Werner Rammensee

Dissolution Rates of Calcite (104) Obtained by Scanning Force Microscopy: Microtopography-Based Dissolution Kinetics on Surfaces with Anisotropic Step Velocities

This paper reports on our use of scanning force microscopy (SFM) to study calcite dissolution rates. Until now, calculation of rates has been limited to surfaces on which steps retreat at an isotropic velocity. More commonly, on surfaces where steps in different directions retreat at different velocities, the rate depends on the velocities and the densities of the differently oriented steps. Here, we show that calculating rates from SFM image sequences is possible for anisotropic surfaces if the ratio of step lengths in different directions is constant in the analyzed images (e.g., by considering steps at nonintersecting pits exclusively). In contrast to nonintersecting pits, high velocity steps are formed at intersecting pits on the calcite (104) surface. At these steps, material can be removed without the slow nucleation of kink sites. The morphology of these steps is not straight and they become easily pinned by particles or impurities. Therefore, measuring the velocity of step retreat directly in the images fails, but calculating the dissolution rate of surface regions with high velocity steps is still possible. Dissolution rate is roughly similar in both the deep etch pits and the areas with high velocity steps at intersecting pits. Consequently, we suggest that the formation of high velocity steps contributes considerably to the weak enhancement of the rate with increasing dislocation density because additional etch pits within the intersectional regions do not significantly increase the rate. The calculated dissolution rate of ∼1.5 × 10−6 mol m−2 s−1 on the calcite (104) surface in water at 24°C and pCO2 = 10−3.5 atm corresponds well to the rates obtained from batch experiments. Thus, SFM can be regarded as an instrument capable of acquiring rates even on surfaces with anisotropic step velocities.

In situ investigation of growth and dissolution on the (010) surface of gypsum by Scanning Force Microscopy

Abstract The kinetics of crystal growth and dissolution on the (010) surface of gypsum were investigated in situ by Scanning Force Microscopy (SFM). It could be shown that growth and dissolution on the (010) surface in aqueous solution is a layer-by-layer process. Monolayer steps parallel to [001], [100], and [101], move with a distinctive anisotropy in velocity. During dissolution experiments, etch pits develop at surface regions, where local structural defects may occur. Therefore, etch pits are getting deeper at the same lateral position. As a consequence of the anisotropy in velocity of step movement, the etch pits are elongated in [001]. Isolated holes remain stable over a long period during growth experiments. Such holes can occur at the same position in several overgrowing monolayers. This kind of memory effect might also be explained by local structural defects. Local surface topography has a strong influence on the velocity of step movement. At surface regions with a high step density, the velocity of monolayer step movement is reduced compared to isolated monolayer steps. Isolated [100] monolayer steps move with a velocity of up to 30.0 nm s−1, whereas [010] steps move with up to 9.5 nm s−1, and [001] steps 2.5 nm s−1, in an undersaturated aqueous solution (9.8 mmol L−1). Imaging monolayer steps with lateral molecular resolution reveals the molecular arrangement at mono-layer steps. Individual kink sites can be observed. The kink site formation energy along [001] monolayer steps is 4.1 ± 0.7 KJ mol−1 in saturated aqueous solution. Observed kink site density agrees with predicted values from Monte Carlo simulations.

Dissolution rates and activation energy for dissolution of brucite (001) : A new method based on the microtopography of crystal surfaces

Abstract Scanning Force Microscopy (SFM) was used to develop a method which can provide quantitative kinetic data of crystal growth and dissolution. Based on observations of single crystal faces in monolayer resolution, morphology and temperature dependent growth and dissolution rates can be obtained. From these kinetic data activation energies can be calculated and compared with existing theories. The experimental method works by extracting grown or dissolved terrace areas and step densities from image sequences taken at different temperatures. As an example, the method is applied for dissolution on the brucite (001) surface in acidic water (pH 2.7) within the temperature range of 21 to 35°C. At these conditions the dissolution rate depends nonlinearly on the step density and gives evidence for interstep interaction. The calculated activation energy for dissolution is 60 ± 12 kJ mol−1. With this high activation energy, dissolution cannot be regarded as a transport-controlled process, and is therefore surface controlled.

Elemental and isotopic fractionations produced through evaporation of the Allende CV chondrilte: Implications for the origin of HAL-type hibonite inclusions

