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Vitreous materials are quite routinely found in natural settings. Most of them are aluminosilicates, which often occur in large deposits. Considering the geological formations in which naturally occurring vitreous aluminosilicates are found, they have generally remained stable for more than 1 Ma on the earth's surface, even in different geological and climatic environments. These non-crystalline solids played a very important role in the development of ancient human civilizations, long before the introduction of metallic tools. Today, however, the properties of natural glasses are of interest to mankind for completely different reasons. For example, industrial glasses are used today for encapsulating toxic wastes, especially radioactive waste, which remains active for centuries or more, in order to prevent the unwanted transfer of harmful materials to the environment. The chemical compositions of industrially produced glasses are in large part different from the compositions of natural glasses. Little is quantitatively known about the stability of industrial glasses over very long periods of time (>10,000 years). However, the physical and chemical stability of natural aluminosilicate glasses is known to extend over very long periods of time.The advancement of technological design to prevent or at least minimize the melt down of toxic waste during the encapsulation process is currently a major challenge, using glasses of natural chemical composition. Brecciated glass, which is found frequently in natural settings, provides a special clue to the possibility of producing vitreous solids by sintering glass fragments without melting the cullets. It is essential to prevent melting of the cullets because the melt has the potential of chemically reacting with the toxic waste.This paper summarizes the geological, chemical, and physical facts concerning naturally produced glasses, and seeks to establish a recognized database for further research in the domain of understanding the glass-forming processes that occur in nature. Furthermore, the authors hope to stimulate research into the utilization of natural resources that to solve the problem of storing of toxic waste safely.Major and trace element data have been collected over the past 100 years. These data constitute a sufficient basis for the chemical characterization of natural glasses. More information about the major elements is not required, in order to understand the chemical properties of these materials. On the other hand, large gaps in compositional data exist where other related components are concerned: e.g., in the case of “water-species”, with its different forms of bonding in silicates or oxygen (oxygen fugacity), CO2-, sulphur - or hydrocarbons (methane)-, hydrogen-, chlorine-and fluorine-species. All these components have a significant impact on the properties of glasses, even when present only in minor quantities. Glass textures and crystal morphologies reflect the processes of nucleation and crystal growth in a glass-forming matrix during the cooling and reheating cycles which are currently not thoroughly understood. In nature, the processes that led to the formation of vitreous materials are very different from those used in the production of industrial glasses. The different genetic conditions under which glass formation occurs permit differentiation between magmatic and metamorphic vitreous solids. Sedimentary and biogenetic processes also contribute to the formation of non-crystalline solids. 相似文献
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Jim Watkins Michael Manga Christian Huber Michael Martin 《Contributions to Mineralogy and Petrology》2009,157(2):163-172
Spherulites are spherical clusters of radiating crystals that occur naturally in rhyolitic obsidian. The growth of spherulites
requires diffusion and uptake of crystal forming components from the host rhyolite melt or glass, and rejection of non-crystal
forming components from the crystallizing region. Water concentration profiles measured by synchrotron-source Fourier transform
spectroscopy reveal that water is expelled into the surrounding matrix during spherulite growth, and that it diffuses outward
ahead of the advancing crystalline front. We compare these profiles to models of water diffusion in rhyolite to estimate timescales
for spherulite growth. Using a diffusion-controlled growth law, we find that spherulites can grow on the order of days to
months at temperatures above the glass transition. The diffusion-controlled growth law also accounts for spherulite size distribution,
spherulite growth below the glass transition, and why spherulitic glasses are not completely devitrified.
An erratum to this article can be found at 相似文献
3.
