Dead Sea water level decline), although gypsum will continuously precipitate, the degree of supersaturation will increase furthermore due to progressively slower kinetics.     

Crystal nucleation and growth produced by continuous decompression of Pinatubo magma

High-temperature decompression experiments demonstrate that crystal textures preserve a record of the style and rate of magmatic ascent. To reinforce this link, we performed a suite of isothermal decompression experiments using starting material from the climactic 1991 Pinatubo eruption. We decompressed experiments from 220 MPa to final, quench pressures of 75 or 30 MPa using continuous decompression rates of 100, 30, 10, 3, 1, and 0.3 MPa h^?1. Amphibole, clinopyroxene, and plagioclase crystallized during the experiments, with plagioclase microlites dominating the assemblage. Total microlite number densities range from 10^7.6±0.4 up to 10^8.2±0.2 cm^?3, with plagioclase accounting for up to 65% of the total number. Plagioclase microlite area increased systematically from 19?±?8 to 937?±?487 µm² with increasing experiment duration. Our textures provide time-integrated records of crystal kinetics. Average nucleation and areal growth rates of plagioclase are highest in the fastest decompressions (~?10^7.5 cm^?3 h^?1 and 10.1?±?4.1 µm² h^?1, respectively) and more than an order of magnitude lower in the slowest experiments (~?10^5.5 cm^?3 h^?1 and 0.8?±?0.2 µm² h^?1, respectively). Both nucleation and growth rates are highest at high degrees of disequilibrium. We find that peak supersaturation-dependent instantaneous rates are generally more than an order of magnitude faster than average rates. We use those instantaneous nucleation and growth rates to introduce an iterative model to evaluate the effects of different decompression rates, decompression paths (continuous, single-step or multistep), and the presence of phenocrysts on final crystallinity and microlite size distribution.     

Numerical simulation of diffusion‐controlled nucleation and growth of porphyroblasts

This article presents the conceptual basis for a new numerical model of diffusion‐controlled nucleation and growth of porphyroblasts, describes its implementation, and illustrates the range of crystallization behaviours encompassed by it. The model differs from previous efforts principally in its ability to track explicitly the evolution in time and space of the chemical affinity for reaction in the intergranular medium, which provides a more accurate characterization of nucleation suppression in the vicinity of pre‐existing crystals and of growth suppression due to competition for nutrients among neighbouring crystals. Critical new features of these numerical simulations include: maintenance of local equilibrium for fluid in contact with reactants or products; persistence of reactants until they are eliminated by dissolution due to reaction progress and local diffusive flux; nucleation rates that vary as the local reaction affinity evolves; complex initial distributions of reactants if desired; and the flexibility to encompass any porphyroblast‐forming reaction for which changes in free energy as a function of time and temperature are specified. Model results reveal that radial growth rates remain proportional to the square‐root of time in diffusion systems buffered by persistent reactants; they document the interchangeable effects of diffusivity, porosity, and solubility on material fluxes and thus growth rates; and they illustrate the offsetting textural effects of rates of diffusion, nucleation, and heating. The initial distribution of reactants is found to exert a first‐order effect on crystal size distributions, confirming their limited utility for diagnosing crystallization mechanisms. These numerical simulations yield novel and rigorous confirmation of the textural effects of nucleation‐site saturation and variation in interconnected porosity, and reproduce with high fidelity much of the textural and chemical information gathered from natural specimens.     

The effect of metamorphic fluid flow on the nucleation and growth of garnets from Troms, North Norway

A spatial association is observed between the size distribution of garnet porphyroblasts and the size distribution of quartz veins in greenschist facies metapelites from Troms, North Norway. The size distribution of quartz veins reflects the flow regime of metamorphic fluids. The hypothesis that the flow regime of metamorphic fluids is also responsible for the size distribution of garnet crystals was tested by ascribing empirical acceleration parameters to the nucleation and growth rates of garnet crystals.
In regions where fluid flow was interpreted as pervasive', acceleration parameters for nucleation were high, whereas in regions where fluid flow was interpreted as channelled', acceleration parameters for growth were high. Accelerated crystal growth is further implied from the chemical zoning and crystal morphologies of garnets collected near discrete veins.
This spatial association may imply that fluid flow can be instrumental in controlling garnet crystallization. Fluid flow could affect garnet crystallization kinetics by facilitating thermal advection and/or mass transfer. In the study area, rhodochrosite (MnCO₃) veins provide evidence for mass transfer of Mn by fluid flow. An influx of Mn would expand the stability field of garnet to lower temperatures. The resulting thermal overstep could accelerate nucleation and/or growth of garnets.
The corollary of this study is that size distributions and chemical zoning of garnets, or other porphyroblast phases, can be used to study metamorphic fluid flow.     

The crystallographic fabric and texture of siderite in concretions: implications for siderite nucleation and growth processes

The crystallographic fabric of siderite in siderite concretions has been determined for upper Carboniferous (Westphalian‐A) non‐marine concretions and lower Jurassic (Pliensbachian) marine concretions. Compositional zoning indicates that individual siderite crystals grew over a period of changing pore water chemistry, consistent with the concretions being initially a diffuse patch of cement, which grew progressively. The siderite crystallographic fabric was analysed using the anisotropy of magnetic susceptibility, which is carried by paramagnetic siderite. The siderite concretions from marine and non‐marine formations exhibit differences in fabric style, although both display increases in the degree of preferred siderite c‐axis orientation towards the concretion margins. The Westphalian non‐marine siderites show a preferred orientation of siderite c‐axes in the bedding plane, whereas the Pliensbachian marine siderites have a preferred orientation of c‐axes perpendicular to the bedding. In addition, a single marine concretion shows evidence of earlier formed, inclined girdle‐type fabrics, which are intergrown with later formed vertical c‐axis siderite fabrics. The marine and non‐marine fabrics are both apparently controlled by substrate processes at the site of nucleation, which was probably clay mineral surfaces. Siderite nucleation processes on the substrate were most probably controlled by the (bio?) chemistry of the pore waters, which altered the morphology and crystallographic orientation of the forming carbonate. The preferred crystallographic orientation of siderite results from the orientation of the nucleation substrate. Fabric changes across the concretions partially mimic the progressive compaction‐induced alignment of the clay substrates, while the concretion grew during burial.     

