Mineralogical Evolution Research Articles

Effect of proteinaceous binder on pollution-induced sulfation of lime-based tempera paints

Even though, atmospheric SO2 contamination has been reduced over the last decades in Europe, practical experience has shown that sulfation of lime-based materials (i.e., lime-based tempera paint, lime mortar and plaster, as well as limestone) is still of importance, affecting monuments exposed to polluted air in urban centers. In order to evaluate the effect of organic binders (i.e., rabbit skin glue or egg yolk) on the chemical weathering resistance of lime-based materials, tempera paint dosimeters (i.e., paints containing calcite or mixtures of portlandite and calcite) were subjected to long-term outdoor exposure and accelerated SO2-aging. SO2-aging caused important morphological changes of the paint surface on the nano- and microscale. However, sulfation was significantly delayed in the presence of the organic binder. Furthermore, paints containing portlandite and calcite transformed faster into calcium sulfite hemihydrate and gypsum than paints containing only calcite. Calcium sulfite hemihydrate formation onto calcite always preceded non-epitaxial gypsum crystallization after dissolution of the sulfite precursor phase. These results suggest that a passivating product layer will not form onto calcite, and so sulfation will continue under suitable environmental conditions until all calcite is transformed into gypsum. Nevertheless, the organic binder strongly affected the mineralogical evolution of paints containing portlandite, resulting in the formation of organic-inorganic hybrid materials similar to biominerals. These hybrid materials generally have superior weathering resistance and might explain the absence of any clear signs of sulfation after prolonged outdoor exposure. The selection of lime-based tempera paints for conservation interventions must be made considering the prevailing exposure conditions. In polluted dry environments where carbonation of portlandite will be significantly delayed, the use of calcite-based tempera paints might be preferable, while tempera paints containing portlandite would be more suitable in humid climates where carbonation is fast, resulting in the formation of weather resistant hybrid materials.

Petrophysical and mineralogical evolution of weathered crystalline basement in western Uganda: Implications for fluid transfer and storage

The effects of weathering on the mineralogical and petrophysical properties of the western Uganda Archaean crystalline basement were investigated using outcrop samples. This study aimed to characterize a typical composite vertical weathering profile by the integration of data from samples with variable degree of alteration, ranging from fresh to completely weathered. A detailed description of samples is proposed, using different observation methods (e.g., x-ray computed tomography scanning and quantitative evaluation of minerals by scanning electron microscopy analysis system). Petrophysical analyses were carried out to fully describe the porous network of the samples (e.g., mercury porosimetry [φ] and permeability [k] measurements). A geometric factor reported in the literature is also used to describe the variable complexity of the porous network through the weathering profile. This multimethod study revealed two distinct domains in the weathering profile, separated by a sharp property transition. The lower part of the profile is characterized by dominant planar pore structures and represents a high transfer zone for fluid circulation (φ ≤ 8%–10%; k ≤ 500 md). The upper part of the profile is characterized by high clay content and dominant vacuole-shaped matrix porosity. This represents a high fluid storage zone (φ a 30%; k ≤ 1 d). Both parts are locally preserved from complete clay plugging by hydrodynamic drainage, resulting from a long-term geomorphic evolution. In the case of preservation of such a composite weathering profile after basement burial in a sedimentary basin with hydrocarbon potential, these domains may represent a significant area for petroleum exploration. Properties described in this study indicate that the weathered basement in such a basin can act as a large reservoir for hydrocarbons or as a favorable migration zone at the interface between the basement and the sedimentary cover. Finally, several proxies are proposed to help for the recognition of these two domains from cutting analyses and well logging.

Mineralogical evolution of ceramic clays during heating. An ex/in situ X-ray diffraction method comparison study

