Si-rich Glass Research Articles

Incorporation of Zn in the destabilization products of muscovite at 1175 C under disequilibrium conditions, and implications for heavy metal sequestration

This work reports on the thermal decomposition of muscovite within a granite powder doped with 8.5 wt% ZnO and heated during 10 min to 68 h at 1175 °C, and the implications for the sequestration of Zn, and other heavy metalts in such decomposition products. Samples were characterized using analytical scanning and transmission electron microscopy. After 10 min, muscovite is completely pseudomorphosed by Si-rich glass, spinel structure phases, and minor mullite. Spinel phases incorporate Zn, but their compositions depend on their position within the muscovite pseudomorphs. Al-rich oxides crystallize at the core of the pseudomorphs while Zn-Al spinels are located at the rims. The most Al-rich spinels have compositions close to γ-Al2O3, a metastable transition alumina, with up to 5 wt% MgO, 2 wt% Fe2O3, 4 wt% ZnO, and 9 wt% SiO2. The most Zn-rich spinels show compositions intermediate between Al2O3 and gahnite (ZnAl2O4), with up to 31 wt% ZnO and significant contents of MgO (3 wt%), Fe2O3 (5 wt%), and SiO2 (10 wt%). After 68 h, stable spinels are gahnite close to the end-member composition with MgO and Fe2O3 contents below ca. 5 wt%, and SiO2 contents ca. 1 wt%. These results support the existence of a metastable solid solution between γ-Al2O3 and gahnite. This experimental work shows that Zn can be incorporated in spinel structures after heating at 1175 °C during short durations and Zn is preferentially incorporated in the muscovite pseudomorphs as opposed to the Qtz-Fds glass. Consequently, the thermal breakdown of phyllosilicates can be a viable process to immobilize heavy metals such as Zn.

CCSEM investigation of respirable quartz in air samples collected during power plant maintenance activities

In an earlier investigation, computer-controlled scanning electron microscopy (CCSEM) and X-ray diffraction (XRD) were used to investigate respirable quartz in several bulk coal fly-ash samples. It was shown that the XRD result for quartz in bulk fly-ash was much larger than the amount of respirable quartz determined by CCSEM because the XRD signal was derived primarily from quartz particles too large to be classified as respirable. In the current paper, CCSEM and XRD were used to investigate coal fly-ash collected using cyclone-filtered personal and area samplers at two different power plants during maintenance activities. The amounts of respirable quartz determined by the two methods were found to be in good agreement, confirming the need for cyclone-filtered samples. The amounts of respirable quartz determined by CCSEM ranged from 1.5 to 3.5vol.% for the plant burning a Southwest sub-bituminous coal and from 0.6 to 1.9vol.% for the plant burning a Mid-West bituminous coal. Very small amounts (average∼0.2vol.% for both plants) of Si-rich glass or amorphous silica were also detected by CCSEM. This paper is important because of the health implications associated with inhaling quartz particles. It demonstrates that the amounts of respirable quartz to which power plant workers are exposed can be accurately measured by two very different measurements on samples collected in cyclone-filtered samplers designed to emulate the inhalation process of human airways.

Crystallization, melt inclusion, and redox history of a Martian meteorite: Olivine-phyric shergottite Larkman Nunatak 06319

