Ralf Milke

Stability of corundum + quartz relative to kyanite and sillimanite at high temperature and pressure

Abstract Although natural occurrences of corundum + quartz ± aluminosilicate are known, internally consistent thermodynamic databases suggest that they do not represent a stable assemblage. This observation has motivated two sets of experiments. In the first set, the equilibrium kyanite = corundum + quartz has been reversed (±5 °C; ±25 MPa) at 600 °C/320 MPa and 700 °C/535 MPa (externally heated cold-seal hydrothermal autoclaves); 800 °C/775 MPa (gas apparatus; NaCl and CaF2 furnace assemblies, non end-load piston-cylinder press); and at 900 ∞C/1075 MPa and 1000 °C/1325 MPa (CaF2 furnace assembly, non end-load piston-cylinder press). These reversals imply an enthalpy of formation from the elements for kyanite of -2594.75 kJ/mol. The slope of the equilibrium curve also confirms both volume and experimental CP data for kyanite. These reversals can serve as a useful calibration for the piston-cylinder press using NaCl furnace assemblies in the 700-800 ∞C and 500- 1000 MPa range and indicate a friction correction for CaF2 furnace assemblies of 75-100 MPa over 800-1000 °C and 1000-1500 MPa. The second set of experiments (1200 °C and 2000 MPa) investigated the growth of kyanite along corundum-quartz grain interfaces. In experiments where no fluid was present, except adsorbed H2O, kyanite did not nucleate and grow. In experiments with 2 wt% H2O, kyanite formed and grew preferentially in the pores surrounding the corundum grains parallel to the corundum-quartz interface and along quartz grain boundaries. Due to a large DVR, reaction halos around the corundum grains never become closed to fluid migration. This suggests that in nature, fluids are channeled to these reaction sites via porous reaction halos surrounding the corundum grains and indicates that, under such circumstances, formation of kyanite is self-promoting and probably goes to completion quickly. The stability of sillimanite relative to corundum + quartz is also discussed, from the standpoints of what is predicted by internally consistent mineral databases and what is observed in nature.

Metastability of sillimanite relative to corundum and quartz in the kyanite stability field: Competition between stable and metastable reactions

Abstract The formation of sillimanite, under metastable conditions, relative to corundum and quartz has been defined experimentally, approximately 700 to 800 MPa inside the kyanite stability field thereby allowing for the approximate location of the metastable Sil = Cnd + Qtz equilibrium to be outlined in P-T space from 600 to 800 °C. Experiments involved using a NaCl assembly with a graphite furnace in a two-piston-cylinder apparatus. The thermocouple tip was in direct contact with the flat-lying, folded Pt capsule thereby minimizing thermal gradients to <5 °C. Charges consisted of equimolar amounts of gem-quality sillimanite, corundum, and quartz, plus H2O as a flux, placed in a 1.3 cm long Pt capsule that was arc-welded shut and folded. During the course of the experiment, the metastability of the assemblage Sil-Cnd-Qtz implies that Sil ↔ Cnd + Qtz is, at some point, in direct competition with Cnd + Qtz → Ky and Sil → Ky. Early during the experiment it may be assumed that a steady state between dissolution and growth rates is established. However, due to the sluggish nucleation of kyanite, there is a P-T dependent induction period during which Cnd + Qtz → Sil is the controlling reaction. Once kyanite does appear, the reaction proceeds very fast to kyanite via reactions Cnd + Qtz → Ky and Sil → Ky. The kyanite surface area is probably a major factor in controlling the overall reaction rates. Under constant P and T, the system evolves from metastable sillimanite formation to sillimanite consumption, which is only dependent on the kyanite surface area. Similar competition between stable and metastable reactions could occur during contact metamorphism where metastable mineral growth is observed. The relative sluggishness of all three reactions under relatively dry conditions could help to explain the persistence of metastable corundum + quartz ± Al2SiO5 assemblages in nature.

Australian sedimentary opal-A and its associated minerals: Implications for natural silica sphere formation

Abstract The vast majority of precious opal on the world market comes from opal fields in the Great Artesian Basin of Australia pointing to very special prerequisites for amorphous silica to consolidate in a way that leads to the famous play-of-color. We analyzed 20 opal-A samples from the Andamooka (South Australia) and Yowah (Queensland) precious opal fields, using petrographic microscopy, XRPD, SEM, and EPMA to identify and characterize opaline silica, the mineral assemblage, and the host rock. Opal-A consists of submicrometer-sized silica spheres with an average diameter of 140-320 nm. The average diameter of monodisperse spheres is 140-290 nm with a relative standard deviation (RSD) of <6%. Polydisperse spheres show an average diameter of 160-320 nm with a RSD larger than 10%. This dichotomy in size is reflected by the Na/K ratio at both localities. Monodisperse spheres show values below 1.2 while polydisperse ones show a ratio larger than 3.0, whereas other contaminations with higher valence show no correlations at all. We therefore suggest that the jump in Na/K signals a fundamental change of pH and salinity of the silica-bearing mineralizing fluids. Judging from the pH stability of the host rock minerals with predominating alunite, kaolinite, illite and gypsum, and omnipresent barite and anatase we conclude that the dominant late-stage mineralization leading to precious opal happened at acidic pH. Our findings indicate that the host rocks and associated minerals are the key to unravel the complex history of opal-forming solutions. A quantitative opal classification based on sphere diameters and their variability, decoupled from gemological properties, is to be established.