Abstract Through evaporation of samples from the Allende carbonaceous chondrite we have produced a series of residues that show correlated variations in mineralogy, chemistry, and isotopic compositions. Major and minor elements are evaporated in the order (Fe, Mn, Cr) → (Mg, Si) → (Ca, Ti) → (Al) and their loss is reflected in the mineralogy of the remaining samples. Residues of low to moderate degrees of evaporation consist of increasingly Mg-rich olivine and silicate glass. After complete evaporation of Fe, Mg, and Si at approximately 96% mass loss, the residues consist of very fine-grained Ca aluminates. Evaporation at higher temperatures produced three residues that contain hibonite and a less refractory CaAl glass. Magnesium, Si, Ca, Ti, and O isotopes show mass-dependent fractionations that are consistent with Rayleigh-type distillation. The rare earth elements and other refractory trace elements are enriched in the residues up to ∼100 × CI, although several elements (V, Ba, Ce) are depleted due to their increased volatilities under oxidizing conditions. The most refractory residues also exhibit depletions in Eu, an element that is volatile under reducing conditions, but is as refractory as the other light rare earth elements under oxidizing conditions. The apparently contradictory presence of both Ce and Eu depletions in the residues is a result of changing evaporation dynamics in the course of the experiments: the release of large amounts of O during evaporation of the major element oxides creates “locally oxidizing” conditions in the samples; later, after most major elements have been vaporized, the local sample environments become more “reducing.” The three hibonite-bearing residues share many chemical and isotopic characteristics with five HAL-type hibonite inclusions for which an origin as distillation residues has been proposed; our data show that many of the unique features of these inclusions can be produced in a single evaporation event. Strong similarities between the hibonite-bearing residues and the hibonite inclusions HAL and DH-H1 suggest that the evaporation histories of these inclusions may be roughly comparable to those of our residues.

Solid-solid reactions mediated by a gas phase; an experimental study of reaction progress and the role of surfaces in the system olivine+iron metal

Abstract The intergranular fluid involved in solid-solid reactions is tacitly assumed to be a melt or a (C-O-H-S-Cl-F)-bearing phase. We have studied the system olivine+metal using diffusion couple experiments, in situ reaction progress monitoring using Knudsen-cell mass spectrometry, and thermodynamic-kinetic analysis to show that a dry vapor phase coexisting with solids (silicates, oxides, metals) has all the characteristics of a classical petrologic ‘‘intergranular fluid,’’ and it is a viable transport agent for major rock-forming elements such as Mg, Fe, or Si in many petrologic situations. Some of the major conclusions of the work are: (1) ignoring the vapor phase leads to incorrect estimation of degrees of freedom and consequently, incorrect interpretations of mineral assemblages and zonation; (2) normally refractory elements such as Mg may in some cases be more volatile than O2; and (3) reaction modeling using free-energy minimization allows the main parameters controlling reaction progress, pathway, and products (assemblage, abundance of phases, and composition) to be identified. These parameters include: available reactive surface area; volume of the reaction system; diffusion rates in the product solid; temperature; and relative rates of reaction to transport (in/out of the system). Components other than those appearing explicitly in the mass-balance equations (e.g., ƒO₂ in the olivine+metal system) O2 may play an important role. Transport of Mg in the vapor phase away from local reaction sites explains the compositional zoning of olivine around FeNi-metal inclusions and simultaneously provides a mechanism for the growth of at least some of the fayalite-rich rims in Allende and other meteorites of the CV3-class. Similar considerations may play a role in terrestrial problems where metal and silicate coexist, e.g., the primitive terrestrial magma ocean and the ‘‘D’’ layer.

Growth and dissolution on the CaF2 (111) surface observed by scanning force microscopy

Abstract Scanning force microscopy (SFM) was used for in situ growth and dissolution experiments on the CaF 2 (111) surface in various aqueous solutions in order to investigate chemical processes within the CaF 2 -water interface. Apart from growth and dissolution, a further process, the formation of protrusions, takes place in the interfacial region. These protrusions are inhomogeneously distributed but uniform in height (2.5–4 nm). The areal density of protrusions depends on the pH value and degree of saturation of the solution. In addition, experiments with in situ exchanged solutions show that the areal density depends further on the sequence of application of the solutions. These and other observations indicate that surface defects which are considered to form surface hydroxyl groups lead to the formation of protrusions. Therefore, we conclude that the protrusions represent surface precipitates which consist of multinuclear calcium (aquo) hydroxo complexes connected to surface hydroxyl groups. The existence of these hydroxo complexes cannot be explained by classical equilibrium thermodynamics of bulk reactions. Their formation is enabled by the different dielectric constant in the electrical double layer. Depending on the composition of solution and the defect density, the complexes reduce the active surface area of CaF 2 and therefore affect the growth and dissolution rates.

Apophyllite (001) surface alteration in aqueous solutions studied by HAFM

Depending on pH and temperature, two different types of surface reactions occur on the apophyllite (001) surface in aqueous HCl-solutions at temperatures from 20 to 130 °C. At low pH, laterally spreading hillocks cover the surface. The hillocks are softer than the pristine surface, chemical analysis shows a depletion in Ca + K, and the spreading velocity of hillocks depends on pH. This indicates a change in chemical bond strength, non-stoichiometric dissolution and a mechanism involving protons. External disturbances such as the AFM scanning tip cause the upper surface layers to peel off revealing that the active sites of hillock formation are between the silicate layers of apophyllite. The observed process can therefore be described by a penetrative ion-replacement reaction which proceeds well below the surface monolayer. By this ion-replacement, the silicate layers eventually become destabilised. The observed reaction, therefore, is equivalent to an incongruent dissolution process. Despite structural similarities, this process is only superficially similar to the ion-exchange occurring in clay minerals or zeolites. In these minerals, the structural backbone is not destabilized. At a more neutral pH and high temperatures, step retreat and etch pit formation can be observed on the apophyllite (001) surface thus indicating a more congruent dissolution mechanism.