The Larkman Nunatak (LAR) 06319 olivine-phyric shergottite is composed of zoned megacrysts of olivine (Fo 76–55 from core to rim), pyroxene (from core to rim En 70Fs 25Wo 5, En 50Fs 25Wo 25, and En 45Fs 45Wo 10), and Cr-rich spinel in a matrix of maskelynite (An 52Ab 45), pyroxene (En 30–40Fs 40–55Wo 10–25,), olivine (Fo 50), Fe–Ti oxides, sulfides, phosphates, Si-rich glass, and baddeleyite. LAR 06319 experienced equilibration shock pressures of 30–35 GPa based on the presence of localized shock melts, mechanical deformation of olivine and pyroxene, and complete transformation of plagioclase to maskelynite with no relict birefringence. The various phases and textures of this picritic basalt can be explained by closed system differentiation of a shergottitic melt. Recalculated parent melt compositions obtained from melt inclusions located in the core of the olivine megacrysts (Fo >72) resemble those of other shergottite parent melts and whole-rock compositions, albeit with a lower Ca content. These compositions were used in the MELTS software to reproduce the crystallization sequence. Four types of spinel and two types of ilmenite reflect changes in oxygen fugacity during igneous differentiation. Detailed oxybarometry using olivine-pyroxene-spinel and ilmenite-titanomagnetite assemblages indicates initial crystallization of the megacrysts at 2 log units below the Fayalite-Magnetite-Quartz buffer (FMQ – 2), followed by crystallization of the groundmass over a range of FMQ – 1 to FMQ + 0.3. Variation is nearly continuous throughout the differentiation sequence. LAR 06319 is the first member of the enriched shergottite subgroup whose bulk composition, and that of melt inclusions in its most primitive olivines, approximates that of the parental melt. The study of this picritic basalt indicates that oxidation of more than two log units of FMQ can occur during magmatic fractional crystallization and ascent. Some part of the wide range of oxygen fugacities recorded in shergottites may consequently be due to this process. The relatively reduced conditions at the beginning of the crystallization sequence of LAR 06319 may imply that the enriched shergottite mantle reservoir is slightly more reduced than previously thought. As a result, the total range of Martian mantle oxygen fugacities is probably limited to FMQ − 4 to − 2. This narrow range could have been generated during the slow crystallization of a magma ocean, a process favored to explain the origin of shergottite mantle reservoirs.

Oxidation of Short Carbon Fiber Reinforced ZrB<sub>2</sub>-SiC Ceramics under Atmospheric and Oxyacetylene Torch Conditions

The oxidation behavior of ZrB2-20 vol.% SiC (ZS) and ZrB2-20 vol.% SiC containing 20 vol.% short carbon fiber (ZSC) was studied using thermal gravimetric analysis and oxyacetylene torch test. It was shown that weight gains changed from 3.71 wt.% for ZS to 4.57 wt.% for ZSC after heating 10°C /min to 1450°C in air. A thin layer of Si-rich glass and then a depletion layer of SiC was found on the cross section of both materials and carbon fiber of ZSC exposed in air was oxidized. Under oxyacetylene, an average mass loss of 0.8 wt.% for ZS and 0.9 wt.% for ZSC was measured after 180 seconds. After exposure, an oxidized layer with the formation of ZrO2 and SiO2 was found on the surface of both materials. Meanwhile, fiber in the surface of ZSC appeared oxidized and removed.

The metasomatic record in the shallow peridotite mantle beneath Grenada (Lesser Antilles arc)

The composition and geochemical signatures of the mantle wedge beneath the Lesser Antilles arc are documented by the ultramafic xenoliths included in alkali basalts (M-series) on Grenada. Xenoliths consist of harzburgites, lherzolites, dunites and subordinate wehrlites and pyroxenites. Primary minerals phases are olivine, low-Al and high-Al orthopyroxene, clinopyroxene and Cr-Spinel. In addition to the primary assemblage, Grenada xenoliths contain metasomatic phases such as Al-rich clinopyroxene, plagioclase, Al-rich spinel, pargasitic amphibole and Si- and Al-rich glasses. The trace-element signatures of pyroxenes and glasses have been determined on selected samples by LA-ICP-MS. Pyroxenes from both lherzolite and harzburgite xenoliths have U-shaped rare earth element (REE) profiles, unusually high Th, U and Sr concentrations and large negative Nb, Ta and Zr, and Hf anomalies. The geochemical signatures of metasomatic clinopyroxene are different from those reported for clinopyroxene from fluid-metasomatised mantle wedge, and are clearly distinct from those of clinopyroxene in equilibrium with host lavas. Si-rich glasses show a narrow compositional range, with trace-element characteristics closely similar to those of reacted pyroxenes. This, along with the general lack of chemical gradients of LILE and LREE over more compatible elements suggests dacitic glasses represent the products of in-situ melting caused by temperature increase before and during the uptake of xenoliths by host lavas. Dacitic melts are believed to represent local re-melts of regions metasomatically enriched by earlier arc magmas that had stalled, fractionated, and solidified in the upper mantle. These local re-melts thus reflect the metasomatic component formed by earlier arc-related metasomatic agents and liable to be re-mobilised. This also appears to be the easiest way to explain the compositional similarities between erupted arc lavas and the metasomatised peridotites. The results of this study suggest that the mantle wedge beneath the Lesser Antilles underwent complex peridotite–melt reaction processes operated by sub-arc melts and, later on, by magmas similar in compositions to the host alkali basalts. The majority of the compositional range of erupted Grenada magmas, but adakites found at surface, can be obtained by the interaction of basalts, possibly formed by hydrous melting of MORB-source mantle, with the overlying mantle wedge.

