Supersaturation Of Fluids Research Articles

Micro- to nano-sized solid inclusions in magnetite record skarn reactions

Abstract. Magnetite is a widespread ore mineral in skarn systems and usually hosts a wide variety of inclusions. Micro- to nano-sized solid inclusions in magnetite are unique tools to track the evolutionary processes of its host mineral and, subsequently, to constrain the timing of the mineralization event. In this study, we characterize micro- to nano-sized solid inclusions in magnetite from the La Víbora magnesian skarn (Málaga, Spain) using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM energy-dispersive X-ray spectrometry (EDS) analyses and compositional mapping expose two types of nano-inclusions oriented along the (111) of magnetite: type 1 includes dolomite, spinel, and Mg–Fe–Al silicate, and type 2 is made up of Mg–Fe–Al silicates enveloping the Mg-bearing amorphous silica phase. High-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), and fast Fourier transform (FFT) patterns reveal that the majority of the solid inclusions display non-oriented matrices compared to the host magnetite, precluding the possibility of sub-solidus processes. Instead, these inclusions are thought to preserve skarn mineral assemblages that were entrapped during the growth of magnetite. However, the local supersaturation of fluids trapped in the boundary layer of crystallizing magnetite is evidenced by coherent lattice orientation of precipitated and host magnetite and by the occurrence of an Mg-bearing amorphous silica phase. Our findings reveal that skarn reactions observed at field and microscopic scales are also recorded in nano-sized inclusions within magnetite. These observations underscore the significance of micro- to nano-scale solid inclusions in magnetite to decipher overprinted skarn reactions as well as constraining the timing of Fe mineralization events in skarns.

Controls of temperature and mineral growth rate on Mg incorporation in aragonite

The incorporation of Mg in aragonite was experimentally investigated as a function of mineral growth rate at 5, 15 and 25 °C using the constant addition technique. Aragonite growth rate was found to be the major parameter affecting the Mg partitioning coefficient (i.e. DMg=(MgCa)solid(MgCa)fluid) between aragonite and fluid, whereas the effect of temperature is smaller but measurable. At similar surface normalized growth rates, DMg values decrease as temperature increases from 5 to 25 °C. The magnitude of decrease as a function of temperature is similar for all the experiments of this study where growth rate varied in the range 10-8.6 ≤ rp ≤ 10-7.1 (mol/m2/s). The combined effect of aragonite growth rate and temperature on DMg can be described by the linear equation:Log DMg = 0.583(±0.020) Log rp − 0.026(±0.001) T + 0.863(±0.153); R2 = 0.97.where T is the temperature in degrees Celsius.The increase of DMg values at decreasing temperatures in experiments conducted at similar growth rates is consistent with the increase of fluid supersaturation with respect to aragonite. Thus, it can be inferred that increased Mg incorporation at higher supersaturation is associated with the greater presence of defect sites on the growing mineral surface, similar to the incorporation of other incompatible ions in carbonate minerals. Overall, the relationship between Mg content of aragonite with the degree of saturation of the fluid with respect to this mineral phase suggests that DMg values or Mg/Ca ratio in natural aragonites can be used as a proxy for saturation degree of the formation fluid with respect to CaCO3 minerals.

Copper-Arsenic Nanoparticles in Hematite: Fingerprinting Fluid-Mineral Interaction