Experimental study of argon sorption in quartz: Evidence for argon incompatibility

Abstract We have conducted Ar sorption experiments on two varieties of natural quartz minerals (Q112, Q113) and a synthetic one (QHE37). Runs were performed in an internally heated pressure vessel at 1300°C and pressures of up to 8000 bar for run times between 1 and 12 d, on grain sizes ranging from 11–20 to 60–80 μm. Run products were analysed by gas chromatography (GC), Knudsen cell mass spectroscopy (KMS), electron microprobe (EMP), and scanning electron microscopy (SEM). Release spectra of Ar desorption were monitored by KMS. For sample Q112 and QHE37 two release signals are observed (500–1000°C and 1200–1600°C). When two grain sizes of the same specimen (QHE37) are analysed, the high temperature peak does not vary whereas the low temperature peak is significantly increased with decreasing grain size, suggesting desorption of surface bonded Ar. Argon contents from the high temperature peak indicate an Ar sorption of 28 ppm (QHE37) and 48 ppm (Q112) at 4070 bar. Specimen Q113 does not exhibit low temperature release, nevertheless, its Ar content is higher in smaller grains (11–20 μm: 431 ppm, 60–80 pm: 128 ppm), while increasing the duration of the experiments from ∼ 1 d to ∼ 10 d does not change the Ar content. This apparently erratic behaviour suggests an extrinsic control for Ar sorption. EMP analysis of all samples at the μm scale reveals heterogeneous Ar distribution. A few enriched spots with Ar up to 4000 ppm are observed (e.g., Q113) compared to a background concentration below the detection limit of ∼30 ppm. The average concentration measured by EMP is fairly similar to the high temperature step of bulk analytical methods (KMS or GC). We can conclude that bulk measurements of the sorption of Ar do not document equilibrium dissolution. Assuming the Ar diffusivity to be fast enough to permit saturation of at least the 11–20 μm grain fraction after ∼ 10 d, at 8000 bar and 1300°C, an upper bound of Ar solubility can be given as 30 ppm. In contrast, bulk methods yield variable average Ar concentrations which depend on experimental conditions. This indicates that solubilities measured by bulk methods grossly overestimate the true solubility. A quartz /melt partition coefficient of less than 0.006 can be derived.

ON THE MECHANISMS OF APOPHYLLITE ALTERATION IN AQUEOUS SOLUTIONS. A COMBINED AFM, XPS AND MAS NMR STUDY

Apophyllite, a hydrous K-Ca-phyllosilicate, reacts with acidic aqueous solutions at room temperature. Various analytical methods have been applied to study the mechanism of the reaction and its characteristics, i.e. the changes in chemical composition, modifications in crystal structure and alterations in surface morphology. In contact with acidic solution, protonation of the terminal, non-bridging oxygen at the silicate tetrahedra takes place and the interlayer cations K+ and Ca2+ are removed. The protonation and ion removal causes the interlayer spacing to increase. Atomic force microscopy shows that the increase takes place discontinuously and, therefore, reflects a discontinuous reaction that comprises a two- or three-step protonation. Additionally, three structurally different protonation sites have been detected by nuclear magnetic resonance spectroscopy which also differ in the amount of close-by hydrogen, although in pristine apophyllite all terminal oxygen positions at silicate tetrahedra are structurally equivalent. In many clay minerals such structurally different protonation sites have not been detected so far. Thus, the multi-step protonation process in apophyllite clearly demonstrates the vast sensitivity of the protonation reaction on small structural variations in phyllosilicates.

Organized multilayers of polydiacetylenes prepared by electrostatic self-assembly

New types of polydiacetylene multilayer are presented. The first type is based on electrostatic self-organization of diacetylene bolaamphiphiles and polyelectrolytes on a charged substrate followed by subsequent ultraviolet (UV) polymerization. The second type is prepared by direct adsorption of a water-soluble polydiacetylene and a polyelectrolyte in alternating sequence. The monomeric diacetylenes are of general formula X-(CH2)9-C triple bond C-C triple bond C-(CH2)9-X, with X being a sulfate (1a), phosphate (2) or pyridinium (3) head group. The polydiacetylene (1b) chosen for the multilayer is obtained by γ irradiation of the corresponding diacetylene monomer 1a. It is found that all diacetylene derivatives are well suited for building up self-assembled multilayers and that two of the monomers (1a, 2) can be polymerized on the substrate, while 3 is photo-inactive. The morphology of the multilayers is studied by scanning force microscopy and discussed. The smoothest surface topology is found for multilayers built up from the polydiacetylene 1b and a cationic polyelectrolyte in alternating sequence, while the largest unevenness is found when the anionic diacetylene 1a is alternatingly adsorbed with the cationic bolaamphiphile 3 followed by subsequent UV polymerization on the substrate.