Abstract The ceramic properties of clay materials heated at high temperature are largely dependent on the mineral reactions, which usually are followed by XRD analysis on pressed pieces that are heated at different temperatures. Nevertheless, the use of “in-situ” high temperature X-ray diffraction (HTXRD) to follow the evolution of the crystalline phases under controlled temperature provides more precision. In this paper, the mineralogical changes produced during firing (RT-1000 °C) of two clay materials of very different mineralogical composition, frequently used in the manufacture of bricks in the SE Spain, have been studied by XRD classical procedure and with HTXRD, and the results obtained were compared. Although the phases identified by both X-ray analyses were the same, temperatures for the mineralogical reactions seem to be different, probably due to the experimental conditions. The patterns obtained by ex-situ XRD were at a higher speed than those by in-situ HTXRD, and probably the equilibrium was not achieved for the most reactions. Because of the expansion of the mineral structure parameters occurred during heating the mineral reflections do not match with those found in the database JPCDS, neither the position nor intensities of some stable phases at high temperature. For the optimization of the interpretation of the HTXRD patterns, and to facilitate the kinetics of the new phases, it is necessary to work at a high integration time per step and a slow heating rate. The in-situ HTXRD investigations of ceramic materials can save time respect to the classical XRD study of test-pieces heated at different temperatures, and lead immediate information of the transformation occurring on heating, which can be useful to improve the most suitable firing temperature in the industry.

The origin of secondary heavy rare earth element enrichment in carbonatites: Constraints from the evolution of the Huanglongpu district, China

The silico‑carbonatite dykes of the Huanglongpu area, Lesser Qinling, China, are unusual in that they are quartz-bearing, Mo-mineralised and enriched in the heavy rare earth elements (HREE) relative to typical carbonatites. The textures of REE minerals indicate crystallisation of monazite-(Ce), bastnäsite-(Ce), parisite-(Ce) and aeschynite-(Ce) as magmatic phases. Burbankite was also potentially an early crystallising phase. Monazite-(Ce) was subsequently altered to produce a second generation of apatite, which was in turn replaced and overgrown by britholite-(Ce), accompanied by the formation of allanite-(Ce). Bastnäsite and parisite where replaced by synchysite-(Ce) and röntgenite-(Ce). Aeschynite-(Ce) was altered to uranopyrochlore and then pyrochlore with uraninite inclusions. The mineralogical evolution reflects the evolution from magmatic carbonatite, to more silica-rich conditions during early hydrothermal processes, to fully hydrothermal conditions accompanied by the formation of sulphate minerals. Each alteration stage resulted in the preferential leaching of the LREE and enrichment in the HREE. Mass balance considerations indicate hydrothermal fluids must have contributed HREE to the mineralisation. The evolution of the fluorcarbonate mineral assemblage requires an increase in aCa2+ and aCO32− in the metasomatic fluid (where a is activity), and breakdown of HREE-enriched calcite may have been the HREE source. Leaching in the presence of strong, LREE-selective ligands (Cl−) may account for the depletion in late stage minerals in the LREE, but cannot account for subsequent preferential HREE addition. Fluid inclusion data indicate the presence of sulphate-rich brines during alteration, and hence sulphate complexation may have been important for preferential HREE transport. Alongside HREE-enriched magmatic sources, and enrichment during magmatic processes, late stage alteration with non-LREE-selective ligands may be critical in forming HREE-enriched carbonatites.

Evolution of porosity and clay mineralogy associated with chemical weathering of black shale: A case study of Lower Cambrian black shale in Chongqing, China

This work aimed to evaluate the evolution of pore networks and clay mineral in black shale weathering systems in Chongqing, China. The pore-size distributions, relationships between fractal dimensions and minerals, and clay mineral weathering pathways were evaluated for three weathering profiles, A, B, and C, which were characterized as weak, weak to moderate, and moderate to intense based on the Chemical Index of Alteration. It was found as the shale density decreased, the porosity and specific surface area increased with decreasing depth during weathering. The largest pore diameter peak in a pore-size distribution graph shifted further right as the degree of weathering increased. Micro-transition pores (<100 nm) dominated the parent shale, while mesopores (0.1–1 μm) and macropores (>1 μm) dominated the saprock and regolith zones. The surface fractal dimension of the micro-transition pores was positively correlated with the pyrite and carbonate content, but negatively correlated with secondary clay mineral content. By contrast, the surface fractal dimension of the macropores was negatively correlated with pyrite and carbonate content and positively correlated with secondary clay mineral content. These relationships indicate that micro-transition pores at the surface became smoother and macropores became rougher during weathering. Inherited micas, including illite, predominated the clay fractions along transects as weathering proceeded. The pedogenic minerals (i.e., smectite, vermiculite, kaolinite, and mixed-layer minerals) were minor components in the saprock and regolith zones. Smectite formation was likely due to direct transformation from mica or illite under strongly acidic conditions. A second possibility is transformation proceeded through mixed-layer mineral intermediates, i.e., mica-smectite/illite-smectite and mica-vermiculite/illite-vermiculite. The weathering of clay minerals in black shale from weak to intense weathering stages was mica/illite → mica-smectite/illite-smectite or mica-vermiculite/illite-vermiculite → “chlorite” (including vermiculite, hydroxy-interlayered vermiculite?, and chlorite?) → smectite → kaolinite → gibbsite. This study revealed pore geometry evolution depends on transport-limited solutions that facilitate mineral breakdown and formation. The clay mineral weathering pathways were controlled by the weathering environment and duration.