Heating effects of the matrix of experimentally shocked Murchison CM chondrite: Comparison with micrometeorites

Abstract— Micrometeorites have been significantly altered or melted by heating, which has been mainly ascribed to aerodynamic drag during atmospheric entry. However, if a major fraction of micrometeorites are produced by impacts on porous asteroids, they may have experienced shock heating before contact with the Earth's atmosphere (Tomeoka et al. 2003). A transmission electron microscope (TEM) study of the matrix of Murchison CM chondrite experimentally shocked at pressures of 10–49 GPa shows that its mineralogy and texture change dramatically, mainly due to shock heating, with the progressive shock pressures. Tochilinite is completely decomposed to an amorphous material at 10 GPa. Fe‐Mg serpentine is partially decomposed and decreases in amount with increasing pressure from 10 to 30 GPa and is completely decomposed at 36 GPa. At 49 GPa, the matrix is extensively melted and consists mostly of aggregates of equigranular grains of Fe‐rich olivine and less abundant low‐Ca pyroxene embedded in Si‐rich glass. The mineralogy and texture of the shocked samples are similar to those of some types of micrometeorites. In particular, the samples shocked at 10 and 21 GPa are similar to the phyllosilicate (serpentine)‐rich micrometeorites, and the sample shocked at 49 GPa is similar to the olivine‐rich micrometeorites. The shock heating effects also resemble the effects of pulse‐heating experiments on the CI and CM chondrite matrices that were conducted to simulate atmospheric entry heating. We suggest that micrometeorites derived from porous asteroids are likely to go through both shock and atmospheric‐entry heating processes.

Cryptic crustal contamination of MORB primitive melts recorded in olivine-hosted glass and mineral inclusions

The origin of compositional heterogeneities among the magmas parental to mid-ocean ridge basalts (MORB) was investigated using a single rock piece of the olivine-phyric basalt from 43 degrees N, Mid-Atlantic Ridge (AII D11-177). The exceptional feature of this sample is presence of very primitive olivine crystals (90-91 mol% Fo) that are significantly variable in terms of CaO (0.15-0.35 wt%). A population of low-Ca olivine (0.15-0.25 wt% CaO) is also notably distinct from high-Ca olivine population in AII D11-177, and primitive MORB olivine in general, in having unusual assemblage of trapped mineral and glass inclusions. Mineral inclusions are represented by high-magnesian (Mg# 90.7-91.1 mol%) orthopyroxene and Cr-spinel, distinctly enriched in TiO2 (up to 5 wt%, c.f. < 1 wt% in common MORB spinel). Glass inclusions associated with orthopyroxene and high-Ti Cr-spinel have andesitic compositions (53-58 wt% SiO2). Compared to the pillow-rim glass and "normal" MORB inclusions, the Si-rich glass inclusions in low-Ca olivine have strongly reduced Ca and elevated concentrations of Ti, Na, K, P, Cl, and highly incompatible trace elements. Strong variability is recorded among glass inclusions within a single olivine phenocrysts. We argue that the observed compositional anomalies are mineralogically controlled, and thus may arise from the interaction between hot MORB magmas and crystal cumulates in the oceanic crust or magma chamber.

Investigating the origin of anorthitic plagioclase through a combination of experiments and natural observations

