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1.
Enthalpies of solution in molten 2PbO·B2O3 at ～988 K have been measured for diaplectic labradorite glass from the Manicouagan impact crater and a fused glass formed from the same material. The enthalpies of solution of the diaplectic and fusion-formed glasses are 4,347 and 2,023 cal mol?1, respectively. The more endothermic enthalpy of solution of the diaplectic glass indicates a greater relative energetic stability of about 2.3 kcal mol?1. The data are consistent with Diemann and Arndt's (1984) structural model that suggests the diaplectic glass is more ordered than fusion-formed glass and with the presence of crystallites. Comparison of data to enthalpies of solution of crystalline labradorite (Carpenter et al. 1985) indicates a maximum percentage of crystalline relics of ～15–18%, also consistent with Diemann and Arndt's (1984) estimate of <17%. Thus the diaplectic glass is intermediate in thermochemical properties between normal glass and crystal (much closer to glass) and does not represent any state more unstable than normal fusion-formed glass.  相似文献   

2.
Raman microprobe spectra were made on three post shock, diaplectic plagioclase feldspars. Optical and X-ray diffraction studies indicated that feldspars maintained a partially or totally crystalline state after having passed through the mixed phase zone of Hugoniot response to shock waves (15–38 GPa). The appearance of uniquely glass-type spectra occurs at different shock pressures for each specimen according to its atomic structural arrangement, below 38 GPa for mosaic structured labradorite, near 40 GPa for anorthite and above 50 GPa for the highly ordered low albite. The diaplectic anorthite and labradorite glasses give spectra which indicate the presence of two glass types. Shifts in the band envelope frequencies compared to spectra of fused glass and statically pressure densified glass suggest that these glasses have specific structural arrangements. These differences suggest that the shock and fusion glass-forming processes are not exactly identical. The results from material shocked in the mixed phase region of Hugoniot response show that the phase transitions are effected at different pressures depending upon the feldspar structural type.  相似文献   

3.
Physical properties and the crystallization behavior of natural diaplectic labradorite glass of the shocked anorthosite from the Manicouagan impact crater have been studied. Glasses prepared by laboratory fusion of this anorthosite and a synthetic An55 plagioclase composition were used for comparison. The close similarities in the mid-and far-infrared spectra of the diaplectic and fused glasses indicate a comparable degree of short-range order and lack of long-range order in their structures. They also show an identical viscosity-temperature relation, reflecting a similar and probably high degree of coherence of the (Si,Al)O4 tetrahedra in the network. However, striking differences exist in the crystallization characteristics. Diffusion-controlled crystallization takes place in the fused glass between about 900 and 1,400° C and proceeds generally by the advance of dendritic crystal layers from the surface into the interior of the samples. By contrast, diffusion plays a minor, if any, role in the crystallization of the diaplectic glass, which, on annealing between 800 and 1,000° C reverts to the original plagioclase structure and the primary mineral grains are restored. From the present experimental results it is suggested that high shock-induced temperatures cause onset of the melting process in the compressed crystalline labradorite. However, due to the extremely short duration of the transient high-temperature excursions, the crystal-melt transition does not come to completion. Instead, a disordered transitional state of the compressed material is frozen-in which is recovered after pressure release as diaplectic glass. Its structure thus represents a frozen-in disordered state intermediate between the structures of the crystalline labradorite and its melt. It appears that the diaplectic glass structure is rather inhomogeneous, thereby reflecting the heterogeneous deformational and thermal conditions associated with shock compression.  相似文献   