Metal nanoparticles (NP) in minerals are an emerging field of research. Development of advanced analytical techniques such as Z-contrast imaging and mapping using high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) allows unparalleled insights at the nanoscale. Moreover, the technique provides a link between micron-scale textures and chemical patterns if the sample is extracted in situ from a location of petrogenetic interest. Here we use HAADF STEM imaging and energy-dispersive X-ray spectrometry (EDX) mapping/spot analysis on focused ion beam prepared foils to characterise atypical Cu-As-zoned and weave-twinned hematite from the Olympic Dam deposit, South Australia. We aim to determine the role of solid-solution versus the presence of discrete included NPs in the observed zoning and to understand Cu-As-enrichment processes. Relative to the grain surface, the Cu-As bands extend in depth as (sub)vertical trails of opposite orientation, with Si-bearing hematite NP inclusions on one side and coarser cavities (up to hundreds of nm) on the other. The latter host Cu and Cu-As NPs, contain mappable K, Cl, and C, and display internal voids with rounded morphologies. Aside from STEM-EDX mapping, the agglomeration of native copper NPs was also assessed by high-resolution imaging. Collectively, such characteristics, corroborated with the geometrical outlines and negative crystal shapes of the cavities, infer that these are opened fluid inclusions with NPs attached to inclusion walls. Hematite along the trails features distinct nanoscale domains with lattice defects (twins, 2-fold superstructuring) relative to hematite outside the trails, indicating this is a nanoprecipitate formed during replacement processes, i.e., coupled dissolution and reprecipitation reactions (CDRR). Transient porosity intrinsically developed during CDRR can trap fluids and metals. Needle-shaped and platelet Cu-As NPs are also observed along (sub)horizontal bands along which Si, Al and K is traceable along the margins. The same signature is depicted along nm-wide planes crosscutting at 60° and offsetting (012)-twins in weave-twinned hematite. High-resolution imaging shows linear and planar defects, kink deformation along the twin planes, misorientation and lattice dilation around duplexes of Si-Al-K-planes. Such defects are evidence of strain, induced during fluid percolation along channels that become wider and host sericite platelets, as well as Cl-K-bearing inclusions, comparable with those from the Cu-As-zoned hematite, although without metal NPs. The Cu-As-bands mapped in hematite correspond to discrete NPs formed during interaction with fluids that changed in composition from alkali-silicic to Cl- and metal-bearing brines, and to fluid rates that evolved from slow infiltration to erratic inflow controlled by fault-valve mechanism pumping. This explains the presence of Cu-As NPs hosted either along Si-Al-K-planes (fluid supersaturation), or in fluid inclusions (phase separation during depressurisation) as well as the common signatures observed in hematite with variable degrees of fluid-mineral interaction. The invoked fluids are typical of hydrolytic alteration and the fluid pumping mechanism is feasible via fault (re)activation. Using a nanoscale approach, we show that fluid-mineral interaction can be fingerprinted at the (atomic) scale at which element exchange occurs.

Surface Wetting Controls Calcium Carbonate Crystallization Kinetics

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.

Calcite sealing in a fractured geothermal reservoir: Insights from combined EBSD and chemistry mapping

Fractures play an important role as fluid flow pathways in geothermal resources hosted in indurated greywacke basement of the Taupo Volcanic Zone, New Zealand, including the Kawerau Geothermal Field. Over time, the permeability of such geothermal reservoirs can be degraded by fracture sealing as minerals deposit out of transported geothermal fluids. Calcite is one such fracture sealing mineral. This study, for the first time, utilises combined data from electron backscatter diffraction and chemical mapping to characterise calcite vein fill morphologies, and gain insight into the mechanisms of calcite fracture sealing in the Kawerau Geothermal Field. Two calcite sealing mechanisms are identified 1) asymmetrical syntaxial growth of calcite, inferred by the presence of single, twinned, calcite crystals spanning the entire fracture width, and 2) 3D, interlocking growth of bladed vein calcite into free space as determined from chemical and crystallographic orientation mapping. This study also identifies other potential uses of combined EBSD and chemical mapping to understand geothermal field evolution including, potentially informing on levels of fluid supersaturation from the study of calcite lattice distortion, and providing information on a reservoir's history of stress, strain, and deformation through investigation of calcite crystal deformation and twinning patterns.