The origin of anorthitic plagioclase in volcanic rocks worldwide remains an enigma. Arenal volcano, Costa Rica, provides a good example of the problem: up to An 96 plagioclase are found within moderately evolved basaltic andesites. We follow up a recent suggestion [Lundstrom, C.C., Boudreau, A., Pertermann, M., 2005. Diffusion-reaction in a thermal gradient: Implications for the genesis of anorthitic plagioclase, high alumina basalt and igneous mineral layering. Earth Planet. Sci. Lett. 237, 829–854] that diffusion-reaction processes occurring between ascending magmas and gabbroic wall rocks might provide a means of producing anorthitic plagioclase without requiring exotic, ultra-calcic melt compositions by integrating observations from three analytical methods: (1) laboratory diffusion-reaction experiments in the diopside–albite–anorthite (DAA) system; (2) transmission electron microscopy (TEM) of plagioclase grains from both a diffusion-reaction experiment and an Arenal lava; and (3) Sr isotope microstratigraphy of phenocrysts and megacrysts from natural lavas from the current eruption at Arenal. Two plagioclase saturated DAA melts, juxtaposed in experiments at either 1 bar pressure (anhydrous) or at 0.5 GPa pressure (hydrous) for 3–5 days at 1244 or 1125 °C, illustrate the dramatic effect of H 2O on facilitating plagioclase reaction. In the presence of wet melt, enrichment in anorthitic plagioclase occurs at the material interface while in the presence of the dry 1 bar melt, plagioclase shows no reaction. TEM analyses of a homogeneous anorthitic plagioclase from a diffusion-reaction experiment indicates heterogeneous columns of more anorthite-rich plagioclase at the < 100 nm scale while a homogeneous anorthitic plagioclase phenocryst from a 1968 lava from Arenal found much of the crystal to be composed of 20–50 μm diameter subgrains, having ∼ 1 μm diameter tubes filled with Fe, K, Si-rich glass. Large core–rim variations in 87Sr/ 86Sr occur within three plagioclase phenocrysts from both 1968 and 1999 Arenal lavas. Although core compositions vary widely (0.7038–0.7052), all three phenocrysts have similar 87Sr/ 86Sr rim compositions (0.7043). The variability in 87Sr/ 86Sr at the phenocryst scale but homogeneity in 87Sr/ 86Sr at the bulk scale in Arenal lavas [Ryder, C.H., Gill, J.B., Tepley III, F.J., Ramos, F., Reagan, M., this volume. Closed to open differentiation at Arenal volcano (1968–2003). J. Volcanol. Geotherm. Res.] could be explained by erupted lavas representing a near steady state balance between ascending magmas and a flux derived from diffusion-reaction processes occurring with surrounding crustal gabbros.

The effect of crystallinity on dissolution rates and CO 2 consumption capacity of silicates

Comparison of measured far-from-equilibrium dissolution rates of natural glasses and silicate minerals at 25 °C and pH 4 reveals the systematic effects of crystallinity and elemental composition on these rates. Rates for both minerals and glasses decrease with increasing Si:O ratio, but glass dissolution rates are faster than corresponding mineral rates. The difference between glass and mineral dissolution rates increases with increasing Si:O ratio; ultra-mafic glasses (Si:O ⩽ 0.28) dissolve at similar rates as correspondingly compositioned minerals, but Si-rich glasses such as rhyolite (Si:O ∼ 0.40) dissolve ⩾1.6 orders of magnitude faster than corresponding minerals. This behaviour is interpreted to stem from the effect of Si–O polymerisation on silicate dissolution rates. The rate controlling step of dissolution for silicate minerals and glasses for which Si:O > 0.28 is the breaking of Si–O bonds. Owing to rapid quenching, natural glasses will exhibit less polymerisation and less ordering of Si–O bonds than minerals, making them less resistant to dissolution. Dissolution rates summarized in this study are used to determine the Ca release rates of natural rocks at far-from-equilibrium conditions, which in turn are used to estimate their CO 2 consumption capacity. Results indicate that Ca release rates for glasses are faster than those of corresponding rocks. This difference is, however, significantly less than the corresponding difference between glass and mineral bulk dissolution rates. This is due to the presence of Ca in relatively reactive minerals. In both cases, Ca release rates increase by ∼two orders of magnitude from high to low Si:O ratios (e.g., from granite to gabbro or from rhyolitic to basaltic glass), illustrating the important role of Si-poor silicates in the long-term global CO 2 cycle.

