Glass Alteration Research Articles

Alteration rate of medieval potash-lime silicate glass as a function of pH and temperature: A low pH-dependent dissolution

Potash lime silicate glasses were extensively used during the Middle Ages as components of stained glass windows. Their external faces have been subjected to alteration in the atmospheric medium for centuries, leading to the development of thick alteration layers on the surfaces. This alteration is mainly caused by water in liquid (rainfall) or gaseous (vapor) form. In order to implement geochemical models simulating the long-term behavior of these glasses by coupling the different atmospheric factors, it is necessary to study mechanisms and kinetics of alteration induced by these different hygroscopic situations (rainfall and relative humidity). The aim of this study is here to evaluate the impact of liquid water on stained glass windows and especially to determine alteration rate laws of dissolution and interdiffusion as a function of pH and temperature. For that, a medieval model glass was altered in aqueous solution in batch experiments at different pH values (from 2 to 11), temperatures (from 5 to 70 °C) and saturation states of the solution. From the elemental concentrations in solution, the release rate of elements was calculated and the pH- and T-dependencies were determined. The dissolution and interdiffusion rate laws can now be used to simulate the alteration of a medieval glass submitted to a rainfall event. Moreover, this study highlights that the Si-K-Ca medieval glass presents a very low pH-dependency at alkaline pH, which can be explained by its composition.

Petrograpic and material observations of basaltic lapilli tuff, 1979 and 2017 Surtsey drill cores, Iceland

Petrographic studies of thin sections from the 1979 and 2017 Surtsey drill cores provide new insights into microstructural features in basaltic lapilli tuff sampled from the principal structural and hydrothermal zones of the volcano. These describe narrow rims of fine ash on altered glass pyroclasts in thin sections of the 2017 cores, characteristics of granular and microtubular structures in the original thin sections of the 1979 core, and glass alteration in diverse environments. The narrow ash rims follow the outlines of glass pyroclasts in the subaerial tuff cone and in submarine and sub-seafloor deposits; they suggest complex eruptive and depositional processes. The tubular microstructures resemble endolithic microborings in older oceanic basalt; they suggest possible microbial activity. Tubule lengths indicate rapid growth rates, up to 30 µm in ~15 years. Comparisons of glass alteration in thin sections prepared immediately after drilling in 1979 and 2017 indicate differential time-lapse alteration processes in the structural and hydrothermal zones of the volcano. In contrast, thin sections of the 1979 core prepared after 38 years in the repository reveal labile glass alteration during archival storage. The oven-dry density of the sub-seafloor lapilli tuff decreases in 2017 samples with high porosity and water absorption and increases in 2017 samples with a compact ash matrix and lower water absorption. The petrographic descriptions and material measurements provide a foundational reference for further investigations of explosive eruption and deposition of basaltic tephra at Surtsey and the subsequent alteration of these deposits in the volcanic environment and, potentially, the curatorial environment.

Near-field corrosion interactions between glass and corrosion resistant alloys

This study explores the corrosion interactions between model nuclear waste glass materials and corrosion resistant alloys, under accelerated conditions that simulate the near field of a nuclear waste repository. The interactions between the corrosion of stainless steel (SS) 316, alloy G30, or alloy 625, and international simple glass or soda-lime silica glass are systematically studied. The dissimilar materials were exposed in close proximity to each other in different electrolytes at 90 °C. After exposure, the glass surface exposed near metals showed different regimes of corrosion, with distinct surface morphologies and chemistries that were likely affected by the local environment created by the localized corrosion of metals. Surface and solution analyses showed that the corrosion rate of glass was enhanced by the presence of metals. Infrared spectroscopy data suggested the local build-up of stresses in the contact area of glass, which may lead to the mechanical instability of the glass alteration layer. On the other hand, the effect of glass on metal corrosion is strongly dependent on the leaching solution. In electrolytes containing abundant aggressive anions such as Cl−, glass seems to suppress the localized corrosion of SS by the precipitation of a Si-rich surface film that protects the SS substrate from solutions. However, in less aggressive electrolytes, the corrosion rate of SS was increased by the presence of glass corrosion products. Overall, our study showed that the hidden and localized damage on glass in contact with metals may enhance the release rate of glass components compared to typical uniform glass corrosion.