On the effect of aqueous Ca on magnesite growth – Insight into trace element inhibition of carbonate mineral precipitation

Motivated by the strong effect of aqueous Mg on calcite growth rates, this study used hydrothermal atomic force microscopy (HAFM) and hydrothermal mixed-flow reactor (HMFR) experiments to explore the effect of aqueous Ca on magnesite growth kinetics at 100°C and pH ∼7.7. Obtuse step velocities on (104) surfaces during magnesite growth were measured to be 4±3nm/s at fluid saturation states, equal to the ion activity quotient divided by the equilibrium constant for the magnesite hydrolysis reaction, of 86–117. These rates do not vary systematically with aqueous Ca concentration up to 3×10−3mol/kg. Magnesite growth rates determined by HAFM are found to be negligibly affected by the presence of aqueous Ca at these saturation states and are largely consistent with those previously reported in aqueous Ca-free systems by Saldi et al. (2009) and Gautier et al. (2015). Similarly, magnesite growth rates measured by HMFR exhibit no systematic variation on aqueous Ca concentrations. Rates in this study, however, were extended to higher degrees of fluid supersaturation with respect to magnesite than previous studies. All measured HMFR rates can be accurately described taking account the combined effects of both the spiral growth and two dimensional nucleation/growth mechanisms. Despite the lack of a clear effect of aqueous Ca on magnesite growth rates, Raman spectroscopy confirmed the incorporation of up to 8mol percent of Ca2+ into the growing magnesite structure.

Serpentinization and Fluid Pathways in Tectonically Exhumed Peridotites from the Southwest Indian Ridge (62-65 E)

Peridotites exhumed in the footwall of axial detachment faults at slow-spreading ridges are highly serpentinized. Most mid-ocean ridge detachment settings are magmatically active and hydrous fluid circulation in and near the fault has been shown to be influenced by the presence of melt or magmatic lithologies. Our working area along the Southwest Indian Ridge (62–65°E) is nearly amagmatic and represents an end-member to study the hydrous alteration of exhumed peridotites without these magmatic influences. We use an integrated petrological approach combining microstructural, mineralogical and chemical observations to unravel the sequence of serpentinization in 272 dredged samples of variably serpentinized peridotites and to document the circulation of serpentinizing fluids in and near the exhumation faults. We find that serpentine recrystallization and veins overprint the initial serpentinite mesh texture in ∼25% of the samples. Oxygen isotope data suggest that this sequence developed at relatively high temperatures (271–336°C) and under increasing fluid–rock ratios, from near stoichiometry for mesh texture formation to >10 during recrystallization. Increasing fluid supersaturation relative to serpentine favors the replacement of mesh texture lizardite by chrysotile and polygonal or polyhedral serpentine. We attribute local recrystallization into antigorite to moderate Si-metasomatism, possibly following pyroxene serpentinization. We do not observe the more pronounced Si-metasomatism leading to talc replacing serpentine that is reported for the more magmatically active Mid-Atlantic Ridge detachment settings and is attributed to prior leaching of magmatic rocks. Scales of preferential fluid pathways in our samples evolved from pervasive and close-spaced (<500 µm) microfractures during the formation of the initial serpentine mesh texture, to centimeter-thick planar domains of enhanced fluid flux, spaced at ∼10 cm intervals and probably grouped in corridors that may be up to ∼100 m across. Serpentine minerals are enriched in some fluid-mobile elements (Cl, B, U) relative to the peridotite protolith, and several elements (Al, Fe, Si, Cu, As, Sb, REE) are redistributed at the millimeter to decimeter scale. Serpentinizing fluids were seawater-derived, probably mildly alkaline (small to no europium anomalies), reducing and H2-enriched (formation of magnetite). These fluids may have been similar to, though warmer than, those venting at the ultramafic-hosted Lost City hydrothermal fluid (30°N, Mid-Atlantic Ridge).

Numerical Simulation of Pore Size Dependent Anhydrite Precipitation in Geothermal Reservoirs

Porosity and permeability of reservoirs are key parameters for an economical use of hydrogeothermal energy. Both may be significantly reduced by precipitation of minerals. The Allermöhe borehole near Hamburg (Germany) represents a case in which precipitation of anhydrite resulted in a failed reservoir development. We introduce a new numerical approach for better understanding of this, considering both the spatial variations of supersaturation on the pore scale and the pore space structure. Besides pore size this requires consideration of the effects of surface charge density on the fluid supersaturation in porous media and of the role of fluid flow velocity.