Petrogenesis of lunar meteorite EET 96008

Lunar meteorite EET 96008 is a fragmental breccia that predominantly consists of basaltic mineral clasts (0.5–2 mm), along with minor lithic fragments and breccia clasts. The matrix consists mainly of smaller mineral fragments (<0.5 mm), bound by glassy cement, the majority of which are pyroxene and plagioclase. The pyroxene possesses extensive exsolution lamellae. These lamellae, up to 1 μm in width, are atypical for mare-basalts. One of the distinguishing textures of EET 96008 is the presence of small pockets (∼400 × 500 μm) of mesostasis areas consisting of coarse (∼20 μm) intergrowths of ferroaugite, fayalite and Si-rich glass. Laths of ilmenite, armalcolite, apatite and whitlockite are also distributed in these areas. Ilmenite grains are abundant and dispersed throughout the thin sections. Chromite and ulvöspinel are present but in minor abundance. Troilite, generally rare in this rock, occurs as several grains in one pyroxene crystal. FeNi metal is conspicuously absent from this meteorite. The molar Fe/Mn ratio in olivines and pyroxenes and the age of the meteorite are evidence for a lunar origin. The mineralogy of EET 96008 shows close affinity to a mare-basalt source, albeit with possible minor highland/non-mare components. The bulk-rock, major-, trace- and rare-earth-element (REE) contents are similar to that of very low-titanium (VLT) basalts, which have experienced extreme fractional crystallization to the point of silicate liquid immiscibility. Mineralogical and textural features of this sample suggest that at least some of the breccia components were derived from a slow-cooled magma. The mineralogy and petrology of EET 96008 is strikingly similar to the lunar meteorite EET 87521, and we support the conclusion that EET 96008 and EET 87521 should be paired. Isochron ages of 3530 ± 270 Ma for apatite and 3519 ± 100 Ma for whitlockite of this rock are consistent with derivation from a mare-basalt precursor. These ages are within error of the low-Ti basalts, dated from the Apollo 12 and 15 sites. The whole-rock, platinum-group-element (PGE) contents of EET 96008 overlap with pristine low-Ti mare basalts, suggesting the presence of only a minimal extraterrestrial component.

Accretion, dispersal, and reaccumulation of the Bishunpur (LL3.1) brecciated chondrite: Evidence from troilite–silicate–metal inclusions and chondrule rims

A set of troilite–silicate–metal (TSM) inclusions and chondrule rims in the Bishunpur (LL3.1) chondrite provide information regarding impact brecciation of small bodies in the early solar system. The TSM inclusions and chondrule rims consist of numerous angular to subrounded silicate grains that are individually enclosed by fine networks of troilite. FeNi metal also occurs in the troilite matrix. The silicates include olivine (Fo 55–98), low-Ca pyroxene (En 78–98), and high-Ca pyroxene (En 48–68Wo 11–32). Al- and Si-rich glass coexists with the silicates. Relatively coarse silicate grains are apparently fragments of chondrules typical of petrologic type-3 chondrites. Troilite fills all available cracks and pores in the silicate grains. Some of the TSM inclusions and rims are themselves surrounded by fine-grained silicate-rich rims (FGR). The TSM inclusions and rims texturally resemble the troilite-rich regions in the Smyer H-chondrite breccia. They probably formed by shock-induced mobilization of troilite during an impact event on a primitive asteroidal body. Because silicates in the TSM inclusions and rims have highly unequilibrated compositions, their precursor was presumably type-3 chondritic material like Bishunpur itself. The TSM inclusions and the chondrules with the TSM rims were fragmented and dispersed after the impact-induced compaction, then reaccreted onto the Bishunpur parent body. FGR probably formed around the TSM inclusions and rims, as well as around some chondrules, during the reaccumulation process. Components of most type-2 and 3 chondrites probably experienced similar processing, i.e., dispersal of unconsolidated materials and subsequent reaccumulation.

Frictional melt pulses during a ∼1.1 Ma earthquake along the Alpine Fault, New Zealand

Our understanding of frictional melting that occurs during large earthquake slip events has been hampered by the extremely fine-grain size of frictionally fused rock, and the lack of detailed high-resolution microscopy studies that describe features at the crystal lattice scale. In such a study, we here report the complex nature of melting and crystallization in a symmetrically layered, pseudotachylyte vein from the Alpine Fault, New Zealand. Two melt pulses are recognized, attributed to successive, but rapid, injections of frictionally generated material. The initial injection, preserved at the vein margins, was proximally derived and contains a high concentration of clasts and a Si-rich glass. This was quickly followed by a second generation of a more distally derived, melt-dominated fraction, which was injected into the weak vein center. Whereas fragments of wall-rock biotite are preserved in the marginal zones, neocrystallized microlitic biotite characterizes both margins and center. The vein biotite is different in composition, microstructure and polytypism from the metamorphic biotite of the wall rock. In all melt layers, newly formed biotite shows notable signs of syn-flow crystallization, strain features and erosion at crystal–glass contacts, with breakdown of neocrystallized microlites along both crystal edges and faces. These characteristics imply cyclic pulses of heating, melting and crystallization occurred during a single, large earthquake episode, and probably reflects the stick-slip propagation properties of coseismic faulting. 40Ar/ 39Ar total gas ages from the vein center give a 1.11±0.04 Ma date for cyclic melting which, based on current exhumation rates, occurred at a crustal of depth ∼11 km.