Chemical weathering in Antarctica: an example of igneous rock particles in Big Lachman Lake sediments, James Ross Island

Lacustrine sediments of the Big Lachman Lake (James Ross Island, Antarctica) are dominated by clay- and silt-sized particles of varied petrographic composition. These are of volcanic origin and are derived predominantly from the Mendel Formation. The intensity of chemical weathering of volcanic rock particles in the sediments was expressed by the Chemical Index of Alteration (CIA), which reflects alteration of feldspars and formation of clay minerals. The observed mineral assemblage of montmorillonite, illite, and kaolinite probably originated from alteration of volcanic glass and from alteration of feldspars to clay minerals. CIA weathering rates from the lacustrine sediments are similar to those of Early Jurrasic high-Ti volcanic rocks of the Mount Poster Formation and Middle Jurrasic high-Ti volcanic rocks of the Mapple Formation from the Antarctic Peninsula. However, the sediments show a good fit in OIB-normalized incompatible trace-element patterns only with the high-Ti volcanic rocks from the Middle Jurassic rhyolites of the Mapple Formation, while the fit with Cenozoic basalts of the James Ross Island Volcanic Group is poorer. Rocks from the Antarctic Peninsula were probably transported through the Prince Gustav channel by small glaciers during the Neogene and Quaternary.

Mineralogical and geochemical analysis of sodium bentonites in continental settings: The Uspallata Group (Triassic) of the Cuyana Basin, Mendoza province, Argentina

The Triassic Uspallata Group of the Cuyana rift Basin is characterized by continental deposits including alluvial, fluvial and lacustrine systems, mainly composed of volcaniclastic (reworked) and pyroclastic sediments. The present work focuses on the ash fall tuffs levels (bentonites) in order to understand the origin of their sodium composition since classic worldwide natural sodium bentonites link with marine environments as a source of sodium. The mineralogical attributes and relationship with diagenetic alterations have been explored by petrography, X-ray diffraction, and geochemical analysis. The tuff levels, recorded in the Cerro de las Cabras, Potrerillos and Cacheuta formations, provide the most reliable representation of the composition of the original material and diagenetic alterations due to these levels are devoid of detrital input. Accordingly, they represent key elements to explore the origin of huge bentonite deposits in the basin. In the study units, Na-montmorillonite and analcime, found at concentrations of up to or more than 50 vol%, grew at the expense of vitric components and represent authigenic minerals developed under early diagenesis. Following sedimentological and paleogeographic evidence the alteration of volcanic glass occurred in hydrologically open lacustrine systems that reached saline-alkaline conditions. The changes in the (Na + K)/H+ ratio influenced the formation of Na-montmorillonite and analcime through the succession. In addition, kaolinite is an important alteration mineral that fills cavities, replaces the feldspar grains, and probably the early diagenetic minerals. The origin of kaolinite is due to the combined effects of decomposition of organic matter and incursion of meteoric freshwater during late-stage telogenesis which generated the acidification of pore waters. Based on geochemical proxies, geochronological data and the geodynamic context the input of high volumes of pyroclastic material may thus be the result of explosive volcanism from an arc-subduction complex located to the west at these latitudes. The present work suggests that the composition of Na-rich pyroclastic ashes accumulated in hydrologically-open lake systems which conditioned the formation of sodium bentonites in continental settings.

Comparative structural investigations of nuclear waste glass alteration layers and sol-gel synthesized aerogels

While various glass alteration layer formation mechanisms have been debated in recent years, the glass alteration community generally agrees that more information on physical properties of the alteration layers is needed to further the understanding of their impacts on overall glass alteration. In this work, pore volumes and solid structures of glass (International Simple Glass, ISG) alteration layers formed in solutions of various pH conditions in initially dilute conditions at 90 °C are evaluated with positron annihilation spectroscopy, small-angle X-ray scattering, and scanning transmission electron microscopy. Pore volumes of alteration layers formed at pH 9 were found to be at their lowest near the surfaces of the alteration layers. Solid structures of alteration layers are compared with those of synthetic aerogels of comparable compositions produced under various pH conditions. Alteration layers formed at pH 11 on ISG were shown to contain large structures (>10 nm) similar to synthetic aerogels created under neutral and basic conditions whereas alteration layers formed at pH 9 did not. Available dissolved silica species defined by silica solubility were proposed to have the greatest impact on alteration layer structure.