Subaqueous effusive to low-explosive eruptive product composed of blocky and fluidal fragments of glass and sparsely crystalline glass of obsidian lava are present in fine-grained volcaniclastic debris flow conglomerate and turbidite, occurring in a deep marine sequence of thin- and even-bedded grey siliceous micritic limestone of the Chanda Limestone. The siliceous limestone conformably encloses stratiform manganese deposit. Obsidian glass fragments consist of coherent and amorphous glassy groundmass containing patches of devitrified fibrous glass, oriented microlites and microphenocrysts of quartz and K-feldspars. Euhedral crystals, resorbed-rounded and embayed crystals, spherical vesicles, microlites, and flow texture are distinctive volcanic features observed in these fragments chemically similar to obsidian lava.The blocky fragments represent the products of hydroclastic fragmentation whereas the fludial one may indicate formation under steam explosion. Gravitational instability of the resulting slurry of glass fragments and the ambient sea water-sediments at the site of eruption causes gravity flows that carries the fragments along with epiclasts away from vent to a distal site for final deposition. This study records maiden evidence of volcanism in the Chanda Limestone of the Neoproterozoic Penganga Group and infers rifting and partial melting of the sialic crust and silicic lava flow during deposition of limestone unusually rich in silica. The inferred tectono-sedimentary setting, facies association and mode of occurrence of the manganese deposit suggest a hydrothermal source for the metal. 相似文献
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K.?ShimodaEmail author M.?Okuno Y.?Syono M.?Kikuchi K.?Fukuoka M.?Koyano S.?Katayama 《Physics and Chemistry of Minerals》2004,31(8):532-542
Shock-recovery experiments for obsidian and its fused glass have been carried out with pressure up to 35 GPa. Structural evolution accompanying the shock compression was investigated using X-ray diffraction technique, Raman and infrared spectroscopy. The densities of obsidian and its fused glass increased with applied shock pressure up to 25 GPa. Densification reached a maximum of 4.7 and 3.6% for obsidian and its fused glass, respectively. The densification mechanism is attributed to reduction of the T–O–T angle, and changes in ring statistics in the structure. Density reduction observed at greater than 25 GPa of applied shock pressure is due to partial annealing of the high-density glass structures brought by high post-shock residual temperature. The density of fused glass is almost equal to its original value at 35 GPa while the shocked obsidian has a slightly lower value than its original value. Amorphization of crystallites present in the obsidian due to shock compression is probably the cause of the density decrease. The structural evolution observed in shock-compressed obsidian and its fused glass can be explained by densification resulting from average T–O–T angle reduction and increase of small rings, and subsequent structural relaxation by high post-shock temperature at applied shock compression above 25 GPa. 相似文献
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It is proposed that fault textures in two dissected rhyolitic conduits in Iceland preserve evidence for shallow seismogenic faulting within rising magma during the emplacement of highly viscous lava flows. Detailed field and petrographic analysis of such textures may shed light on the origin of long-period and hybrid volcanic earthquakes at active volcanoes. There is evidence at each conduit investigated for multiple seismogenic cycles, each of which involved four distinct evolutionary phases. In phase 1, shear fracture of unrelaxed magma was triggered by shear stress accumulation during viscous flow, forming the angular fracture networks that initiated faulting cycles. Transient pressure gradients were generated as the fractures opened, which led to fluidisation and clastic deposition of fine-grained particles that were derived from the fracture walls by abrasion. Fracture networks then progressively coalesced and rotated during subsequent slip (phase 2), developing into cataclasite zones with evidence for multiple localised slip events, fluidisation and grain size reduction. Phase 2 textures closely resemble those formed on seismogenic tectonic faults characterised by friction-controlled stick-slip behaviour. Increasing cohesion of cataclasites then led to aseismic, distributed ductile deformation (phase 3) and generated deformed cataclasite zones, which are enriched in metallic oxide microlites and resemble glassy pseudotachylite. Continued annealing and deformation eventually erased all structures in the cataclasite and formed microlite-rich flow bands in obsidian (phase 4). Overall, the mixed brittle–ductile textures formed in the magma appear similar to those formed in lower crustal rocks close to the brittle–ductile transition, with the rheological response mediated by strain-rate variations and frictional heating. Fault processes in highly viscous magma are compared with those elsewhere in the crust, and this comparison is used to appraise existing models of volcano seismic activity. Based on the textures observed, it is suggested that patterns of long-period and hybrid earthquakes at silicic lava domes reflect friction-controlled stick-slip movement and eventual healing of fault zones in magma, which are an accelerated and smaller-scale analogue of tectonic faults.Editorial responsibility: J. Stix 相似文献
7.
The pyroclastic deposits of the 1300 B.P. eruption of Newberry Volcano, OR, USA, contain minor amounts of obsidian (1–6 wt.%).
The volatile (H2O and CO2) contents and textures of these clasts vary considerably. FTIR measurements of H2O in obsidian pyroclasts range from 0.1 to 1.5 wt.% indicating equilibration pressures ≤20 MPa. CO2 contents are low (<10 ppm) except in clasts that also contain xenolith powder that provided a local CO2 source. Obsidian clasts exhibit a range of color and textural types that changed in relative proportion as the eruption progressed.
Together these data indicate that there were multiple origins of obsidian and that the dominant source changed during the
eruption. Early in the eruption, obsidian was almost entirely black or grey (microlite-bearing) and probably derived from
dikes or wall rock fractures filled with vanguard magma or tuffisite that, together with wall rocks, were eroded and incorporated
into the eruption column as the vent widened. Later in the eruption, following a brief cessation of activity, the proportion
of obsidian to wallrock lithic clasts increased and new types of obsidian dominated, types that represent remnants of a shallow
conduit plug, welded fallback material from within the conduit, and sheared and degassed magma from near the conduit walls.