A new Martian meteorite from Morocco: the nakhlite North West Africa 817

North West Africa (NWA 817) is a single stone of 104 g found in the Sahara (Morocco) by meteorite hunters in November 2000. The meteorite is an unbrecciated, medium-grained olivine-bearing clinopyroxenite with a cumulate texture. It consists of zoned euhedral subcalcic augite (Wo 42En 38–22Fs 20–36), olivine spanning a wide range of compositions (from Fa 56 in the core to Fa 86) with rare magmatic inclusions and an intercumulus mesostasis made of Fe-bearing albitic plagioclase, Si-rich glass, Ti-magnetite with unusual skeletal growth morphologies containing ilmenite exsolutions, acicular pyroxene, olivine and cristobalite. Trace minerals are sulfide droplets and Cl-apatite. Mineral modes (in vol%) are augite 69%, olivine 10%, mesostasis 20% and Fe–Ti oxides 1%. Pervasive alteration produced a reddish clay mineral (hydrous ferrous silicate) in both olivine crystals and the mesostasis. The major element composition of NWA 817 is very similar to that of the other nakhlites: high FeO, MgO and CaO concentrations reflect the abundance of cumulus augite and olivine. Key element ratios such as FeO/MnO (=37), Na/Al (=0.40), K/La (=449), Ga/Al (=3.9×10 −4) and oxygen isotopic composition (Δ 17O=+0.37‰) are clear evidence for a Martian origin. The incompatible trace element pattern as in Nakhla displays a strong light rare earth element enrichment relative to chondrite (La n /Yb n =4.89). However, when compared to the other nakhlites, NWA 817 has specific features: (1) a higher modal proportion of mesostasis; (2) quench textures of Ti-magnetite and Fe-rich clinopyroxene; (3) more Mg-rich olivine core compositions whereas the augite core composition is identical for all nakhlites; (4) a stronger Fe enrichment toward crystal rims of these cumulus minerals. The intercumulus minerals (Ti-magnetite with skeletal growth morphology, acicular chains of clinopyroxene and Fe 3+-rich feldspar) indicate rapid crystallization in response to a high degree of undercooling at the end of the sample story.

The effect of melt distribution on the rheology of plagioclase rocks

Synthetic plagioclase aggregates were fabricated by hot isostatic pressing glass powder with An60Ab40 composition at a confining pressure of 300MPa and a temperature of 1223K. The residual glass content (≈15vol.%) was reduced to 1–3vol.% by annealing the specimens at temperatures between 1423 and 1523K at 0.1MPa. High-resolution electron microscopy showed that the residual glass was confined to grain triple junctions. When these samples were annealed at 0.1MPa and above the solidus (1620K), they also contained silica-rich amorphous phases at grain boundaries. Other samples were hot pressed at 1423K; they often have grain boundaries containing thin amorphous films 5–50nm wide. The chemical composition of the films differs from pure labradorite with up to 80wt.% SiO2. The volume fraction of Si-rich glass increases up to ≈2vol.% for samples that contain ≈1wt.% water. Uniaxial creep experiments were performed at temperatures of 1373–1638K and stresses between 2 and 50MPa. The plagioclase aggregates deformed by grain size sensitive creep with a stress exponent n≈1. The activation energy for grain boundary diffusion controlled creep of the partially molten aggregates was 400±20kJmol−1. It remained unchanged with varying glass content and glass topology. The strength of samples containing glass along grain boundaries is 4–10 times less than without amorphous grain boundary films. The transition between diffusion and dislocation creep is shifted towards higher stresses for samples that contain glass films at grain boundaries.