Mineralogical Evolution of the Paleogene Formations in the Kyzyltokoy Basin, Kyrgyzstan: Implications for the Formation of Glauconite

AbstractAlthough several hypotheses for the formation of glauconite have been proposed, the sedimentary environment and mechanism of glauconitization are still poorly understood. In this contribution, the mineralogy and chemical compositions of sediments from Paleogene formations (Fms) in the Kyzyltokoy basin (Kyrgyzstan) were examined to better understand glauconitization processes. The samples were analyzed using microscopic petrography, X-ray diffraction (XRD), electron probe microanalysis (EPMA), and X-ray fluorescence (XRF). Interlayered diatomite-argillaceous rocks were newly identified within the diatomites of the Isfara Fm. Glauconite from the Kyzyltokoy basin displayed two stages of maturity: 1) early stage (nascent) glauconite grains composed of ∼3.5% K2O and ~8% FeOT; 2) late-stage (highly evolved) glauconite grains composed of 7–9% K2O and ~27% FeOT. The early stage glauconite grains in the Hanabad Fm green clay (green clay is clay with a greenish color) indicate interruptions in glauconitization processes, whereas the (highly) evolved glauconite grains show a completed glauconitization process along the contact between the Hanabad and Sumsar Fms. Hematite was detected in the red clay (clay with reddish color) of the Sumsar Fm and probably formed by glauconite disintegration. Accordingly, the Paleogene Fms depositional conditions were of three types: 1) beginning of glauconitization with interruptions, 2) completion of glauconitization, and 3) glauconite disintegration. Glauconitization in the Kyzyltokoy basin, thus, likely occurred via a combination of dissolution, precipitation, and recrystallization processes.

Deciphering mineralogical changes and carbonation development during hydration and ageing of a consolidated ternary blended cement paste.

To understand the main properties of cement, a ubiquitous material, a sound description of its chemistry and mineralogy, including its reactivity in aggressive environments and its mechanical properties, is vital. In particular, the porosity distribution and associated sample carbonation, both of which affect cement's properties and durability, should be quantified accurately, and their kinetics and mechanisms of formation known both in detail and in situ. However, traditional methods of cement mineralogy analysis (e.g. chemical mapping) involve sample preparation (e.g. slicing) that can be destructive and/or expose cement to the atmosphere, leading to preparation artefacts (e.g. dehydration). In addition, the kinetics of mineralogical development during hydration, and associated porosity development, cannot be examined. To circumvent these issues, X-ray diffraction computed tomography (XRD-CT) has been used. This allowed the mineralogy of ternary blended cement composed of clinker, fly ash and blast furnace slag to be deciphered. Consistent with previous results obtained for both powdered samples and dilute systems, it was possible, using a consolidated cement paste (with a water-to-solid ratio akin to that used in civil engineering), to determine that the mineralogy consists of alite (only detected in the in situ hydration experiment), calcite, calcium silicate hydrates (C-S-H), ettringite, mullite, portlandite, and an amorphous fraction of unreacted slag and fly ash. Mineralogical evolution during the first hydration steps indicated fast ferrite reactivity. Insights were also gained into how the cement porosity evolves over time and into associated spatially and time-resolved carbonation mechanisms. It was observed that macroporosity developed in less than 30 h of hydration, with pore sizes reaching about 100-150 µm in width. Carbonation was not observed for this time scale, but was found to affect the first 100 µm of cement located around macropores in a sample cured for six months. Regarding this carbonation, the only mineral detected was calcite.