Far from equilibrium basaltic glass alteration: The influence of Fe redox state and thermal history on element mobilization

Elemental release from basaltic glasses at far from equilibrium conditions was investigated as a function of the Fe redox state (Fe(II)/Fetot = 0.35 and 0.80) and thermal history (quenched ↔ annealed). A flow-through column setup was used to ensure disequilibrium of basaltic glass and solution during the entire runtime. Percolation experiments were performed at 25 °C for up to 500 h with intermediate sample collection. Two different pH values were adjusted, and the effect of organic matter was tested by adding oxalic acid. Element concentrations in the percolate were measured by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES).After an initial high release of elements from fresh glass surface, steady state was achieved after 240–430 h for Si at pH 5–7 and after 170–240 h for Al at pH 2. At near neutral conditions (pH 5–7) mobilization of Si is relatively high, while Fe and possibly Al are retained in precipitates. Under more acidic conditions (pH 2), the Si concentration of the solutions was very low compared to the other main constituents of the glass. Large amounts of Si-rich residues are formed after glass dissolution at pH 2. On the other hand, Fe (and Mn) is very mobile under acidic conditions, favored by complex formation with oxalic acid and chlorine. In the initial phase of the pH 2 experiments, the element release from reduced glasses is higher than from oxidized glasses. However, this trend is reversed when approaching steady state. Higher dissolution rates for oxidized glasses are predicted due to the progressive replacement of strong Si-O-Si with weaker Fe(III)-O-Si. At pH 5–7 the concentrations of elements in the percolate are too low to establish a systematic difference between oxidized and reduced glasses. Looking at the total amount of mobilized elements, the thermal history of the glasses has no significant effect in the case of oxalate-free solutions, but a noticeable increase of element release in the case of rapidly quenched glasses was observed when using 1 mM oxalic acid solution. The strong effect of oxalate on dissolution of quenched glasses is probably related to the more open glass network structure in quenched glasses.

Acceleration of glass alteration rates induced by zeolite seeds at controlled pH

As the glass-water reaction proceeds, certain glass compositions undergo a delayed acceleration in the alteration rate – a phenomenon that is commonly referred to as Stage III behavior – which generally coincides with zeolite formation. However, studying Stage III in the laboratory is difficult because the time scale required to observe Stage III behavior can vary from months to several years, and the solution conditions that initiate this acceleration remain poorly understood. Consequently, the development of a corrosion test method capable of quantifying Stage III rates in a timely manner would be of great benefit to both the scientific community and policy makers concerned with nuclear waste disposal. In this work, we induced and measured the acceleration in the corrosion rate of the AFCI glass by seeding static corrosion tests with zeolite Na–P2 at imposed pH90°C values ranging from 9.5 to 11.5. The log10 induced Stage III rates increased linearly with increasing pH and were measured to be between the forward and residual rates of AFCI. Tests designed to compare induced Stage III rates obtained for different seeding times showed that the induced Stage III rates were faster in the absence of an alteration layer. In summary, the use of the seeded test method allows for a quantification of induced Stage III rates in a relatively short, one-month time period.

Nanostructure of CaO-(Na2O)-Al2O3-SiO2-H2O Gels Revealed by Multinuclear Solid-State Magic Angle Spinning and Multiple Quantum Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy

(Calcium,alkali)-aluminosilicate gel frameworks are the basis of modern cements and alkali-activated materials for sustainable infrastructure and radioactive waste immobilization and also find application in glass alteration, mineral weathering, and zeolite synthesis. Here, we resolve the nanostructure of these gels that dictates mass transport, solubility, and mechanical properties. The key structural motifs comprising hydrous (calcium,alkali)-aluminosilicate gels are identified via 17O, 23Na, and 27Al triple-quantum magic angle spinning and 29Si magic angle spinning nuclear magnetic resonance spectroscopy of a novel class of stoichiometrically controlled 17O-enriched multiphase gels. Increased Ca content promotes low-Al, high-Ca chain-structured “C-S-H-type” products exhibiting significant nanostructural ordering, low levels of chain cross-linking, predominant Ca coordination of nonbridging oxygen atoms, and an increase in proton association with CaO layers to form Ca–OH sites. Al substitution is identified in multiple sites in the silicate chains, including cross-linking, bridging, and pairing tetrahedra. Increased Al content increases the proportion of cross-linking sites and gel disorder. The large increase in SiIV–O–AlIV sites increases the relative amounts of Na+ and AlV species charge-balancing AlO4– tetrahedra and results in the formation of an additional disordered low-calcium, framework-structured alkali aluminosilicate (“N-A-S-H-type”) gel, with high Al and Na contents. Changes in bulk composition significantly alter the nanostructures of the C-S-H-type and N-A-S-H-type gels. Mean SiIV–O–AlIV bond angles for each type of AlIV site environment are highly consistent, with compositional changes dictating the relative proportions of individual AlIV species but not altering the local structure of each AlIV site. These findings reveal the molecular interactions governing the (calcium,alkali)-aluminosilicate gel nanostructure, which are crucial in controlling material properties and durability.

