Kilian Pollok

Mineral replacement reactions in solid solution-aqueous solution systems: Volume changes, reactions paths and end-points using the example of model salt systems

The volume change of solid phases associated with dissolution and precipitation reactions during mineral replacement is a critical factor for the advancement of the reaction boundary. Contributing parameters to the overall volume change of a replacement reaction are the molar volume of parent and product and their solubility ratio within a given solution. Based on simple model salt systems, the contribution of solubility to volume change is quantitatively determined. For NaCl-KCl as an example of a binary salt system without solid solution, the relative volume changes can be calculated for various reaction paths using the slope of the solubility from a simple solubility diagram. For KBr-KCl as an example of a binary salt system with complete solid solution, the determination of the solubility curve is based on a modified Lippmann phase diagram called a solubility phase diagram. It allows a quantitative calculation of the relative volume change based on the solid solution-aqueous solution (SS-AS) relationships for variable solution compositions and reaction paths in the salt-water system. Reaction kinetics, textures and the compositional evolution of replacements in both salt systems can be conclusively explained by the relative volume change on the basis of experimentally constrained reaction paths. The analogy from simple model system to replacement reactions at the Earths surface and crustal conditions (for example in apatites or feldspars) may offer insights to successfully describe volume changes and porosity generation in mineral reactions on the basis of solubility data towards a more quantitative modeling of interface-coupled dissolution-precipitation reactions.

Low-temperature aqueous alteration of crystalline pyrochlore: correspondence between nature and experiment

Abstract Pyrochlore has been considered as a waste form to immobilize high-level nuclear waste such as excess weapons-grade plutonium. In order to study the aqueous stability of pyrochlore, we have carried out hydrothermal experiments with a natural microlite (Nb + Ta > 2Ti; Ta ≥ Nb) at 175°C in a neutral and acidic solution for 14 days. The starting material and the experimental products were studied by electron microprobe, backscattered electron (BSE) imaging, powder infrared (IR) and micro-Raman spectroscopy, and powder X-ray diffractrometry (XRD). The microlite has small U (~200 ppm) and Th (~ 800 ppm) contents and is crystalline. The hydrothermal treatment in a solution containing 1 mol/l HCl and 1 mol/l CaCl2 causes the partial replacement of the microlite (up to ~10 mm) by a new pyrochlore phase. This new phase is characterized by a larger unit-cell volume and contains a large number of vacancies at the A site (A = Ca, Na) as well as anion vacancies, molecular water, and possibly OH groups. Analyses of the experimental fluid further revealed that U was also selectively lost to the solution. Treatment in pure water did not produce reaction zones detectable by BSE imaging or powder XRD. However, significant spectral changes in powder IR spectra of the reaction product and the detection of Na and Ca in the solution indicate that the microlite has also reacted in pure water. The experimental alteration features bear a remarkable resemblance to those seen in naturally altered microlite samples, suggesting that short-term experimental results can be applied to natural systems and vice versa in order to evaluate the long-term stability of a pyrochlore waste form.

Translation interface modulation in NC-pyrrhotites: Direct imaging by TEM and a model toward understanding partially disordered structural states

Abstract The crystallographic complexity of “hexagonal” or “intermediate” pyrrhotites (Fe1-xS with 0.125 > x > 0.080) is a long-standing and challenging problem. Integral (e.g., 5C) and non-integral NC type structures found within this group at ambient temperatures are characterized by sharp but complicated electron diffraction patterns, which were found to be interpretable in terms of a translation interface modulation (TIM) superstructure. Transmission electron microscopy (TEM) dark-field images obtained using superstructure reflections show dense arrangements of stripes, which can be interpreted as arrays of closely spaced anti-phase domain boundaries (APB). The displacement vector at the interface is R = 1/8[001] of a metrically hexagonal 4C superstructure cell and the involved translations are solely confined to the Fe sublattice. The vacancy arrangement of the APB-free monoclinic 4C-pyrrhotite serves as a base of the TIM superstructure and therefore NC structures can be regarded as two super-imposed ordering phenomena relating to the arrangements of individual vacancies and APBs, respectively. APBs are chemically non-conservative and govern the higher Fe/S ratios of intermediate NC-pyrrhotites. If oriented strictly parallel to (001) the APBs can be regarded as completely filled Fe double layers within the 4C stacking sequence. However, direct imaging of APBs shows waviness and variable disorder on mesoscopic scales, yielding essentially aperiodic structures. A high degree of self-organization among APBs has been observed within apparent diffusion profiles around exsolved troilite lamellae and along interfaces with 4C-pyrrhotite, where complicated eightfold node arrangements occur. Our TEM observations indicate that all NC-type pyrrhotites can be treated by the TIM approach and that the concepts of polytypism and polysomatism in pyrrhotite are not fully capable in representing the observed structural complexities.