4.
The textural relationships and structural states of optically isotropic labradorite from the Manicouagan, Quebec, impact structure have been examined by light (optical) and transmission electron (TEM) microscopy. Two distinct diaplectic glasses have been recognized based on their contrasting morphology, timing and the inferred modes of formation. The earliest isotropic bands and grain-scale isotropism (maskelynite) optically exhibit a gradational,in situ transformation from crystalline plagioclase with preservation of relict textures (twins, grain boundaries). The same transformation from crystalline to amorphous structure is observed in TEM to occur heterogeneously at scales on the order of the unit cell. The progressive transformation of optical properties reflects an increase in the volume fraction and eventual coalescence of these amorphous units. This maskelynite-type diaplectic glass is interpreted to form in the solid-state directly from crystalline material during the compressional phase of the shock wave. The other isotropic material occurs in spatially discrete tensiongashes and planar deformation features (PDFs) that overprint the maskelynite-type glass. This second type of diaplectic glass (PDF-type) is developed homogeneously within a given glass band and exhibits sharp crystal-glass boundaries, in contrast to the gradational boundaries of the maskelynite-type glass. PDF-type glass is interpreted to form by melting in tensional release zones during passage of the rarefaction wave. These observations emphasize the ability of naturally shocked rocks to preserve subtle evidence of variations in the shock process from highly transient events.  相似文献   

5.
29Si MAS NMR experiments have been carried out to determine the silica species distribution (Q distribution) in albite, NaAlSi3O8, and anorthite, CaAl2Si2O8, composition glasses (designated albite and anorthite glass). Our results indicate that the Q distribution of albite glass contains all five possible silica species and shows a tendency towards high Q3 and Q4 concentrations, whereas anorthite glass does not contain Q4 and has a high Q0 concentration. Rationalizations are made in terms of the observed Q distributions to explain differences in devitrification behavior of these two glasses. 27Al MAS NMR data for these glasses suggest that differences in devitrification behavior between these two glasses should be ascribed to small growth rates rather than small nucleation rates of crystalline albite from albite glass.  相似文献   

6.
Raman vibrational spectra and X-ray diffractometer scans were obtained from experimentally shocked samples of oligoclase (An19) and andesine (An49). Some 11 oligoclase and 15 andesine targets were shocked between 24 and 40 GPa to address the transition from crystalline to diaplectic states and to explore differences in the structural state of diaplectic feldspar glasses (maskelynite) as a function of peak shock stress. Thy symmetrical VS (T-O-T) (T=Si or Al) stretch bands are the most persistent. They disappear, however, in the noise of an unusually strong luminescent spectrum at > 32 GPa in the oligoclase and at > 30 GPa in the andesine; i.e., at pressures where transition to diaplectic glass is complete. The Raman investigations yield a maskelynite structure that is probably one of a multitude of very small domains with some order, but with a large range of local properties on the scale of small domains, either in heterogeneous size-distribution of domains or in their detailed order, if not both. This results in a very large number of Raman photon-phonon frequencies unlike glasses derived from quenched melts. Our study corroborates conclusions by others, that diaplectic glasses may be the quench products of very dense, disordered phases that exist during shock compression and that subsequently relax to these unusually dense glasses that are only known from shock processes. An origin by relaxation of highly ordered, genuine high pressure polymorphs possessing the structure of hollandite is unlikely, as no evidence for any six-fold Si-coordination was found. Detailed luminescent emission spectra were taken of the oligoclase samples and they show disappearance of the IR band and a strengthening of the green band (the blue band could not be detected with a primary radiation of wavelength 448 nm). This supports previous views that the disappearance of IR emission is most likely caused by shock-induced changes of the crystal field near Fe3+ sites, rather than due to quenching by Fe2+. The X-ray studies were primarily intended to explore whether differences in structural states of maskelynite occur on sufficiently large scales to be detected by standard diffractometry methods. This is not the case. X-ray diffractometer patterns are grossly similar, if not identical, in samples shocked between 30 and 40 GPa and may not be used to fine-tune the shock histories of naturally produced diaplectic glasses.  相似文献   