Occurrence, composition, and origin of analcime in sedimentary rocks of non-marine petroliferous basins in China

Abstract Natural analcime, a highly brittle aluminosilicate mineral that dissolves easily during diagenesis, occurs in the sedimentary rocks of eight main non-marine petroliferous basins in China. This mineral has been well documented for the past 30 years and has proved to be significant for the formation of high-quality hydrocarbon reservoirs. The present study fully reviews analcime occurrences, chemical compositions, Si/Al ratios, origin and controlling factors, and influence on the hydrocarbon reservoir quality in Permian to Paleogene sedimentary rocks. Previous research has shown that analcime is commonly distributed within argillaceous laminae (l-type); occurs as intergranular cement (c-type), replacement (r-type), and crack filling (f-type) forms; or is closely intergrown with other authigenic/exhalative minerals (a-type). Analcime in lacustrine basins is slightly Al-rich and Na-poor, and its Si/Al ratios range from 1.95 to 2.80. The geochemical compositions of the analcime- and ankerite-bearing exhalites are characterised by enrichment in light rare earth elements and 13C and depletion in heavy rare earth elements and 18O, with negative Eu and positive Ce anomalies. Diagenetic analcime forms by the alteration of volcanic glass, clays, or zeolite precursors in saline-alkaline lakes or during buried diagenetic stages in sedimentary rocks. In addition, it coexists with other authigenic zeolite, carbonate, and clay minerals as well as albite. Hydrothermal analcime presumably precipitates from hydrothermal fluids and is not dependent on a local precursor mineral or volcanic glass; this type is commonly related to exhalative mineralisation of hydrothermal springs. The formation of natural analcime is controlled by various factors such as the composition of the parent material, temperature prevalent during crystallisation, chemistry of the fluid phase, and thermodynamic characteristics of the geological environment. As zeolites are formed in alkaline fluid phases, they become unstable in acidic pore water. Secondary pores created by analcime mineral dissolution, and intercrystalline micropores among analcime minerals, are highly significant in hydrocarbon reservoirs. Analcime-bearing mudstone, siltstone, dolostone, and exhalite are favourable lithofacies for unconventional hydrocarbon accumulation owing to its high brittleness and selective dissolution of analcime and other associated minerals.

In situ nuclear-glass corrosion under geological repository conditions

Silicate glasses are durable materials but laboratory experiments reveal that elements that derive from their environment may induce high corrosion rates and reduce their capacity to confine high-level radioactive waste. This study investigates nuclear-glass corrosion in geological media using an in situ diffusion experiment and multi-component diffusion modelling. The model highlights that the pH imposed by the Callovo–Oxfordian (COx) claystone host rock supports secondary-phase precipitation and increases glass corrosion compared with pure water. Elements from the COx rock (mainly Mg and Fe) form secondary phases with Si provided by the glass, which delay the establishment of a passivating interface. The presence of elements (Mg and Fe) that sustain glass alteration does not prevent a significant decrease in the glass-alteration rate, mainly due to the limited species transport that drives system reactivity. These improvements in the understanding of glass corrosion in its environment provide further insights for predictive modelling over larger timescales and space.