A study of alteration phases on glass-bonded zeolite and sodaiate using the vapor hydration test

AbstractThis paper focuses on the reaction of glass-bonded zeolite and sodalite ceramic waste forms, naturally occurring sodalite, and their respective alteration phases that formed after reaction in a Vapor Hydration Test (VHT). Scanning electron microscope (SEM) analysis of the glass-bonded zeolite and sodalite samples indicate that NaCI is the major alteration phase present, along with KCI, Na-Al-Si, and Na-K-Ca-Al-Si phases. The order of precipitation of salt crystals from ceramic waste form occurred in the sequence NaCl ⇒ KC1 ⇒ CaCl2 and MgCl2. We also found aluminum silicate phases containing Ca, Mg, K-Na-Ca, K-Mg, Na-K-Mg, K-Ca-Na-Mg, as well as CaCO3, SiO2. The ratio of Si/Al in the alteration phases was noted to increase from approximately 1.5 to 5 during the progressive alteration of the samples. This trend indicates a relative increase in the contribution of silicon from the relatively Si-rich glass regions of the samples over time. Phases enriched in Cs, Ba and rare earth elements (RE) were also observed on the altered samples. Alteration phases initially appeared along the micro-fractures during the early stages of alteration. The number of micro-fractures, density of alteration phase cover, and grain size of alteration phases increased with reaction time and temperature.

Mechanisms of orthopyroxene dissolution in silica-undersaturated melts at 1 atmosphere and implications for the origin of silica-rich glass in mantle xenoliths

Experiments dissolving orthopyroxene (En93) in a variety of Si-undersaturated alkaline melts at 1 atmosphere and variable f O2 demonstrate that orthopyroxene dissolves to form olivine, Si-rich melt and clinopyroxene. These phases form a texturally and chemically distinct boundary layer around the partly dissolved orthopyroxene crystals. The occurrence of clinopyroxene in the boundary layer is due to inward diffusion of Ca from the solvent melt to the boundary layer causing clinopyroxene saturation. Compositional profiles through the solvent and the boundary layer for a number of experiments demonstrate rapid diffusion of cations across the boundary layer – solvent interface. SiO2 diffuses outward from the boundary layer whereas CaO and Al2O3 diffuse toward the Si-enriched boundary layer melt. The rate of Al diffusion is slower under reducing conditions compared to the rates in experiments performed in air. Concentrations of FeO and MgO in the boundary layer and solvent are approximately equal indicating rapid diffusion and attainment of equilibrium despite ongoing crystallisation of clinopyroxene within the boundary layer. The behaviour of Na2O and K2O is strongly affected by f O2. Under reducing conditions Na2O and K2O concentrations are approximately equal in the boundary layer and solvent indicating normal diffusion down the concentration gradient and attainment of equilibrium. Under oxidising conditions, K2O and to a lesser extent Na2O, have compositional profiles indicative of uphill diffusion likely due to their preference for more polymerised Si- and Al-rich melts. Under reduced conditions Al-enrichment in the boundary layer melt is not as extreme and uphill diffusion did not occur. The composition of the solvent melt after the experiments indicates that it was contaminated by the boundary layer by convective mixing due to the onset of hydrodynamic instabilities brought on by density and viscosity contrasts between the two melts. Despite using a wide variety of solvent melt compositions we find that the boundary layer melts converge toward a common composition at high SiO2 contents. The composition of glass generated by orthopyroxene dissolution at 1 atmosphere is similar in many respects to Si-rich glass found in many orthopyroxene-rich mantle xenoliths that have been attributed to high pressure in situ processes including mantle metasomatism. The results of this study suggest that at least some Si-rich melts are likely to have formed by dissolution of xenolith orthopyroxene at low pressure possibly by their Si-undersaturated host magmas.

Carbon Content and Isotopic Composition of K/T Impact Glasses from Haiti

A microscope study of dark brown impact-produced glasses from the K/T boundary beds of Beloc (Haiti) has revealed the presence of bubbles (50–300 μm in size). A simple experiment to release any enclosed gases proved positive in some cases and demonstrated pressure in the bubbles equal to or greater than atmospheric. Stepped combustion analyses combined with static mass spectrometry was used to verify that the gas bubbles contained CO 2; the content and stable carbon isotopic composition of this gas was determined along with background measurements of the glass itself. Results indicated that the brown Si-rich glasses were heterogeneous in carbon content ranging from 0.007–2.62 wt% carbon, but that sharp releases of gas were afforded at relatively low temperatures with a carbon isotopic composition up to −0.8‰, similar to that of marine carbonate. The glasses had been cleaned and acid-etched to remove contamination, ensuring that any carbon released was indigenous to the glass. The Ca-rich yellow glasses were analysed in the same way and released 0.265–0.343 wt% carbon with a δ 13C of −29‰, which is quite different from the Si-rich glasses. The proposed source crater for these impact glasses is the Chicxulub crater, which has a pre-impact stratigraphy containing carbonate. Vaporisation of the carbonate during the impact would have released large quantities of CO 2 into the atmosphere, which could have been incorporated into the glasses as bubbles. Alternatively, the bubbles could have been formed by degassing of the glasses themselves during cooling. This would account for the marine carbonate carbon isotopic signature identified during stepped heating of the high-Si glass. We believe this provides a further link between the impact glasses of Haiti and the Chicxulub impact crater although our results for the high Ca glasses proved inconclusive.