7.
Samples of single crystal calcic plagioclase (labradorite, An63, from Chihuahua, Mexico) have been shock-loaded to pressures up to 496 kbar. Optical and electron microscopic studies of the recovered samples show the effects of increasing shock pressures on this mineral. At pressures up to 287 kbar, the recovered specimens are still essentially crystalline, with only a trace amount of optically unresolvable glass present at 287 kbar. Samples recovered after shock-loading to pressures between 300 and 400 kbar are almost 100% diaplectic glasses; that is formed by shock transformation presumably in the solid-state. Above about 400 kbar, glasses with refractive indices similar to thermally fused glass were produced. The general behavior of the index of refraction with shock pressures agrees closely with previous work, however, the absence of planar features is striking. At pressures less than 300 kbar, the most prominent physical feature is the pervasive irregular fracturing caused by the shock crushing, although some (001) and (010) cleavages are observed. No fine-scale shock deformation structures, i.e. planar features, were noted in any of the specimens. We conclude, in contrast to previous studies of shocked rocks that planar features are not necessarily definitive shock indicators, in contrast to diaplectic glass (e.g., maskelynite) and high-pressure phases, but are rather likely indicative of the local heterogeneous dynamic stress experienced by plagioclase grains within shocked rocks.  相似文献   

8.
The short range distribution of interatomic distances in three feldspar glasses has been determined by X-ray radial distribution analysis. The resulting radial distribution functions (RDF's) are interpreted by comparison with RDF's calculated for various quasi-crystalline models of the glass structure.The experimental RDF's of the alkali feldspar glasses were found to be inconsistent with the four-membered rings of tetrahedra associated with crystalline feldspars; the structures of these glasses are probably based on interconnected six-membered rings of the type found in tridymite, nepheline, or kalsilite. In contrast, the RDF of calcic feldspar glass is consistent with a four-membered ring structure of the type found in crystalline anorthite. T-O bond lengths (T = Si,Al) increase from 1.60 Å in SiO2 glass [J. H. Konnert and J. Karle (1973) Acta Cryst.A29, 702–710] to 1.63 Å in the alkali feldspar glasses to 1.66 Å in the calcic feldspar glass due to the substitution of Al for Si in the tetrahedra] sites. The T-O-T bond angles inferred from the RDF peak positions are 151° in SiO2 glass (see reference above), 146° in the alkali feldspar glasses, and 143° in the calcic feldspar glass. Detail in the RDF at distances greater than 5 Å suggests that the alkali feldspar glasses have a higher degree of long range order than the calcic feldspar glasses.Assuming that the structural details of our feldspar glasses are similar to those of the melts, the observed structural differences between the alkali feldspar and calcic feldspar glasses helps explain the differences in crystallization kinetics of anhydrous feldspar composition melts. Structural interpretations of some thermodynamic and rheologic phenomena associated with feldspar melts are also presented based on these results.  相似文献   

9.
The glass transition temperatures and the thermal expansions both below and above the glass transition temperature region of synthetic glasses of compositions close to those of plagioclases have been determined. The linear thermal expansion coefficient of the rigid glasses decreases on average from 7.4×10–6/dgC for albite glass to 4.9×10–6/dgC for glass close to anorthite composition. The glass transition temperature of the glasses initially decreases from 763° C for albite glass to 752°C for An9.7 glass and then increases nearly linearly with further increasing anorthite content to 813° C for glass close to anorthite composition.—Measurements made for comparison on a glass prepared from Madagascar orthoclase yielded a linear thermal expansion coefficient of 6.1×10–6/dgC and a glass transition temperature of 905° C.The variations in thermal expansion and glass transition temperature of the feldspar glasses with composition are discussed in terms of structural changes which are assumed to be associated with cation replacement.  相似文献   

10.
The accuracy and precision of absolute C concentrations measured by beta track autoradiography (BTA) have been evaluated by synthesizing 14CO2-bearing glasses (0.5–2.0 wt.% CO2) of albite and diopside composition and comparing the amount of CO2 introduced into the glass to the amount measured by BTA. Measured CO2 contents in diopside glass are less than loaded CO2 contents, but agree within experimental error; measured contents in albite glass are greater than loaded contents by 10–50%, which exceeds experimental error in many cases. Carbon isotope fractionation between calcite and CO2 vapor is not significant above 1000°C at 0.1 or 3.0 GPa and cannot explain the discrepancies. The discrepancies represent an error in BTA which has not been previously recognized and is apparently related to beta absorption.Based on this study, CO2 concentrations in albite measured by BTA should be corrected downward by 40%, which would bring such solubilities into better agreement with those determined by infrared spectroscopy. CO2 solubilities in diopside melts measured by BTA are in agreement with phase equilibria and are accurate to within 20%. Partition coefficients measured by BTA may be subject to analytical errors inherent to autoradiography. Lack of awareness of these potential errors casts doubt on the accuracy of these measurements in many cases.  相似文献   