Experimental study of phlogopite reaction rim formation on olivine in phonolite melts: Kinetics, reaction rates, and residence times

Abstract Experiments were conducted to reproduce reaction rims of phlogopite ± diopside around olivine that have been observed within a wide range of potassic melts, including phonolite. Phlogopite is also a common secondary phase formed at the expense of olivine during metasomatic events involving K2Oand H2O-rich fluids or melts. Piston-cylinder experiments where olivine single crystals were reacted with synthetic phonolite melt at 10.7-14.7 kbar and 950-1000 °C recreate the mineralogy and textures documented in natural samples. Rim growth is parabolic with time, indicating a diffusion-controlled reaction. Fast diffusion in the melt and varying compositions across the phlogopite reaction rims suggest that diffusion through the rims, along grain boundaries is rate limiting. Reaction rates dramatically increase with temperature as well as the bulk water content of the sample charge. This is because of increasing amounts of atomically bound hydrous species along the grain boundaries that increase the rates of diffusion and thereby the rates of rim growth. Atomically bound hydrous species increase the rates of rim growth by lowering the activation energy for diffusion and by increasing the solubility of diffusing species in the grain boundary region. Transmission electron microscopy shows the presence of isolated pores and open grain boundaries. Most of these may have opened during quenching, but there is some evidence to suggest that a free fluid phase may have been locally present in experiments with high melt water contents (>8 wt%). The measured rim growth rates at different conditions are used to estimate residence times of reacting olivine crystals in natural systems.

Biogenic overgrowth on detrital pyrite in ca. 3.2 Ga Archean paleosols

Regionally traceable paleosols in the lower Moodies Group of the Barberton greenstone belt (ca. 3.22 Ga, northeastern South Africa and Swaziland) contain locally abundant silicified nodules, originally composed of pedogenic carbonates and sulfates, interbedded with heavy-mineral laminae dominated by pyrite. Pyrite grains show rounded detrital cores and secondary idiomorphic rims with trace element concentrations and δ 34 S ratios clearly different from those of the cores. While cores have low Co and Ni concentrations and high Co/Ni ratios, rims show as much as 5.5 wt% of these elements and low Co/Ni ratios, reflecting the weathering of nearby ultramafic rocks. In-situ sulfur isotope analyses of pyrite cores show δ 34 S VCDT (Vienna Canyon Diablo troilite) values between +5‰ and −5‰, while the rims show δ 34 S VCDT values between −20‰ and −24.5‰, suggesting biogenic fractionation of sulfur. The close spatial association and microtextural evidence for nearly contemporaneous formation of the pedogenic sulfate nodules and the secondary pyrite rims suggests microbial processing of sulfur in the paleosols, which provided reduced and 34 S-depleted sulfur for the growth of authigenic pyrite. This indicates that vadose-zone soil-forming processes in the Archean involved not only physical and chemical modification of moist, unconsolidated sediment in a terrestrial environment but also already included its microbiological modification.

Silica nanoparticle aggregation in calcite replacement reactions

Natural nanoparticles are fundamental building blocks of Earth’s bio- and geosphere. Amorphous silica nanoparticles are ubiquitous in nature, but fundamental knowledge of their interaction mechanisms and role in mineral replacement reactions is limited. Here we show how silica nanoparticles replace Cretaceous calcite bivalve shells in a volume- and texture-preserving process. Electron tomography reveals that mineral replacement transfers calcite crystallographic orientations to twinned photonic crystals composed of face-centered cubic silica sphere stacks. During the face-specific replacement process, silica nanoparticles continuously nucleate, aggregate, and form a lattice of uniform spheres parallel to calcite low-energy facets. We explain the replacement process with a new model that unifies recently proposed, probably universal mechanisms of interface-coupled dissolution-precipitation and aggregation-based crystallization; both key mechanisms in geological processes and nanomaterials design and synthesis.