In-situ Raman spectroscopy of amorphous calcium phosphate to crystalline hydroxyapatite transformation

Amorphous calcium phosphate (Ca3(PO4)2xnH2O; n?=?3–4.5; ACP) is a precursor phase of the mineral hydroxyapatite (Ca5(PO4)3(OH); HAP) that in natural settings occurs during both authigenic and biogenic mineral formation. In aqueous solutions ACP transforms rapidly to the crystalline phase. The transformation rate is highly dependent on the prevailing physico-chemical conditions, most likely on: Ca & PO4 concentration, pH and temperature. In this study, we conducted a calcium phosphate precipitation experiment at 20?°C and pH 9.2, in order to study the temporal evolution of the phosphate mineralogy. We monitored and assessed the transformation process of ACP to crystalline HAP using highly time-resolved in-situ Raman spectroscopy at 100 spectra per hour, in combination with solution chemistry and XRD data. Transformation of ACP to crystalline HAP occurred within 18?h, as it is illustrated in a clear peak shift in Raman spectra from 950?cm-1 to 960?cm-1 as well as in a sharpening of the 960?cm-1 peak. The advantages of this method are:•In-situ Raman spectroscopy facilitates quasi – continuous monitoring of phase transitions.•It is an easy to handle and non-invasive method.

Mineralogical stabilization of Ternesite in Belite Sulfo-Aluminate Clinker elaborated from limestone, shale and phosphogypsum

This paper investigates the mineralogical evolution of sulfoaluminate clinker elaborated from moroccan prime materials limestone, shale and phosphogypsum as a byproduct from phosphoric acid factories. The advantage of the production of this type of clinker is related to the low clinkerisation temperature which is known around 1250°C, and to less consumption quantity of limestone thus enabling less CO2 emissions during the decarbonation process compared to that of Portland cement. In this study we determine the stability conditions of belite sulfoaluminate clinker containing belite (C2S) ye’elimite (C4A3$) and ternesite (C5S2$). The hydration compounds of this clinker are also investigated. The monitoring of the synthesized and hydrated phases is performed by X-Ray Diffraction and Infrared spectroscopy. The results show the formation of ternesite at 800°C and the stabilization of clinker containing y’elminite, belite and ternesite at temperatures between 1100 and 1250°C.

Mineralogical stabilization of Ternesite in Belite Sulfo-Aluminate Clinker elaborated from limestone, shale and phosphogypsum

This paper investigates the mineralogical evolution of sulfoaluminate clinker elaborated from moroccan prime materials limestone, shale and phosphogypsum as a byproduct from phosphoric acid factories. The advantage of the production of this type of clinker is related to the low clinkerisation temperature which is known around 1250°C, and to less consumption quantity of limestone thus enabling less CO2 emissions during the decarbonation process compared to that of Portland cement. In this study we determine the stability conditions of belite sulfoaluminate clinker containing belite (C2S) ye’elimite (C4A3$) and ternesite (C5S2$). The hydration compounds of this clinker are also investigated. The monitoring of the synthesized and hydrated phases is performed by X-Ray Diffraction and Infrared spectroscopy. The results show the formation of ternesite at 800°C and the stabilization of clinker containing y’elminite, belite and ternesite at temperatures between 1100 and 1250°C.

Mineralogical evolution of cement pastes at early ages based on thermogravimetric analysis (TG)

Ordinary thermogravimetric analysis (TG) and high-resolution TG tests were carried out on three different Portland cement pastes to study the phases present during the first day of hydration. Tests were run at 1, 6, 12 and 24 h of hydration, in order to determine the phases at these ages. High-resolution TG tests were used to separate decompositions presented in the 100–200 °C interval. The non-evaporable water determined by TG was used to determine hydration degree for the different ages. The effect of particle size distribution (PSD) on mineralogical evolution was established, as well as the addition of calcite as mineralogical filler. Finer PSD and calcite addition accelerate the hydration process, increasing the hydration degree on the first day of reaction between water and cement. According to high-resolution TG results, it was demonstrated that ettringite was the only decomposed phase in the 100–200 °C interval during the first 6 h of hydration for all studied cements. C-S-H phase starts to appear in all cements after 12 h of hydration.