Reactive transport modeling of glass alteration in a fractured vitrified nuclear glass canister: From upscaling to experimental validation

Performance assessment of geological nuclear waste repositories entails modeling of the long-term evolution of the aqueous alterations of the fractured nuclear glass block, because the time scales under consideration are of several thousands of years and hence beyond the range of any direct experimental perspectives. In this study, our objective is to bridge the gap between the reservoir-scale flow and transport simulations and the micron-scale modeling of the glass-water interfacial processes by providing quantitative evaluation of the aqueous alteration of glass at the block scale. In particular, calculations of the equivalent diffusive, hydraulic, and alteration kinetics properties and reactive transport simulations are discussed. Prior to performing reactive transport modeling at the scale of the glass canister, the preferred upscaling techniques were first applied to a synthetic fracture network system with ends to compare the results of the borosilicate glass alteration with the discrete fracture modeling and the equivalent porous medium approach. The evolution of the altered glass obtained from reactive transport modeling applied to several realizations of the equivalent fracture network tessellation was compared to the experimental data of the aqueous alteration test of a nonradioactive full-scale SON68 glass canister. The proposed model agrees with the experimental data and offers, for the first time, an opportunity to better understand the impact of fracturing as the convection due to the radioactivity acting as a heating source on the corrosion of nuclear glass.

Effect of alkaline synthesis conditions on mineralogy, chemistry and surface properties of phillipsite, P and X zeolitic materials prepared from fine powdered perlite by-product

Abstract The fine powdered perlite, a by-product of processing of raw perlite was used for zeolite synthesis. Perlite by-product material (PBM) is not suitable for perlite expansion, due to fine particle size, therefore it has very limited application (recently only as additive to concrete). The conversion of PBM into zeolites is proposed to recover this material (i.e. minimize its accumulation) and to obtain value-added material (i.e. zeolites). Volcanic glass was the main PBM component transformed to zeolites after reaction with NaOH solutions. As volcanic glass alteration proceeded, crystallization of zeolites having lower Si/Al ratio was favored. This was likely due to more rapid increase in the solubility of alumina than the solubility of silica with increasing concentration of reacting NaOH solution. Phillipsite, zeolite P and zeolite X were the main reaction products synthesized from PBM. The concentration of NaOH solution had significant impact on the type of synthesized zeolites whereas reaction temperature and time influenced mainly the quantity of synthesized zeolite species. The highest-grade zeolitic material synthesized from PBM contained 77 wt% of zeolites, 16 wt% of unaltered volcanic glass and 7 wt% of accessory minerals. The small amount of zeolites (11–29 wt%) was synthesized even at the lowest tested temperature of 50 °C. After shortest reaction time (24 h), from 6 to 54 wt% of zeolites was formed. Synthesized materials reached maximum total specific surface area (SBET) of 362 m2/g and cation exchange capacity (CEC) of 371 meq/100g.

Plutonium Desorption from Nuclear Melt Glass-Derived Colloids and Implications for Migration at the Nevada National Security Site, USA.

The migration of low levels of plutonium has been observed at the Nevada National Security Site (NNSS) and attributed to colloids. To better understand the mechanism(s) of colloid-facilitated transport at this site, we performed flow cell desorption experiments with mineral colloid suspensions produced by hydrothermal alteration of NNSS nuclear melt glass, residual material left behind from nuclear testing. Three different colloid suspensions were used: (1) colloidal material from hydrothermal alteration of nuclear melt glass at 140 °C; (2) at 200 °C; and (3) plutonium sorbed to SWy-1 montmorillonite at room temperature. The 140 °C sample contained only montmorillonite, while zeolite and other phases were present in the 200 °C sample. Overall, more plutonium was desorbed from the 140 °C colloids (ca. 9-16%) than from the 200 °C colloids (ca. 4-8%). Furthermore, at the end of the 4.5 day flow cell experiments, the desorption rates for the 140 °C colloids and the Pu-montmorillonite colloids were similar while the desorption rates from the 200 °C colloids were up to an order of magnitude lower. We posit that the formation of zeolites and clays hydrothermally altered at 200 °C may lead to a more stable association of plutonium with colloids, resulting in lower desorption rates. This may give rise to more extensive colloid-facilitated transport and help explain why trace levels of plutonium are found downgradient from their original source decades after a nuclear detonation. Interestingly, in the case of cesium (a co-contaminant of plutonium), no difference was observed between the 140 and 200 °C colloids. This reflects intrinsic differences between cesium and plutonium sorption/desorption behavior (charge, cation size) and suggests that the Cs sorption mechanism (cation exchange) is not similarly affected by colloid formation temperature.