11.
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12.
This paper reports experimental data on the investigation of the chemical composition of condensed silicate matter produced during the high-temperature pulse vaporization of feldspars. The experiments simulated the conditions of vaporization accompanying a high-velocity impact. Samples of albite, bytownite, calcic and sodic labradorite, and sanidine were used in the experiments. The investigation of the condensate layers obtained in the experiments included the determination of element distribution and structural characteristics of the materials using layer-by-layer X-ray photoelectron spectroscopy. It was shown that the vaporization of the samples occurred mainly through the release of complex atom–molecule groups referred to as clusters. “Nepheline,” “wollastonite,” and “sillimanite” clusters were identified as characteristic groups. The thermodynamic evaluation of melt composition at temperatures up to 5000 K performed using the Magma program confirmed high activities of these components in feldspar melts.  相似文献   

13.
The structures of jadeite and albite composition glasses quenched from 10 and 15 kbar, respectively, have been studied using radial distribution analysis. It is found that aluminum does not undergo a coordination increase at these pressures, in agreement with recent Raman and NMR studies of these glass compositions. Radial distribution functions (RDF's) indicate that the structural response of albite and Jadeite composition melts to pressure up to 10–15 kbar has to do primarily with a distortion of the tetrahedral sites, a slight collapse of the tetrahedral rings forming the framework, and a small loss of short range order resulting in a more densely packed framework structure. The RDF's do not provide any clues concerning T-O-T angular changes at these pressures. It is suggested that the observed structural changes should be closely related to those that take place in melts of rhyolitic composition. Such changes in rhyolitic melts would result in a density increase and viscosity decrease with increasing pressure similar to that observed for melts of albite and jadeite composition.  相似文献   

14.
Two kinds of glasses are to be found in the breccias of the Ries basin both which have been generated by shock wave action on the rocks of the crystalline basement: a) Normal glasses, containing flow structures and vesicles; they are formed by shock waves of high energy which after unloading, leave behind material, the temperature of which is above the melting point. b) Diaplectic glasses without vesicles and flow structures, preserving the grain boundaries, cleavages and twin lamellae of the primary minerals; they are formed by shock waves of lower energy which destroy the crystal lattice but after unloading, leave behind material, the temperature of which is below the melting point. — Measurements have been taken of the density and index of refraction of diaplectic quartz- and plagioclase-glasses. The values obtained proved to be higher than those of the normal glasses and lower than those of the crystalline phases. Diaplectic glasses are distinguishable from normal glasses by their physical properties. They represent intermediate stages of structural order between the crystalline and normal glass phases.  相似文献   

15.
The distribution of interatomic distances in amorphous NaAlSi3O8 has been determined at 805°C by X-ray radial distribution analysis to investigate structural differences between the glass (T < 763°C) and the supercooled liquid (763°C < T < 1118°C). Except for slight differences attributable to thermal expansion, no significant changes were observed. The sample crystallized during the course of the experiment, but at least one crystal-free data set was obtained. The transition from the inferred six-membered ring structure of the supercooled liquid to the four-membered ring structure of the crystal was clearly visible in radial distribution function (RDF's) determined before and after crystallization.RDF's were also determined at 25°C for two NaAlSi3O8 glasses with different histories. The first was derived from a melt that had been cooled slowly from 1600 to 32°C above the melting point (Tf = 1118°C) to detect possible repolymerization to a more ‘crystal-like’ structure as the melt approached Tf. The second glass was prepared by holding a single crystal of Amelia albite at 50°C above Tf to see if the crystalline four-membered ring structure was preserved in melts at temperatures just above the liquidus. No significant differences were observed between these two RDF's and one obtained from a glass quenched from 1800°C. These results suggest that in addition to the destruction of formation of a periodic structure, melting and crystallization in NaAlSi3O8 also involves a repolymerization of tetrahedra. This would explain the observed kinetic barrier to melting and crystallization in the anhydrous system and the catalytic effect of small amounts of water or alkali oxide.  相似文献   