Analysis of bubble shapes preserved within obsidian indicates that shear stresses and shear rates varied by over two orders
of magnitude, with maxima of 88 kPa and 10−2.3 s−1, respectively, based on an assumed magma temperature of 850°C. Furthermore, the highest shear rates and stresses, and the
shortest flow times (10–20 min), are preserved in clasts that also contain wall rock. The longest deformation times (5 and
8 h) correspond to two microlite-rich clasts, suggesting that the higher microlite content results from slower ascent rates
and/or longer magma residence times at shallow levels. Differences between obsidian pyroclasts from the Newberry eruption
and those of the Mono Craters may relate to the nature of the conduit feeding the two events. From this comparison, we conclude
that obsidian can provide information on time scales and mechanisms of pre-fragmentation magma ascent. 相似文献
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H. Tuffen D. W. McGarvie H. Pinkerton J. S. Gilbert R. A. Brooker 《Bulletin of Volcanology》2008,70(7):841-860
This paper describes unusual rhyolitic deposits at Dalakvísl, Torfajökull, Iceland that were emplaced during a Quaternary subglacial eruption. Despite its small volume (<0.2 km3), the eruption mechanisms were highly variable and involved both explosive and intrusive phases. The explosive phase involved vesiculation-driven magma fragmentation at the glacier base and generated a pumiceous pyroclastic deposit containing deformed sheets of dense obsidian. Textures suggest that the obsidian was generated by the collapse of partly fragmented foam that was intruding the deposit and water contents indicate quenching at elevated pressures. In contrast, the intrusive phase of the eruption generated vesicle-poor quench hyaloclastites associated with a variety of peperitic lava bodies. The presence of juvenile-rich fluvio-lacustrine sediments is the first documented evidence that meltwater may pond close to the vent during subglacial rhyolite eruptions if the bedrock topography is favourable. In order to explain the variable eruption mechanisms, a conceptual model is presented in which the transition from an explosive to an intrusive eruption was controlled by the space available for fragmentation within the subglacial cavity melted above the vent. When the cavity became completely filled by volcanic deposits, the vent became blocked and rising magma was forced to intrude through poorly consolidated debris. This led to arrested fragmentation and welding of foam domains to form vesicle-poor obsidian lava; the transition to an intrusive eruption has taken place. Although this vent-blocking mechanism is particularly relevant to subglacial eruptions, it may also apply to subaerial rhyolitic eruptions, where patterns of explosive and effusive activity cannot be explained by shallow degassing processes alone. Meanwhile, the variable style of a small-volume subglacial rhyolite eruption further highlights the complex processes that mediate volcano-ice interactions. 相似文献
9.
Color-change processes of a plinian pumice and experimental constraints of color-change kinetics in air of an obsidian 总被引：1，自引：0，他引：1
Mihoko Moriizumi Satoru Nakashima Satoshi Okumura Yuta Yamanoi 《Bulletin of Volcanology》2009,71(1):1-13
Colors of plinian pumices were measured by spectrocolorimetry, and their quantitative color parameters in the L*a*b* color
space were determined. A series of heating experiments of obsidian was conducted to simulate the color-change processes of
rhyolitic glasses. In these experiments, following three stages of color-change processes were observed. Stage I showed a
rapid b* (yellowishness) increase associated with fast dehydration controlled by water diffusivity (D
water). In stage II, a* (reddishness) increase was accompanied by Fe2+ decrease. Both a* increase and Fe2+ decrease can be simulated by a diffusion model. Obtained diffusivity D
oxidation were about two orders of magnitude smaller than D
water
. The a*-value increase after the oxidation in stage III appeared to be quasi-linear with time, indicating the zeroth order
reaction corresponding to the formation of hematite-like structures in rhyolitic glasses. The diffusion-limited a* increase
model in stage II was applied to a natural plinian pumice fall unit to evaluate time periods of color-change processes through
oxidation by air of fragmented rhyolitic materials. 相似文献
10.
Franois-Xavier Le Bourdonnec Grard Poupeau Carlo Lugli 《Comptes Rendus Geoscience》2006,338(16):1150-1157
The contents of Na, Al, Si, K, Ca and Fe of 99 obsidians from the western Mediterranean islands of Lipari, Palmarola, Pantelleria and Sardinia were determined with the energy-dispersive spectrometer of a scanning electron microscope (SEM–EDS). The Na and in a lesser way other elements contents characterize any obsidian source-island. In Sardinia (80 samples), the four Monte Arci obsidian geochemical types can be discriminated from binary diagrams of element contents or by a discriminant analysis based on the six elements measured. It is concluded that SEM–EDS offers a new option for Neolithic obsidian provenance studies in this region. To cite this article: F.-X. Le Bourdonnec et al., C. R. Geoscience 338 (2006). 相似文献
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