Mineralogy,nucleation and growth of dolomite in the laboratory and sedimentary environment: A review

Dolomite [CaMg(CO₃)₂] forms in numerous geological settings, usually as a diagenetic replacement of limestone, and is an important component of petroleum reservoir rocks, rocks hosting base metal deposits and fresh water aquifers. Dolomite is a rhombohedral carbonate with a structure consisting of an ordered arrangement of alternating layers of Ca²⁺ and Mg²⁺ cations interspersed with anion layers normal to the c‐axis. Dolomite has symmetry, lower than the (CaCO₃) symmetry of calcite primarily due to Ca–Mg ordering. High‐magnesium calcite also has symmetry and differs from dolomite in that Ca²⁺ and Mg²⁺ ions are not ordered. High‐magnesium calcite with near‐dolomite stoichiometry (≈50 mol% MgCO₃) has been observed both in nature and in laboratory products and is referred to in the literature as protodolomite or very high‐magnesium calcite. Many dolomites display some degree of cation disorder (Ca²⁺ on Mg²⁺ sites and vice versa), which is detectable using transmission electron microscopy and X‐ray diffractometry. Laboratory syntheses at high temperature and pressure, as well as studies of natural dolomites show that factors affecting dolomite ordering, stoichiometry, nucleation and growth include temperature, alkalinity, pH, concentration of Mg and Ca, Mg to Ca ratio, fluid to rock ratio, mineralogy of the carbonate being replaced, and surface area available for nucleation. In spite of numerous attempts, dolomite has not been synthesized in the laboratory under near‐surface conditions. Examination of published X‐ray diffraction data demonstrates that assertions of dolomite synthesis in the laboratory under near‐ambient conditions by microbial mediation are unsubstantiated. These laboratory products show no evidence of cation ordering and appear to be very high‐magnesium calcite. Elevated‐temperature and elevated‐pressure experiments demonstrate that dolomite nucleation and growth always are preceded by very high‐magnesium calcite formation. It remains to be demonstrated whether microbial‐mediated growth of very high‐magnesium calcite in nature provides a precursor to dolomite nucleation and growth analogous to reaction paths in high‐temperature experiments.     

Investigation of the different binding edge sites for Zn on montmorillonite using P-EXAFS - The strong/weak site concept in the 2SPNE SC/CE sorption model

The 2 site protolysis non electrostatic surface complexation and cation exchange (2SPNE SC/CE) sorption model has been used over the past decade or so to quantitatively describe the uptake of metals with oxidation states from II to VI on 2:1 clay minerals; montmorillonite and illite. One of the main features in this model is that there are two broad categories of amphoteric edge sorption sites; the so called strong (S^SOH) and weak (S^W1OH) sites. Because of their different sorption characteristics, it was expected that the coordination environments of the surface complexes on the two site types would be different. Zn isotherm data on two montmorillonites, Milos and STx-1, were measured and modelled using the 2SPNE SC/CE sorption model. The results were used to define the most favourable experimental conditions under which Zn sorption was either dominated by the strong (S^SOH, ∼2 mmol kg⁻¹) or by the weak sites (S^W1OH, ∼40 mmol kg⁻¹). Highly oriented self-supporting films were prepared for polarised extended X-ray absorption fine structure (P-EXAFS) investigations.Montmorillonites often contain Zn incorporated in the clay matrix. The Zn bound in this form was quantified and the results from the analysis of the P-EXAFS spectra were taken into account in the interpretation of the spectra measured at low Zn loadings (∼2 mmol kg⁻¹) and medium Zn loadings (∼30 mmol kg⁻¹). The Zn spectra on the “strong sites” exhibited a pronounced angular dependency and formed surface complexes in the continuity of the Al-octahedral sheets at the montmorillonite edges. In contrast, the Zn “weak site” spectra showed only a weak angular dependency. The spectroscopic evidence indicates the existence of two distinct groups of edge surface binding sites which is consistent with a multi-site sorption model and in particular with the strong/weak site concept intrinsic to the 2SPNE S/CE sorption model.     

Simulation of the nucleation and growth of simple clay minerals in weathering processes: The NANOKIN code

We present a numerical approach which accounts for nucleation, growth and/or resorption of particles of fixed composition in aqueous solutions, and which involves functionalities suited to the formation of simple clay minerals in weathering processes, such as: formation of non-spherical particles, heterogeneous/homogeneous nucleation, several growth laws, precipitation resulting from the dissolution of primary minerals. The overall model is now embedded into a new numerical code called NANOKIN, in which several optimization procedures have been introduced in order to allow long dynamics to be followed. NANOKIN was applied to the precipitation of Al- bearing minerals from aqueous solutions: halloysite, kaolinite and Ca-montmorillonite. It allowed us to propose a stable scheme for the competitive precipitation of halloysite and kaolinite under two different types of initial conditions: (1) a given initial super-saturation state of the aqueous solution; (2) progressive super-saturation resulting from the kinetic dissolution of the minerals from a granitic rock under weathering conditions. Both yield particle sizes in the micron range, but with distinct crystal size distribution functions. The interplay between kinetic and thermodynamic effects is discussed.