Experimental maturation of Archaea encrusted by Fe-phosphates

Burial is generally detrimental to the preservation of biological signals. It has often been assumed that (bio)mineral-encrusted microorganisms are more resistant to burial-induced degradation than non-encrusted ones over geological timescales. For the present study, we submitted Sulfolobus acidocaldarius experimentally encrusted by amorphous Fe phosphates to constrained temperature conditions (150 °C) under pressure for 1 to 5 days, thereby simulating burial-induced processes. We document the molecular and mineralogical evolution of these assemblages down to the sub-micrometer scale using X-ray diffraction, scanning and transmission electron microscopies and synchrotron-based X-ray absorption near edge structure spectroscopy at the carbon K-edge. The present results demonstrate that the presence of Fe-phosphates enhances the chemical degradation of microbial organic matter. While Fe-phosphates remained amorphous in abiotic controls, crystalline lipscombite (FeIIxFeIII3−x(PO4)2(OH)3−x) entrapping organic matter formed in the presence of S. acidocaldarius cells. Lipscombite textures (framboidal vs. bipyramidal) appeared only controlled by the initial level of encrustation of the cells, suggesting that the initial organic matter to mineral ratio influences the competition between nucleation and crystal growth. Altogether these results highlight the important interplay between minerals and organic matter during fossilization, which should be taken into account when interpreting the fossil record.

Lime-based injection grouts with reduced water content: An assessment of the effects of the water-reducing agents ovalbumin and ethanol on the mineralogical evolution and properties of grouts

Abstract Non-structural lime-based injection grouts with reduced water content were designed for the stabilisation of delaminated plasters in water-sensitive contexts. Two water reducing components were considered: (i) ethanol, as a partial substitute for water, being a less effective solvent for ionic substances, (ii) ovalbumin, a protein found in egg white (it is a water-reducer and air-entrainer). The influence of ethanol and ovalbumin on grouts was assessed in terms of its effect on chemical reactions and formation of phases, the internal structure of the set binders and porosity, through a combination of X-ray powder diffraction coupled with quantitative phase analysis by means of the Rietveld method, scanning electron microscopy with EDS microanalysis and X-ray micro-computed tomography. Water vapour permeability, capillary water absorption and mechanical strength of the materials were tested with standard methods. The results show that both ethanol and ovalbumin affect formation of phases, grout internal structure and porosity, and thus other related physical–mechanical properties. The grouts designed have properties that allow their potential for in situ implementation, with the advantage of water reduction.

Physicochemical and Geotechnical Alterations to MX-80 Bentonite at the Waste Canister Interface in an Engineered Barrier System

The study investigated the basic geomechanical and mineralogical evolution of the bentonite barrier under various experimental boundary conditions which replicated the near-field Thermo-Hydro-Chemico (THC) conditions in a repository. The relationships between the physicochemical alterations and changes in the geotechnical properties have seldom been studied, especially on a consistent dataset. This paper attempts to link the physicochemical properties of Na-bentonite (MX-80) to the macro-scale engineering functionality of the bentonite post THC exposure. Experiments investigated the impact of THC variables on the engineering and physicochemical functionality of the bentonite with respect to its application within a High-Level Waste (HLW) engineered barrier system. Intrinsic alterations to the MX-80 bentonite under relatively short-term exposure to hydrothermal and chemical conditions were measured. Additionally, two long-term tests were conducted under ambient conditions to consider the impact of exposure duration. The intrinsic measurements were then related to the overall performance of the bentonite as a candidate barrier material for application in a UK geological disposal facility. Findings indicate that exposure to thermo-saline-corrosion conditions (i.e., corrosion products derived from structural grade 275 carbon steel) inhibits the free swell capacity and plasticity of the bentonite. However, the measured values remained above the design limits set out for the Swedish multi-barrier concept, from which the UK concept may take a lead. Corrosion alone does not appear to significantly affect the geotechnical measurements compared with the influence of thermal loading and high saline pore water after relatively short-term exposure. Thermal and corrosion exposure displayed no impact on the intrinsic swelling of the smectite component, indicating that no significant structural alteration had occurred. However, when exploring more complex saline solutions i.e., mixed Na, K and Ca, rather than the reference NaCl, divalent cation replacement was observed within the interlayer exchange site. This was accelerated in higher thermal loading conditions.