Blood oxygenation using microbubble suspensions

Microbubbles have been used in a variety of fields and have unique properties, for example shrinking collapse, long lifetime, efficient gas solubility, a negatively charged surface, and the ability to produce free radicals. In medicine, microbubbles have been used mainly as diagnostic aids to scan various organs of the body, and they have recently been investigated for use in drug and gene delivery. However, there have been no reports of blood oxygenation by use of oxygen microbubble fluids without shell reagents. In this study, we demonstrated that nano or microbubbles can achieve oxygen supersaturation of fluids, and may be sufficiently small and safe for infusion into blood vessels. Although Po(2) increases in fluids resulting from use of microbubbles were inhibited by polar solvents, normal saline solution (NSS) was little affected. Thus, NSS is suitable for production of oxygen-rich fluid. In addition, oxygen microbubble NSS effectively improved hypoxic conditions in blood. Thus, use of oxygen microbubble (nanobubble) fluids is a potentially effective novel method for oxygenation of hypoxic tissues, for infection control, and for anticancer treatment.

Magnesium isotope fractionation during hydrous magnesium carbonate precipitation with and without cyanobacteria

The hydrous magnesium carbonates, nesquehonite (MgCO 3·3H 2O) and dypingite (Mg 5(CO 3) 4(OH) 2·5(H 2O)), were precipitated at 25 °C in batch reactors from aqueous solutions containing 0.05 M NaHCO 3 and 0.025 M MgCl 2 and in the presence and absence of live photosynthesizing Gloeocapsa sp. cyanobacteria. Experiments were performed under a variety of conditions; the reactive fluid/bacteria/mineral suspensions were continuously stirred, and/or air bubbled in most experiments, and exposed to various durations of light exposure. Bulk precipitation rates are not affected by the presence of bacteria although the solution pH and the degree of fluid supersaturation with respect to magnesium carbonates increase due to photosynthesis. Lighter Mg isotopes are preferentially incorporated into the precipitated solids in all experiments. Mg isotope fractionation between the mineral and fluid in the abiotic experiments is identical, within uncertainty, to that measured in cyanobacteria-bearing experiments; measured δ 26Mg ranges from −1.54‰ to −1.16‰ in all experiments. Mg isotope fractionation is also found to be independent of reactive solution pH and Mg, CO 3 2−, and biomass concentrations. Taken together, these observations suggest that Gloeocapsa sp. cyanobacterium does not appreciably affect magnesium isotope fractionation between aqueous fluid and hydrous magnesium carbonates.

Morphological and compositional changes in the skeletons of new coral recruits reared in acidified seawater: Insights into the biomineralization response to ocean acidification

We reared primary polyps (new recruits) of the common Atlantic golf ball coral Favia fragum for 8 days at 25°C in seawater with aragonite saturation states ranging from ambient (Ω = 3.71) to strongly undersaturated (Ω = 0.22). Aragonite was accreted by all corals, even those reared in strongly undersaturated seawater. However, significant delays, in both the initiation of calcification and subsequent growth of the primary corallite, occurred in corals reared in treatment tanks relative to those grown at ambient conditions. In addition, we observed progressive changes in the size, shape, orientation, and composition of the aragonite crystals used to build the skeleton. With increasing acidification, densely packed bundles of fine aragonite needles gave way to a disordered aggregate of highly faceted rhombs. The Sr/Ca ratios of the crystals, measured by SIMS ion microprobe, increased by 13%, and Mg/Ca ratios decreased by 45%. By comparing these variations in elemental ratios with results from Rayleigh fractionation calculations, we show that the observed changes in crystal morphology and composition are consistent with a &gt;80% decrease in the amount of aragonite precipitated by the corals from each “batch” of calcifying fluid. This suggests that the saturation state of fluid within the isolated calcifying compartment, while maintained by the coral at levels well above that of the external seawater, decreased systematically and significantly as the saturation state of the external seawater decreased. The inability of the corals in acidified treatments to achieve the levels of calcifying fluid supersaturation that drive rapid crystal growth could reflect a limit in the amount of energy available for the proton pumping required for calcification. If so, then the future impact of ocean acidification on tropical coral ecosystems may depend on the ability of individuals or species to overcome this limitation and achieve the levels of calcifying fluid supersaturation required to ensure rapid growth.