Mechanism and kinetics of forsterite formation in metamorphic siliceous dolomites: Findings from a rock-sample experiment

A cylindrical sample of a tremolite-dolomite marble was used as starting material for a long-term experiment of 184 days at 581 °C and a CO2–H2O-fluid pressure of 100 MPa. The fluid, which was always present in excess, had a composition in the range of XCO2 = 0.21−0.37. During the run forsterite and magnesian calcite formed both within the rock sample as well as on its cylindrical surface. In areas where a tremolite crystal was exposed on the sample surface a 400–500 μm thick forsterite – magnesian calcite reaction rim evolved with a composition of 58 ± 2 mol% forsterite and 42 ± 2 mol% calcite. The dolomite areas of the surface of the rock cylinder are similarly covered by a reaction rim of forsterite plus magnesian calcite. This ca . 300 μm thick rim, however, is composed of 28 ± 5 mol% of forsterite and 72 ± 5 mol% of calcite. From the different compositions of the reaction rims we conclude that the well-known overall reaction: 1 tremolite + 11 dolomite ⇔ 8 forsterite + 13 calcite + 9 CO2 + 1 H2O occurs via the following two partial reactions: (1) 1 tremolite + 2 CO2 + 21 H2O ⇔ 2.5 forsterite + 2 calcite + 5.5 Si(OH)4 · 2 H2O and (2) 11 dolomite + 5.5 Si(OH)4 · 2 H2O ⇔ 5.5 forsterite +11 calcite + 11 CO2 + 22 H2O. All three equations are slightly simplified, because they assume the formation of pure instead of magnesian calcite. The derived partial reactions, which occur as expected via dissolution-transport-precipitation mechanisms, are sequential reactions; they are linked by the production, diffusion, and consumption of the component Si(OH)4 · 2 H2O. In contrast to the two kinds of rims on the rock cylinder’s surface, only one combined forsterite – magnesian calcite reaction rim was formed between tremolite and dolomite in the interior of the sample. The formation of this reaction rim can be explained by a combination of the two partial reactions given above. For the formation of forsterite plus magnesian calcite on the surface of the rock cylinder we come from an interpretation of the experimental results to the conclusion that the diffusion of the component Si(OH)4 · 2 H2O through the forsterite – calcite reaction rim that developed on dolomite is the rate-controlling step in the sequence of the two partial reactions given above. For the formation of forsterite plus calcite within the rock sample it is not possible to derive a rate-controlling step for the overall reaction, because of different diffusion paths of the reaction fluid and the components dissolved in it. The observed partial reactions are crucial for deciphering the formation of the various textures of forsterite – calcite –dolomite (−tremolite) assemblages developed in metamorphic siliceous dolomites. The magnesian calcites of the reaction rims have different compositions, which all deviate considerably from the equilibrium composition according to the calcite-dolomite solid solution. This result sheds light on the difficulty to attain equilibrium between the reactant dolomite and the product magnesian calcite. The consequences of the observed disequilibrium for the calcite–dolomite geothermometry are discussed.

Laboratory Tools to Quantify Biogenic Dissolution of Rocks and Minerals: A Model Rock Biofilm Growing in Percolation Columns

Sub-aerial biofilms (SAB) are ubiquitous, self-sufficient microbial ecosystems found on mineral surfaces at all altitudes and latitudes. SABs, which are the principal causes of weathering on exposed terrestrial surfaces, are characterised by patchy growth dominated by associations of algae, cyanobacteria, fungi and heterotrophic bacteria. A recently developed in vitro system to study colonisation of rocks exposed to air included two key SAB participants - the rock-inhabiting ascomycete Knufia petricola (CBS 123872) and the phototrophic cyanobacterium Nostoc punctiforme ATCC29133. Both partners are genetically tractable and we used them here to study weathering of granite, K-feldspar and plagioclase. Small fragments of the various rocks or minerals (1 to 6 mm) were packed into flow-through columns and incubated with 0.1% glucose and 10 µM thiamine-hydrochloride (90 µL.min-1) to compare weathering with and without biofilms. Dissolution of the minerals was followed by: analysing (i) the degradation products in the effluent from the columns via Inductively Coupled Plasma Spectroscopy and (ii) by studying polished sections of the incubated mineral fragment/grains using scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray analyses. K. petricola/N. punctiforme stimulated release of Ca, Na, Mg and Mn. Analyses of the polished sections confirmed depletion of Ca, Na and K near the surface of the fragments. The abrupt decrease in Ca concentration observed in peripheral areas of plagioclase fragments favoured a dissolution-reprecipitation mechanism. Percolation columns in combination with a model biofilm can thus be used to study weathering in closed systems. Columns can easily be filled with different minerals and biofilms, the effluent as well as grains can be collected after long-term exposure under axenic conditions and easily analysed.

In situ monitoring and ex situ TEM analyses of spinel (MgAl_2O_4) growth between (111)-oriented periclase (MgO) substrates and Al_2O_3 thin films

We investigated the temperature-dependent onset and subsequent reaction kinetics of spinel (MgAl2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}