16.
Shock recovery experiments on melilite samples in the pressure range from 11 to 50.5 GPa have been performed in order to examine the effects of shock waves on this material. The shocked samples were subsequently studied in the transmission electron microscope. All samples displayed the shock-induced amorphous areas, known as diaplectic glass. The amount of diaplectic melilite glass increased from a few percent at 11 GPa to about 85 percent at 50.5 GPa shock pressure. The shock waves also caused deformational effects as planar faults parallel to (001) and dislocations with a density in the order of 1010 cm?2. Regarding the present discussion on the origin and nature of diaplectic glass, diaplectic melilite glass is assumed to be the reversion product of a high-density phase produced in the shock front. Deformed melilites in Ca-Al-rich inclusions from chondritic meteorites studied so far do not contain diaplectic glass. It is assumed that the meteoritic melilites were hot (>1,000° C) and thus plastically deformable by shock waves of rather low amplitudes.  相似文献   

17.
A new microscale oxybarometer for solar system basaltic glasses, based on vanadium K-edge X-ray absorption near edge structure (XANES) spectroscopy, is described. Vanadium is unique among abundant elements in siliceous materials in that it can potentially occur in nature in four valence states: V2+, V3+, V4+ and V5+. Consequently, the vanadium redox system is a robust oxybarometer covering at least six orders of magnitude in buffer-relative oxygen fugacity. The method was calibrated using synthetic glass standards produced under known fO2 and temperature conditions. Correction for temperature differences among standards and unknowns was quantified using microXANES data for isobaric synthetic glass couples. Application of the method to lunar, martian, and terrestrial glasses yielded fO2 estimates from 1.6 log units more reduced than the iron-wüstite (IW) buffer (IW-1.6) for lunar glasses, to IW + 4.0 for terrestrial glass inclusions. The martian and terrestrial results are in good agreement with previous estimates by other methods. The inferred fO2 values for lunar pyroclastic glasses are ∼0.5 log unit more reduced than previous estimates, but the differences are comparable to analytical uncertainties. Micro-extended X-ray absorption fine structure spectra were consistent with the valence states determined by microXANES and provided additional constraints on vanadium site geometry. These results demonstrate the value of this new oxybarometer, which can be applied nondestructively to individual grains in conventional thin sections with ∼ micrometer resolution and ∼100 ppm elemental sensitivity.  相似文献   

18.
The central anorthosite peak of the Maniconagan crater displays characteristic shock deformation and transition phenomena in plagioclase feldspars, scapolithe, apatite and other maphitic minerals. The optical orientation of plagioclases is determined. With increasing shock, a trend to a highly disordered structure of the plagioclase lattice can be observed. Rock fracturing occurs at low pressures. At higher pressures different kinds of isotropisation features and planar deformation structures in plagioclase, scapolithe and apatite can be distinguished. These planar elements can be interpreted as glide planes of low crystallographic indices, set in motion during shock compression by plastic deformation. Their optical orientation is measured. At very high pressures a completely isotropic phase, the s. c. diaplectic glass is formed. The physical properties of diaplectic plagioclase crystals and diaplectic glass are determined which are different from those of an unshocked crystal and its molten plagioclase glass of the same chemical composition. The diaplectic plagioclase phases are apparently mixed phases of molten glass and normal crystal. This can be proved by x-ray and infrared absorption studies. These results are correlated to shock recovery experiments and hugoniot states of plagioclase. All shock effects of plagioclases are classified into three groups according to the low pressure regime, mixed phase regime and high pressure regime. The low pressure regime is characterised by strong fracturing, the mixed phase regime by the development of planar elements, which were transformed during shock compression into the high pressure plagioclase phase with hollandite structure. The latter is converted into diaplectic glass after pressure release. The high pressure regime is characterised by complete transformation of plagioclase into the high pressure phase, which is unstable and reverts completely to the amorphous phase (diaplectic glass or maskelynite) at zero pressure density.