Mineralogy and defect microstructure of an olivine-dominated Itokawa dust particle: evidence for shock metamorphism, collisional fragmentation, and LL chondrite origin

We report here detailed analytical scanning and transmission electron microscopic investigations on an olivine-dominated dust particle (RB-QD04-0042) from the surface of asteroid 25143 Itokawa. The dust particle was returned to Earth by the Hayabusa spacecraft and was made available in the context of the first announcement of opportunity for Hayabusa sample investigation. Multiple thin slices were prepared from the precious particle by means of focused ion beam thinning, providing a unique three-dimensional access to its interior. The 40 × 50 μm sized olivine particle contains a spherical diopside inclusion and an intimate intergrowth of troilite and tetrataenite. The compositions of olivine (Fo69Fa31) and diopside (En48Wo42Fs10), as well as the high Ni content of the sulfide-metal alloy, indicate a LL ordinary chondrite origin in accord with previous classifications. Although no impact crater exists at the surface of RB-QD04-0042, transmission electron microscopy revealed the presence of various shock defects in constituent minerals. These defects are planar fractures and [001] screw dislocations in olivine, multiple {101} deformation twins in tetrataenite and basal (0001) stacking faults in troilite. These diagnostic shock indicators occur only in a small zone on one concave side of the dust particle characterized by a high fracture density. These observations can be explained by a collisional event that spalled off material from the particles surface. Alternatively, the dust particle itself could be a spallation fragment of an impact into a larger regolith target. This suggests that Itokawa dust particles lacking visible microcraters on their surfaces might have still experienced shock metamorphism and were involved in collisional fragmentation that resulted in the formation of regolith.

Nanocrystalline Nickel and Cobalt Sulfides Formed by a Heavy Metal-Tolerant, Sulfate-Reducing Enrichment Culture

A soil enrichment culture of the sulfate-reducers Desulfosporosinus auripigmenti and Citrobacter freundii and of fermentative bacteria from a former uranium-mining site was studied for its metal retention potential by promoting metal sulfide precipitation. The culture could tolerate up to 30 mM Ni and 40 mM Co. XRD and TEM analyses revealed the formation of amorphous NiS together with nanocrystalline, metastable α-NiS, and nanocrystalline cobalt pentlandite. The α-NiS with a grain size of 5 nm shows probably an example of size-dependent phase stability and/or specific biomineralization precipitation paths. Detailed mineralogical characterizations are necessary to correctly assess the mineral inventory and thus metal bioavailability.

Redox stability of As(III) on schwertmannite surfaces

As(III)-enriched mine discharge often drains through Fe(III)-mineral abundant land covers which makes the understanding of its fate and redox behaviour extremely important. We therefore conducted batch kinetic and equilibrium studies at pH 3.0±0.05 in anoxic media coupled with spectroscopic and microscopic examinations at variable conditions to understand possible As(III) binding mechanisms and the redox stability of As(III) on schwertmannite, a prominent ferric mineral in acid mine drainage environments. Schwertmannite acted as an efficient scavenger for As(III) compared to goethite at identical sorbent:solute ratios. As K-edge X-ray absorption near-edge structure (XANES) demonstrated partial oxidation of sorbed As(III) to As(V) on both the minerals depending on the Fe(III)/As(III) ratios (goethite acted as a better oxidant than schwertmannite). Sorbed As(III) and As(V) coordinated in a bidentate binuclear binding mechanism with As(III)/As(V)-O and As(III)/As(V)-Fe interatomic distances as 1.78/1.69 and 3.37/3.31Å, respectively. Scanning (SEM-EDX) and transmission (TEM) electron microscopic, and IR spectroscopic measurements revealed the formation of As-containing surface coatings by sorbed As on schwertmannite.

Where is iron in erionite? A multidisciplinary study on fibrous erionite-Na from Jersey (Nevada, USA)

Fibrous erionite is a mineral fibre of great concern but to date mechanisms by which it induces cyto- and geno-toxic damage, and especially the role of iron associated to this zeolite species, remain poorly understood. One of the reasons is that we still don’t know exactly where iron is in natural erionite. This work is focused on fibrous erionite-Na from Jersey (Nevada, USA) and attempts to draw a general model of occurrence of iron in erionite and relationship with toxicity mechanisms. It was found that iron is present as 6-fold coordinated Fe3+ not part of the zeolite structure. The heterogeneous nature of the sample was revealed as receptacle of different iron-bearing impurities (amorphous iron-rich nanoparticles, micro-particles of iron oxides/hydroxides, and flakes of nontronite). If iron is not part of the structure, its role should be considered irrelevant for erionite toxicity, and other factors like biopersistence should be invoked. An alternative perspective to the proposed model is that iron rich nano-particles and nontronite dissolve in the intracellular acidic environment, leaving a residue of iron atoms at specific surface sites anchored to the windows of the zeolite channels. These sites may be active later as low nuclearity groups.