Alteration of 29Si-doped SON68 borosilicate nuclear waste glass in the presence of near field materials

The leaching of 29Si-doped SON68 French nuclear waste glass was investigated in Callovo-Oxfordian (COx) water in the presence of P235GH stainless steel and COx argillite. Experiments were carried out to understand the effect of near-field materials on the rate of glass dissolution and the migration of 29Si in the claystone core. Tests were performed by using percolation cells containing glass and stainless steel sandwiched between two clay cores in two different combinations: glass/clay at 50 and 90 °C, and glass/steel/clay/at 90 °C. COx water was injected through the percolation cells at 120 bars (solution flow rate 0.05–0.38 mL d−1). Results showed that the presence of steel did not enhance the dissolution rate of the glass despite the retention of 29Si released from the glass on the corrosion products. The glass dissolution rate up to 17 months tends towards 10−4 and 10−3 g m−2 d−1 at 50 and 90 °C, respectively. The glass alteration in the presence of clay leads to the formation of calcite and Mg-silicates at 50 °C in addition to apatite and powellite at 90 °C. The main steel corrosion products were siderite, magnetite, ferrosilicates, pyrite, pyrrhotite, troilite and mackinawite. MgSO4 and CaSO4 were also identified in the presence of steel.

Sol-Gel Synthesis and Characterization of Gels with Compositions Relevant to Hydrated Glass Alteration Layers.

During the processes associated with glass corrosion, porous hydrated glass alteration layers typically form upon exposure to aqueous conditions for extended time periods. The impacts of the alteration layer on glass durability have not been agreed upon in the glass science community. In particular, the formation mechanisms of hydrated glass alteration layers are still largely unknown and require further investigation, but these layers often require months to years to develop and are often too thin to adequately characterize. Meanwhile, sol–gel-derived silicate gels are relatively easy to synthesize in bulk with custom compositions relevant to hydrated glass alteration layers. If alteration layers and synthetic silicate gels demonstrate physical and chemical properties that are sufficiently similar, synthetic silicate gels could be used as analogues for hydrated glass alteration layers in future studies. However, synthetic gels must first be prepared and evaluated before comparisons between glass alteration layers and synthetic silicate gels can be made. This work focuses entirely on the synthesis and observed physical properties of synthetic silicate gels. A future work will compare the characteristics of synthetic gels described in this work with altered waste glass formed in similar pH environments. In this study, synthetic gels were made with custom compositions at various pH values to evaluate the effect of pH on gel structure and morphology. Several other variables were examined also, such as composition, drying, and aging. Gels were produced by sequential additions of organometallic precursors in a single container. Gels were analyzed with several techniques including small-angle X-ray scattering, gas adsorption, and He pycnometry to determine the effects of the variables on physical properties. Results show that gels prepared at pH 3 consistently contained fewer primary particles with diameters larger than 7.2 nm and fewer pores with diameters larger than 30 nm compared to gels synthesized at pH 7 and 9. Composition was shown to have no discernable effect on primary particle and pore sizes at any pH.

Aeolian Dust Provenance in Central East Antarctica During the Holocene: Environmental Constraints From Single‐Grain Raman Spectroscopy

AbstractWe analyzed the single‐grain mineralogical composition of aeolian dust transported to central East Antarctica for provenance purposes. Comparison with data from the last glacial period shows for the first time disappearance of carbonates during the Holocene related to sea level rise and suppressed deflation from the Argentinean continental shelf, exposed during Marine Isotope Stage 2. Zeolites, related to alteration of volcanic glass in the subglacial/periglacial environment of Patagonia, show a similar behavior. The remaining minerals, remarkably similar between the two climatic periods, are compatible with a Pampean and Patagonian provenance, but Holocene data show a more pronounced volcanic and metamorphic imprint and presence of minerals related to warm climate weathering environments compatible with an additional contribution from subtropical latitudes of South America. These results do not imply a major large‐scale reorganization of atmospheric circulation after the last climatic transition.