Grain sorting effects on geochemical characteristics of sulfide mine tailings: a case study

The geochemical evolution of a sulfide mine tailings impoundment in SW Spain was studied. The impoundment was selected because of its small size and its tailings deposition system with a simple discharge point. The objective of this study was to test the hypothesis that mineral segregation associated to hydraulic sorting has significant effects on the geochemical characteristics and the long term weathering. Tailings samples were collected along depth profiles in three sampling points (proximal, central and distal to the point of discharge), and characterized by color, grain size, pH, acid-base account and chemical elements concentration, with the help of routine XRD analysis. Three vertical zones were identified: an upper oxidized zone, a transition intermediate zone, and an unoxidized zone. The analytical results indicate a segregation pattern in the unoxidized tailings based on differences in size and density of tailings grains. Near the discharge point, tailings were coarser and rich in pyrite, whereas the proportion of silicates increased from proximal to distal points. This results in a clear zoning which has consequences on geochemical and mineralogical evolution under weathering, showing substantial differences in the depth of the oxidation front, the acid generation and neutralization capacity, the formation of Fe secondary phases (jarosite) and the total content of the sulfide-related elements (Fe, As, Zn, Cu, Pb and Cd). The results of the study can serve to improve the theoretical bases for the development of conceptual models for predicting environmental impacts associated with sulfide tailings impoundments. Recently, the impoundment has been covered with a soil cover. This fact offers the possibility of new research on its evolution under new conditions.

Fluid-induced transition from banded kyanite- to bimineralic eclogite and implications for the evolution of cratons

Heterogeneous, modally banded kyanite-bearing and bimineralic eclogites from the lithospheric mantle, collected at the Roberts Victor Diamond mine (South Africa), show a reaction texture in which kyanite is consumed. Geothermobarometric calculations using measured mineral compositions in Perple_X allowed the construction of a P-T path showing a steep, cool prograde metamorphic gradient of 2°C/km to reach peak conditions of 5.8GPa and 890°C for the kyanite eclogite. The kyanite-out reaction formed bimineralic eclogite and is probably an integral part of the mineralogical evolution of most archetypal bimineralic eclogites at Roberts Victor and potentially elsewhere. The kyanite-out reaction occured at close to peak pressure (5.3GPa) and was associated with a rise in temperature to 1380°C. Mass balance calculations show that upon breakdown, the kyanite component is fully accommodated in garnet and omphacite via a reaction system with low water fugacity that required restricted fluid influx from metasomatic sources. The δ18O values of garnets are consistently higher than normal mantle values. Each sample has its characteristic trend of δ18O variance between garnets in the kyanite-bearing sections and those in the bimineralic parts covering a range between 5.1‰ and 6.8‰. No systematic change in O-isotope signature exists across the sample population. Differences in garnet trace element signatures between differing lithologies in the eclogites are significant. Grossular-rich garnets coexisting with kyanite have strong positive Eu-anomalies and low Gd/Yb ratios, while more pyrope-rich garnets in the bimineralic sections have lost their positive Eu-anomaly, have higher Gd/Yb ratios and generally higher heavy rare earth element contents. Garnets in the original kyanite-bearing portions thus reflect the provenance of the rocks as metamorphosed gabbros/troctolites. The kyanite-out reaction was most likely triggered by a heating event in the subcratonic lithosphere. As kyanite contains around 100ppm of H2O it is suggested that the kyanite-out reaction, once initiated by heating and restricted metasomatic influx, was promoted by the release of water contained in the kyanite. The steep (high-P low-T) prograde P-T path defining rapid compression at low heating rates is atypical for subduction transport of eclogites into the lithospheric mantle. Such a trajectory is best explained in a model where strong lateral compression forces eclogites downward to higher pressures, supporting models of cratonic lithosphere formation by lateral collision and compression.