Nanoscale effects of strontium on calcite growth: An in situ AFM study in the absence of vital effects

This experimental study presents in situ measurements of step migration rates for layer growth of calcite at various levels of superaturation and fluid Sr concentrations. Our results show that Sr has complex behavior as an impurity. At low concentrations, Sr promotes faster growth. This effect may be associated with slight shifts in calcite solubility when Sr is incorporated or may be due to as yet uncharacterized kinetic effects. At higher concentrations, Sr stops step advancement by pinning kink-sites or step edges. The threshold concentration of Sr needed to halt growth is positively correlated with supersaturation. Addition of Sr to the calcite growth system leads to significant changes in hillock morphology. Hillocks become elongate perpendicular to the projection of the c-glide plane, in contrast to the changes previously reported for Mg. Step edges also become scalloped, and the boundary between the obtuse-stepped flanks disappears and is replaced by a new step direction with edges parallel to [010]. Incorporation of Sr was measured at two supersaturation levels and identical fluid [Sr]. The results indicate a strong positive correlation between fluid supersaturation and crystal Sr content. Further, Sr is strongly fractionated between obtuse- and acute-stepped flanks by a factor of approximately two. The sensitivity of Sr uptake to supersaturation may explain apparently contradictory results in the literature regarding whether Sr uptake in the calcite produced by one-celled marine organisms is controlled by temperature. In addition, Sr contents of natural calcite samples may be good indicators of the levels of supersaturation at which the crystals grew. Results of this investigation demonstrate the importance of understanding impurity-specific interactions with calcite growth surfaces at the microscopic scale. Despite similar chemical behavior in some systems, Mg and Sr clearly have very different effects on calcite growth. If Sr and other impurities are to be used as robust indicators of growth conditions in natural calcite samples, well grounded understanding of the mechanisms of recording trace element signatures in calcite is an essential step toward correctly deciphering paleoenvironmental signals from fossil calcite compositions.

The Serpentinite Multisystem Revisited: Chrysotile Is Metastable

The two rock-forming polymorphs of serpentine Mg3Si2O5(OH)4, lizardite and chrysotile, occur in nature in virtually identical ranges of temperature and pressure, from surficial or near-surficial environments to temperatures perhaps as high as 400°C. Laboratory evidence indicates that lizardite is the more stable at low temperatures, but the difference in their Gibbs free energies is not more than about 2 kJ in the 300-400°C range. Above about 300°C, antigorite + brucite is more stable than both; in other words, chrysotile is nowhere the most stable. The crystal structures of lizardite and chrysotile give rise to contrasting crystallization behaviors and hence modes of occurrence. The hydration of peridotite at low temperature results in the growth of lizardite from olivine, and (commonly topotactically) from chain and sheet silicates, although the MgO-SiO2-H2O (MSH) phase diagram predicts antigorite + talc in bastite. The activity of H2O during serpentinization may be buffered to low values by the solids, making the reaction of olivine to lizardite + brucite a stable one. Conservation of oxygen and inheritance of the Fe2+/Mg exchange potential of olivine lead predictably to the precipitation of a highly magnesian lizardite and magnetite, and to the evolution of H2. Volume expansion is made possible by lizardite's force of crystallization, and it is tentatively suggested that this might account for the a-serpentine orientation (length normal to (001)) of lizardite pseudofibers in mesh rims and hourglass pseudomorphs after olivine. Whereas mineral replacements commonly conserve volume, in massif serpentinites the diffusive loss of Mg and Si needed for volume conservation during serpentinization requires chemical potential gradients that are unlikely to exist. For small bodies of serpentinite, sheared serpentinite, and systems of large water/rock ratio, volume expansion may be much less. Chrysotile is most conspicuously developed in tectonically active environments, where associated lithotypes show marginal greenschist-facies parageneses and antigorite tends to make its first appearance. Chrysotile growth is favored in isotropic stress microenvironments of fluid-filled voids and pores (where it may ultimately crystallize pervasively), and in veins, generally after active hydration in the immediate surroundings has ceased. This nevertheless allows the simultaneous growth of lizardite and chrysotile in adjacent partially and fully serpentinized peridotite, respectively, as in the cores and rims of kernel structures. Although prominent along shear surfaces, chrysotile growth as slip fiber is promoted by the presence of fluid rather than shear stress. Unlike lizardite, whose growth produces the stress associated with expansion, extreme flattening and shear might be expected to destroy the chrysotile structure. Thus, lizardite and chrysotile behave as though they were a stress-antistress mineral pair. Calorimetric, solubility, and reaction-reversal experiments on chrysotile integrate contributions to its free energy arising from surface properties and, most importantly, from its radius-dependent strain energy. Minimally strained chrysotile (r ≈ 90 Å) may in fact be more stable than lizardite, whereas a maximal-radius chrysotile (r ≈ 200 Å) is not. A model for chrysotile in veins cutting lizardite- or antigorite-bearing rock involves nucleation of low-strain chrysotile followed by kinetically favored crystallization of higher-energy layers driven by mild fluid supersaturation maintained by local potential gradients. It is not clear if this explanation adequately accounts for serrate veins and mass-fiber chrysotile. A revised phase diagram for lizardite and antigorite is offered, and possible stable and metastable reactions among the phases in serpentinites are followed on an isobaric diagram of reaction free energy (driving force) as a function of temperature. Composition-induced equilibrium shifts are believed unlikely to be determinative in most occurrences of Mg-rich lizardite and chrysotile. Circumstances of growth rather than temperature and pressure determine the occurrence of chrysotile vis-à-vis lizardite in serpentinites.