A nanoscale study of the formation of Fe-(hydr)oxides in a volcanic regolith: Implications for the understanding of soil forming processes on Earth and Mars

Abstract Iron(hydr)oxides are one of the most important constituents of regoliths and soils derived from volcanic rocks both on Earth and Mars, often giving them their characteristic red color. This study deciphers for the first time an underlying mechanism for the formation of Fe-(hydr)oxides in a regolith which can occur during the weathering of basaltic glass; Fe-(hydr)oxides are prominent alteration products of volcanic regolths. An excellent example of these conditions is the early stage of basaltic glass weathering in the Martian regolith simulant JSC MARS-1A. In this study, a combination of focused ion beam technology and analytic transmission electron microscopy is employed in order to characterize basaltic glass weathering down to the nanometer level. Our results show that the formation of Fe-(hydr)oxide phases such as ferrihydrite, magnetite/maghemite and hematite during alteration of basaltic glass is based on complex and formerly unknown sequences of dissolution-precipitation reactions and pressure induced coalescence, segregation, aggregation, densification and growth processes. The weathering of the glass starts with its dissolution and subsequent precipitation of hydrous amorphous silica-bearing pockets rimmed by nano-size domains of ferrihydrite. An increase in molar volume during this process leads to an overall volume expansion, which promotes (a) growth of the hydrous silica through coalescence of individual pockets, (b) agglomeration of ferrihydrite domains to larger and denser aggregates in between layers or along the surfaces of plagioclase, hydrous amorphous silica and amorphous Al-(hydr)oxides, (c) formation of hematite within dense aggregates of ferrihydrite or as larger nanoparticles within an hydrous amorphous Si-Al-rich phase and (d) the break-up of plagioclase crystals and the replacement of these fragments by an hydrous amorphous Fe-Al-Si-bearing phase. At a later weathering stage, ferrihydrite nano-domains can also transform into magnetite/maghemite nanoparticles, which occur as layers around and on the surface of larger plagioclase crystals. This study also indicates the presence of past nano-environments in close proximity to each other, as for example layers of imogolite and ferrihydrite/hydrous amorphous silica occur only nanometers apart from each other on the opposite sides of unaltered glass. In accord with previous mineralogical studies of JSC MARS-1, the observed bulk and nano-mineralogical composition indicate that early alteration processes of basaltic glass under dry and cold conditions are mainly controlled by the formation of Fe-(hydr)oxides and minor imogolite and kaolinite. Recent mineralogical studies indicate that alteration processes at these conditions may have been the dominant weathering processes over long time periods on the Martian surface.

Frontiers in natural and un-natural glasses: An interdisciplinary dialogue and review

The purpose of this review is to compare and contrast silicate natural glasses with so-called un-natural glasses, bringing together literature and common issues in geology and technical glass science and engineering. The review is organized into three proposed ‘frontiers’ of glass science of interdisciplinary interest: 1) extreme conditions for glass formation, 2) crystallization in complex glasses, and 3) long-term glass alteration. In each frontier, examples of natural (geologic) as well as un-natural (accidental anthropogenic) glasses are presented, and their characteristics are compared and contrasted. Since crystallization is an important consideration, not only fully amorphous glasses but also partially crystallized glasses are considered. Natural glasses discussed include primarily impactites, obsidian, and basaltic glass, though glasses formed by other means are also considered. Un-natural glasses discussed include those created in atomic blasts, vitrified Iron Age hillforts, and slags from metallurgical processes. Conditions for extreme glass formation in different types of glasses formed by shock metamorphism are compared to modern time-resolved shock and high-pressure experiments for observing polyamorphism and densification changes. Silicate magmas producing obsidian and basaltic glasses are presented and the differences in crystallization pathways are discussed as they relate to changes in flow properties and eruptions. Vitrified hillfort materials and combustion natural glasses are then compared with respect to their felsic-mafic liquid phase separation and crystal assemblage. Finally, an appeal is made for the use of natural and un-natural glasses for understanding long-term glass corrosion as a function of composition. Glass corrosion models are considered as applied to both types of glasses, particularly relating to the neo-formed crystals, biologically-induced glass corrosion, and nuclear waste management. It is hoped that this unorthodox presentation will foster dialogue between earth scientists and technical glass scientists and further research into these and other frontiers of natural glass science.

Temperature-dependent mechanisms of the atmospheric alteration of a mixed-alkali lime silicate glass

The atmospheric alteration of a mixed-alkali lime silicate glass is described as a function of time and temperature. The glass hydration kinetics self-accelerates with time and is considerably enhanced with the temperature. At 80 °C, alkali and alkaline earth ions are retained in hydrate-type environments while at 40 °C, they are partially leached out forming surface carbonates, in the order Ca2+>Na+>>K+. The temperature dependency of hydrolysis processes, the important role of the hydration energy of cations and the diminished solvation properties of bound water are considered to discuss experimental observations and advance in the understanding of the specificities of glass atmospheric alteration.