The role of Fe on the formation and diagenesis of interstratified glauconite-smectite and illite-smectite: A case study of Upper Cretaceous shallow-water carbonates

The widespread formation of interstratified glauconite-smectite (Gl-S) and illite-smectite (I-S) in modern and ancient diagenetic settings records the physicochemical conditions prevailing during clay mineral authigenesis. To date, however, significant gaps in our knowledge persist in respect to the influence of interstitial solution chemistry, temperature and reaction kinetics on the evolution of composition, mineralogy and microstructure of Gl-S and I-S. Herein, we present a study on the reaction mechanisms and the physicochemical conditions that led to the precipitation of early diagenetic Gl-S and late diagenetic I-S on a stable carbonate platform during the Cenomanian at Langenstein in the Northern German Basin.The texture and the K-Ar age (95.0±1.8Ma) of the green glauconitized grains revealed that green-clay authigenesis progressed in initially organic-rich, semi-confined micromilieus, i.e., in fecal pellets and in foraminifera, close to the sediment-seawater interface. The composition of Gl-S varied in the range (K+0.20–0.74Na+0–0.10Ca2+0–0.05)0.28–0.75 (Fe3+0.63–1.20Fe2+0.08–0.24Al3+0.19–0.97Mg2+0.29–0.52)2.01–2.12 [Al3+0.09–0.35Si4+3.65–3.91O10](OH2), and depended on the rate of aqueous Fe2+ and K+ ion diffusion, the micromilieu of glauconitization and on the bulk sedimentation rate. The mineralogical, microstructural and chemical changes of the ongoing Gl-S products revealed the following reaction for green-clay authigenesis at Langenstein: Fe(III)-smectite reacted with monosilicic acid, goethite and aqueous K+, Mg2+ and Fe2+ to form glauconite and aqueous Na+, Ca2+ and H+ ions. This process considers complex mineral transformations commonly associated with glauconitization, such as early diagenetic oxidation of organic matter and microbial-catalyzed dissolution of Fe-(oxy)hydroxides, carbonates and detrital silicates. In contrast, the K-Ar age of I-S (68.0±1.6Ma) and its compositional variability, (K+0.29–0.45Na+0–0.10Ca2+0–0.06)0.30–0.55 (Fe3+0.16–0.29Fe2+0–0.10Al3+1.37–1.68Mg2+0.18–0.43)2.00–2.12 [Al3+0.17–0.39Si4+3.61–3.83O10](OH2), indicate a burial diagenetic origin for this mineral phase, rather than transformation of illitic clays into I-S during weathering under warm and humid climatic conditions. The results from kinetic modelling support a diagenetic origin of I-S (50–60%I layers and 50–40%S layers) and imply its formation by the replacement of pre-existing K-feldspar at high pore-fluid activity K/Na ratios and at low Fe2+ concentrations.We propose that the substitution of Al3+ for Fe3+, Fe2+ and Mg2+ in the octahedral sheet shifts the stability field of the kaolinite–Fe-Al-Mg-smectite–Fe-Al-Mg-illite (or glauconite) triple point to much lower monosilicic acid activities, and stabilizes the I-S (or Gl-S) structure. This reaction supports the idea that the (bio)availability of Fe is the rate-limiting factor for glauconitization, which is not the case for the diagenetic growth of I-S, whereby the pore water Fe2+ concentration may be limited by the competing formation of Fe-(oxy)hydroxides and/or Fe-sulfides.

Demise of Ediacaran dolomitic seas marks widespread biomineralization on the Siberian Platform

Abstract The trigger for biomineralization of metazoans in the terminal Ediacaran, ca. 550 Ma, has been suggested to be the rise of oxygenation or an increase in seawater Ca concentration, but geochemical and fossil data have not been fully integrated to demonstrate cause and effect. Here we combine the record of macrofossils with early marine carbonate cement distribution within a relative depth framework for terminal Ediacaran to Cambrian successions on the eastern Siberian Platform, Russia, to interrogate the evolution of seawater chemistry and biotic response. Prior to ca. 545 Ma, the presence of early marine ferroan dolomite cement suggests dominantly ferruginous anoxic “aragonite-dolomite seas”, with a very shallow oxic chemocline that supported mainly soft-bodied macrobiota. After ca. 545 Ma, marine cements changed to aragonite and/or high-Mg calcite, and this coincides with the appearance of widespread aragonite and high-Mg calcite skeletal metazoans, suggesting a profound change in seawater chemistry to “aragonite seas” with a deeper chemocline. By early Cambrian Stage 3, the first marine low-Mg calcite cements appear, coincident with the first low-Mg calcite metazoan skeletons, suggesting a further shift to “calcite seas”. We suggest that this evolution of seawater chemistry was caused by enhanced continental denudation that increased the input of Ca into oceans so progressively lowering Mg/Ca, which, combined with more widespread oxic conditions, facilitated the rise of skeletal animals and in turn influenced the evolution of skeletal mineralogy.