Meinem verehrten Lehrer, Herrn Prof. Dr. W. v. Engelhardt, danke ich für die Unterstützung bei der Bearbeitung des Themas. Herrn Dr. D. Stöffler danke ich für klärende Diskussionen und Ratschläge. Dem Ministère des Richesses Naturelles, Québec, Canada, sowie Herrn M.S. J. Murtaugh sei für die großzügige Unterstützung bei den Geländearbeiten im Manicouagan-Krater gedankt. Die Deutsche Forschungsgemeinschaft hat die Arbeit finanziell unterstützt.  相似文献   

19.
The dissolution kinetics of five chemically complex and five chemically simple sodium silicate glass compositions (Na-Si±Al±B) were determined over a range of solution saturation values by varying the flow-through rates (1-100 mL/d) in a dynamic single-pass flow-through (SPFT) apparatus. The chemically complex borosilicate glasses are representative of prospective hosts for radioactive waste disposal and are characterized by relatively high molar Si/(Si + Al) and Na/(Al + B) ratios (>0.7 and >1.0, respectively). Analysis by X-ray absorption spectroscopy (XAS) indicates that the fraction of ivB to iiiB (N4) varies from 0.66 to 0.70. Despite large differences in bulk chemistry, values of δ29Si peak shift determined by MAS-NMR varies only by about 7 ppm (δ29Si = −94 to −87 ppm), indicating small differences in polymerization state for the glasses. Forward rates of reaction measured in dynamic experiments converge (average log10 rate [40 °C, pH 9] = −1.87 ± 0.79 [g/(m2 d)]) at high values of flow-rate (q) to sample surface area (S). Dissolution rates are independent of total Free Energy of Hydration (FEH) and this model appears to overestimate the impact of excess Na on chemical durability. For borosilicate glass compositions in which molar Na > Al + B, further addition of Na appears to stabilize the glass structure with respect to hydrolysis and dissolution. Compared to other borosilicate and aluminosilicate glasses, the glass specimens from this study dissolve at nearly the same rate (0-∼56×) as the more polymerized glasses, such as vitreous reedmergnerite (NaBSi3O8), albite, and silica. Dissolution of glass follows the order: boroaluminosilicate glass > vitreous reedmergnerite > vitreous albite > silica glass, which is roughly the same order of increasingly negative 29Si chemical shifts. The chemical shift of 29Si is a measure of the extent of bond overlap between Si and O and correlates with the forward rate of reaction. Thus, dissolution appears to be rate-limited by rupture of the Si-O bond, which is consistent with the tenants of Transition State Theory (TST). Therefore, dissolution at far from equilibrium conditions is dependent upon the speed of the rate-controlling elementary reaction and not on the sum of the free energies of hydration of the constituents of boroaluminosilicate glass.  相似文献   

20.
The local, up to second nearest neighbor, around Si atoms in alkali and alkaline earth-silicate glasses has been characterized by SiKβ X-ray emission spectroscopy. Principally two types of Si atoms can be distinguished. These are Si atoms with only other Si atoms as second nearest neighbours, and those with one or more alkali or alkaline earth atoms in their second coordination sphere. The spectroscopic results indicate that the lower molecular weight alkali and alkaline earth-silicate glasses tend towards a bimodal distribution of local Si environments, which is designated Q4-Q0 following the assignment by Engelhardtet al. (1975) for silica species in aqueous solutions. From a different perspective the outcome of these experiments suggests that, though the concentration of bridging oxygens (O(br)) and non-bridging oxygens (O(nbr)) is fixed by the stoichiometry of the glass, the distribution of O(nbr) in the glass varies according to the kind of alkali or alkaline earth atom present. From observed nucleation data on R2O-SiO2 (R = Li, Na, K) glasses it is inferred that a bimodal Q distribution and in particular the presence of Q0 species dominates the internal nucleation process in the alkali and alkaline earth-silicate glasses studied. Using this inference rationalizations can readily be found to explain the observed resistance to thermal shock and devitrification rates of these glasses.  相似文献   

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