CORRELATION OF GRAIN BOUNDARY CHARACTERISTICS WITH ELECTRICAL PROPERTIES IN ZNO-GLASS VARISTORS

Grain boundary glass crystallinity was studied to determine its effect on the electrical properties of ZnO–glass varistors. Si-rich glass at the grain boundary layer transformed into a crystalline zinc silicate phase after heat treatment. However, glass without any SiO2 was not recrystallized and remained amorphous following the same process. Variations in non-ohmic behaviour of the samples with three different glass additives are attributed to their different crystallinity. According to a proposed defect reaction equation, strong pinning of the barrier height was found in ZnO–glass varistors with crystallized glass phase. This would ultimately cause an increase in the α values, grain boundary barrier height, breakdown voltage per grain and device stability. If charged ions such as Vo⋅, are present due to the formation of the crystalline intergranular phase, the grain boundary barrier height and device stability would initially be enhanced by increased density of interface states, later decreasing due to the migration of zinc interstitials Zni⋅⋅ under electrical stress. Furthermore, results of deep-level transient spectroscopy, together with dielectric loss and Auger linescan analyses detected three electron traps: L2 is associated with the energy level of the second ionization of interstitial zinc atoms; and the broadness of the trap L3 is analogous to the energy level of the first ionization of oxygen vacancies; trap L1 has been identified as native defects.

Xenocrystic richterite in an olivine-nephelinite: destabilisation and diffusion phenomena

AbstractA partly destabilised Na-richterite has been found in an olivine-nephelinite from Morocco. The riehterite crystal (600 × 420 μm) is surrounded by a reaction zone (400-700 μm) of K- and Si-rich glass containing small (&lt;50 μm) olivine (Fo80-83%) and endiopside crystals. Outwards, another zone is formed of normal magmatic minerals and circumscribes the original crystal, indicating that the destabilisation event took place at the end of the crystallisation sequence. Estimated ascent time of about 100 hours would have completely decomposed an isolated richterite crystal, which suggests that the amphibole was originally included in a xenolith. A mass-balance calculation shows that the fichterite isovolumic decomposition was accompanied by exchanges with the magma. The loss of Na from the reaction zone and the gain of AI from the magma allowed the precipitation of an analcime-rich zone observed around the destabilised amphibole and the concentration of K in the reaction zone glass. Compositional variations, Fe and Ti increase and Mg, Ca and F decrease at the richterite edge are interpreted as the result of a diffusion process. No alkali gradients are observed. The diffusion phenomenon lasted less than 100 hours and ceased to be operative at a temperature of 900-950°C i.e. just below the solidus temperature. Diffusion coefficients for the amphibole are proposed: e.g 10−9 cm2 s−1 for K2O and 10−10 for FeO at 900°C

Leaching behaviour of selected trace elements in chemically weathered alkaline fly ash

In a laboratory study alkaline fly ash was leached in a series of lysimeters with dilute H 2SO 4. The weathered residues retrieved after leaching were analyzed for major constituents by atomic absorption spectrophotometry and trace elements by instrumental neutron activation analysis. The characteristics of the weathered residues ranged from highly leached acidified material, from which many constituent elements had been mobilized, to minimally leached alkaline material containing accumulations of newly formed secondary minerals. The leaching behavior of constituent trace elements was related to the chemical environment of the leachates and partitioning among two previously identified major phases within parent ash particles and with newly formed secondary minerals. Elements such as Rb, Cs, Pb, Ta, Ti, and Hf were enriched in the highly leached portion of the residue sequence, suggesting association with the resistant internal Si-rich glass matrix of ash particles. Between 50 and 80% of the total Mn, Sb, Th, Cr, Zn, Co, Sc, and rare earth elements was also retained in the highly leached ash residues. About 50% of the total Sr, V, and U, and more than 80% of the total As and B was dissolved from the ash under acidic conditions. With the exception of B, all elements that were mobilized from the acidified ash residues were also attenuated in the alkaline residues in association with one or more of the newly formed secondary minerals.