Experimental Study of Porosity Changes in Shale Caprocks Exposed to CO2-Saturated Brines I: Evolution of Mineralogy, Pore Connectivity, Pore Size Distribution, and Surface Area

Abstract Carbon capture, utilization, and storage, one proposed method of reducing anthropogenic emissions of CO2, relies on low permeability formations, such as shales, above injection formations to prevent upward migration of the injected CO2. Porosity in caprocks evaluated for sealing capacity before injection can be altered by geochemical reactions induced by dissolution of injected CO2 into pore fluids, impacting long-term sealing capacity. Therefore, long-term performance of CO2 sequestration sites may be dependent on both initial distribution and connectivity of pores in caprocks, and on changes induced by geochemical reaction after injection of CO2, which are currently poorly understood. This article presents results from an experimental study of changes to caprock porosity and pore network geometry in two caprock formations under conditions relevant to CO2 sequestration. Pore connectivity and total porosity increased in the Gothic Shale; while total porosity increased but pore connectivity decrea...

The Influence of Salt Loading Density on the Atmospheric Corrosion of Aluminum

The relationship between NaCl on an aluminum surface and its corrosion rate in humid conditions is explored as a means of better understanding the impact of salt loading on atmospheric corrosion in service environments. Corrosion of aluminum and aluminum alloys under atmospheric exposure has been well documented for outdoor conditions.[1-8] These studies characterized effects of environmental severity and exposure factors that may control or enhance corrosion rates. One severity factor, salt deposition rate, results in higher corrosion rates with higher salt deposition rates.[7,14] The same correlative relationship has been reported in the fewer laboratory studies on this topic.[9-12] Figure 1 exemplifies these trends where mass loss of Al 4N, an Al-Mg alloy (AA 6061), and Al 1100 is correlated with salt deposition rate for field and lab accelerated exposures.[13] For all exposures and materials, corrosion rates increased with increasing deposition rates, except for AA 6061, which showed a stifling in corrosion rate in laboratory exposures above 100 mg/m2d. While these studies expose some environmental severity effects and indicate factors that may be controlling corrosion rates, they do not provide insight into the dependence of corrosion rate on salt deposition. Other laboratory studies have elucidated a phenomenon for atmospheric exposures of aluminum, in that corrosion rate diminishes with increased exposure time. Al samples exposed to humid air with 14 or 70 µg/cm2 coverage of NaCl displayed corrosion stifling at 200 h of exposure.[9,10] A thorough explanation to the stifling effect of corrosion over time under fixed NaCl loading conditions has not been fully developed. Possible theories have been put forth, including, the stability or passivation of the surface that shuts down the cathodic region, which is pH and, in turn, CO2 dependent [9,10] or the gettering of NaCl by corrosion product leading to surface drying and depletion of the corrosion aggressor. While there is data supporting the stifling of corrosion with increased ppm of CO2, empirical evidence of pH change is lacking, and there is no evidence to date of NaCl gettering. Also, the hypothesis that increased CO2 results in the stifling of corrosion suggests that corrosion would be independent of NaCl loading density. This presentation explores the effects of selected NaCl loading densities vs. exposure time of Al 1100 and Al 5N samples with isohumidity tests at a range of set RH. Pure Al samples were selected for study to establish a simplified corrosion system to understand atmospheric mechanisms which could then be expanded to common engineering alloys. Both macro and micro scale studies are undertaken to elucidate the possible mechanisms leading to corrosion stifling under fixed NaCl loading conditions. High throughput inkjet salt printing is applied to characterize corrosion rate via coupon-level studies. Gravimetry and optical profilometry are utilized to determine mass/volume loss of salt-loaded coupons exposed to humidity and provide indication of conditions leading to corrosion stifling. Micro scale explorations with in-situ time lapse imaging of micro-pipetted droplets exposed to isohumidity conditions can provide basic indication of electrolyte coverage and stability during exposure. Post-exposure analysis, including optical imaging, pH analysis, SEM, and vibrational spectroscopy will provide characterization of the evolution of electrolyte chemistry and corrosion product mineralogy. Through this work, an understanding of the relationship between corrosion in atmospheric systems versus the variation of a specific environmental severity factor, NaCl loading density, will be further developed.