New insights into the toxicity of mineral fibres: A combined in situ synchrotron μ-XRD and HR-TEM study of chrysotile, crocidolite, and erionite fibres found in the tissues of Sprague-Dawley rats

Along the line of the recent research topic aimed at understanding the in vivo activity of mineral fibres and their mechanisms of toxicity, this work describes the morpho-chemical characteristics of the mineral fibres found in the tissues of Sprague-Dawley rats subjected to intraperitoneal/intrapleural injection of UICC chrysotile, UICC crocidolite and erionite-Na from Nevada (USA). The fibres are studied with in situ synchrotron powder diffraction and high resolution transmission electron microscopy to improve our understanding of the mechanisms of toxicity of these mineral fibres. In contact with the tissues of the rats, chrysotile fibres are prone to dissolve, with leaching of Mg and production of a silica rich relict. On the other hand, crocidolite and erionite-Na fibres are stable even for very long contact times within the tissues of the rats, showing just a thin dissolution amorphous halo. These findings support the model of a lower biopersistence of chrysotile with respect to crocidolite and erionite-Na but the formation of a silica-rich fibrous residue after the pseudo-amorphization of chrysotile may justify a higher cytotoxic potential and intense inflammatory activity of chrysotile in the short term in contact with the lung tissues.

The crystal structure of mineral fibres: 2. Amosite and fibrous anthophyllite

This study reports for the first time crystal-structure data for amosite and fibrous anthophyllite. The chemical composition of the two fibre species was determined from EMPA. Crystal structures were refined using powder-diffraction data, using both laboratory sources and synchrotron radiation. Results were compared with the available literature data for the non-fibrous varieties grunerite and anthophyllite, respectively. The calculated site-occupancies for all samples are in agreement with the chemical compositions calculated from EMPA. The existing structure models of grunerite and orthorhombic anthophyllite also applies to the corresponding fibrous varieties amosite and fibrous anthophyllite, respectively. In amosite, both Fe2+ and Fe3+ atoms are found at the sites M(1), M(2) and M(3) and Fe2+ ions is the only atomic species found at site M(4). Mg is disordered over the C sites with a preference for site M(2). Minor Ca and Na have been assigned to the A site. In fibrous anthophyllite, Mg is the only atomic species found at the M1, M2 and M3 sites. Fe2+, Mg (and minor Mn) have been assigned to the M4 site, whereas minor Ca has been assigned to the A site. In both structures, the environment at the M(4) site in amosite and M4 site is in fibrous anthophyllite highly distorted. This work can be considered a basis for studies aimed at understanding the potential toxicity/pathogenicity of these mineral fibres.

Marine and freshwater micropearls: Biomineralization producing strontium-rich amorphous calcium carbonate inclusions is widespread in the genus Tetraselmis (Chlorophyta)

Unicellular algae play important roles in the biogeochemical cycles of numerous elements, particularly through the biomineralization capacity of certain species (e.g., coccolithophores greatly contributing to the “organic carbon pump” of the oceans), and unidentified actors of these cycles are still being discovered. This is the case of the unicellular alga Tetraselmis cordiformis (Chlorophyta) that was recently discovered to form intracellular mineral inclusions, called micropearls, which had been previously overlooked. These intracellular inclusions of hydrated amorphous calcium carbonates (ACCs) were first described in Lake Geneva (Switzerland) and are the result of a novel biomineralization process. The genus Tetraselmis includes more than 30 species that have been widely studied since the description of the type species in 1878. The present study shows that many other Tetraselmis species share this biomineralization capacity: 10 species out of the 12 tested contained micropearls, including T. chui, T. convolutae, T. levis, T. subcordiformis, T. suecica and T. tetrathele. Our results indicate that micropearls are not randomly distributed inside the Tetraselmis cells but are located preferentially under the plasma membrane and seem to form a definite pattern, which differs among species. In Tetraselmis cells, the biomineralization process seems to systematically start with a rod-shaped nucleus and results in an enrichment of the micropearls in Sr over Ca (the Sr/Ca ratio is more than 200 times higher in the micropearls than in the surrounding water or growth medium). This concentrating capacity varies among species and may be of interest for possible bioremediation techniques regarding radioactive 90Sr water pollution. The Tetraselmis species forming micropearls live in various habitats, indicating that this novel biomineralization process takes place in different environments (marine, brackish and freshwater) and is therefore a widespread phenomenon.