Thaumasite and secondary calcite in some Norwegian concretes

Abstract Thaumasite formation (TF) and limited thaumasite form of sulfate attack (TSA) has recently been detected in several Norwegian sprayed concretes. TF and TSA is frequently associated with contemporaneous and late stage internal calcite formation by: (a) decalcification of calcium silicate hydrate (CSH): (b) decomposition of thaumasite associated with secondary liberation of SO 4 2− and occasional formation of subordinate gypsum; (c) supersaturation of fluids in voids. Popcorn calcite and other textural forms were characteristic for these reactions. Also co-precipitation of popcorn calcite + thaumasite, as well as later stage dissolution of both minerals occurred. The entire process was represented by a drop in pore fluid pH from about 13 towards 5–7. In this paper we study the TF–TSA–carbonation process in several environments: (1) three examples of 2–13 years old steel fibre reinforced sprayed concrete made with Sulfate Resisting Portland Cement (SRPC) and silica fume in contact with carbon-, calcite- and sulfide bearing Alum Shale: (2) two examples of ≈30 years old, and severely damaged, SRPC based sprayed concrete within the Alum Shale: (3) one 16 years old sprayed concrete made with Portland Cement (PC) and possibly fly ash in presence of sulfate bearing ground water and (4) one 10 years old steel fibre reinforced sprayed concrete in a sub-sea tunnel with inflow of somewhat modified seawater. This PC based concrete with silica fume had suffered localised crumbling and mush formation after less than 5 years. The critical factors for thaumasite formation are discussed together with consequences for further deterioration and timing of repair.

Experimental and Theoretical Treatment of Molecular-scale Mechanisms of Crystal Growth on Baryte

We have used Atomic Force Microscopy in a fluid cell with close to molecular resolution to observe different growth types such as two-dimensional nucleation, anisotropic effects during layer-by-layer growth and spiral growth with structure-induced selfinhibition on different faces of baryte. In combination with computer simulations, we were able to resolve these growth mechanisms and their relative importance for overall growth at an atomic level. This knowledge allows us to critically test the relevance of classical crystal growth theories, specifically the relationship between crystal growth mechanisms and fluid supersaturation as well as the effect of surface crystal structure on growth morphology. According to the classical BurtonCabrera-Frank (BCF) theory, we found that spiral growth is the predominant mechanism at the lowest supersaturation, whereas two-dimensional nucleation prevails at somewhat higher supersaturations. Even though Periodic-Bond-Chain (PBC) theory can explain some of the directions of terminating steps of growth islands, computer simulations involving ionic attachment energies are necessary to explain anisotropic effects into different growth directions and finally explain the sector-like shape of these islands. We also demonstrate that this effect leads to a self-inhibition of spiral growth on baryte, although spiral growth is widely regarded as the most important growth mechanism under near-equilibrium conditions. In order to understand the mechanisms of crystal growth at different conditions (e.g. supersaturation, pH), for different minerals and in the presence of different growth inhibitors/chelators, it is necessary to be able to model these processes by using dynamic molecular modelling techniques. In addition to the deterministic approach that explains the shape of the growth islands, we developed a semistatistical Monte-Carlo model which allows to calculate growth of processes such as dynamic attachment/ dissolution, surface diffusion, relaxation of the surface structure and migration of ions in the nearsurface region. The difference between the different minerals (BaSO4, SrSO4, PbSO4) then only arises from applying the respective sets of ionic potentials that were derived from structural data, elastic properties and phonon frequencies of the respective minerals. Once crystal growth in a pure BaSO4 solution is calculated, we introduced the presence of different organic growth inhibitors and chelators as an additional parameter. These play a crucial role in nature and for geotechnical applications. Molecular modelling techniques can be applied to calculate the surface sites to which the organic molecules will be bonded. These processes influence the rates of growth and dissolution and change the shape of growth islands and etch pits as can be verified by using AFM.

Fluid supersaturation and crystallization in porous media

AbstractThe relationship between the supersaturation at the point of crystallization and the rate at which supersaturation increases has been studied from nucleation experiments on barite BaSO4, strontianite SrCO3, witherite BaCO3 and gypsum CaSO4.2H2O. The crystallization experiments have been carried out by the counter-diifusion of cations and anions through a column of porous silica gel transport medium. Nucleation is suppressed in a finely-porous medium resulting in very high values of supersaturation before crystallization from the solution begins. This threshold supersaturation for nucleation depends on the solubility of the salt, the porosity of the medium and the supersaturation rate. Nucleation inhibitors were used to extend the range of supersaturation attainable. In all cases the experimental data fits the general expression: rate of change of supersaturation ∝ (threshold supersaturation)m. These results are compared to previous work from the field of chemical engineering on the relationship between supersaturation, volume and cooling rate in aqueous salt solutions. These experiments have important implications to supersaturation in natural fluids and subsequent crystallization in relation to geological problems including crystallization in low temperature sedimentary environments and fluid-rock ratios in hydrothermal mineral deposits.

The Effect of Supersaturation on Apatite Crystal Formation in Aqueous Solutions at Physiologic pH and Temperature

The importance of supersaturation in the dynamics of apatite precipitation from aqueous solutions is well-established. To determine whether this parameter has a comparable impact on the concomitant development of the textural properties of this phase, such as crystal size and shape, we investigated mineral accretion in synthetic solutions seeded with 0.67 g/L apatite over a range of supersaturations at pH 7.4 and 37 degrees C. A dual specific-ion electrode-controlled titration method was used to maintain the seeded reactions under the following solution conditions: 1.0 to 1.8 mmol/L Ca2+, 0.67 to 1.2 mmol/L total phosphate (PO4), Ca/PO4 (initial) = 1.5, 143 mmol/L KNO3, and 10 mmol/L HEPES. Samples were collected for chemical and textural analyses when the seed apatite was reduced by new accretions to 1/2, 1/4, 1/8, 1/16, and 1/32 of the total solids in suspension. All new accretions were found to be apatitic. At the lowest supersaturation, accretion occurred primarily by growth of the seed crystals. However, at the highest supersaturation examined, the crystals at the end of the experiments were actually smaller, on average, than the original seeds, even though the total mass increased 32-fold. The results suggest that proliferation of new crystals supplanted growth of the seed crystals as supersaturation was increased. The results also suggest that differences in tissue fluid supersaturation may contribute to the large disparity in dimensions between dentin and enamel apatite crystals.