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1.
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   

2.
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   

3.
The intergrowth of diaplectic glass and primitive anorthite has been observed in fragments shocked to 300 kbar. The textures observed by electron microscopy are similar to those produced in shocked quartz crystals. The diffuseness of reflections, related to antiphase domains, show no noticeable change in comparison with unshocked specimens. These results indicate that no gradual change in original domain size occurs and that the transformation of primitive anorthite to diaplectic glass occurs abruptly.  相似文献   

4.
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   

5.
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   

6.
Crystalline rocks from breccias of the Ries basin, Germany, contain highly deformed quartz. Various planar deformation structures could be observed and classified into five different types: (1) Decorated planar elements, (2) Non-decorated planar elements, (3) Homogeneous lamellae, (4) Filled lamellae, (5) Planar fractures. All these structures are parallel to crystallographic planes: {10¯13}, {10¯12}, {10¯11}, {0001},{11¯21}, {11¯22}, {21¯31}, {51¯61}, {10¯10}. The most typical and most abundant planar structures are decorated and nondecorated planar elements parallel to {10¯13} and {10¯12}. Planar fractures are parallel to {0001} and {10¯11} and form at lower stress levels, probably earlier than the planar elements.Quartz containing planar elements, especially of the non-decorated type, has lower density, index of refraction and birefringence than normal quartz. This quartz is apparently a mixture of an amorphous phase and crystalline quartz, the amount of which can be calculated using average density or refractive index.Comparison of planar quartz structures found in tectonites and those produced artificially under static or dynamic high pressure conditions demonstrates that Ries quartz closely resembles deformed quartz recovered from shock wave experiments. The planar structures found in Ries quartz have been formed by shock wave actions with peak pressures in the 100–400 kbar range.Planar elements are explained to be traces of gliding processes during shock loading visible due to the fact that a high pressure phase (stishovite and/or a stishovite-like glass phase) has been produced along the glide planes. Upon pressure release most of the high pressure phase was transformed into an SiO2-glass (diaplectic glass).In comparison with experimental data the amount of residual crystalline quartz as well as type and orientation of planar structures in the quartz grains are clues to estimate the peak pressures responsible for these deformations. Shock waves with peak pressures exceeding about 400 kbar completely transform quartz into diaplectic SiO2-glass.  相似文献   

7.
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   

8.
The microtextures of stishovite and coesite in shocked non-porous lithic clasts from suevite of the Ries impact structure were studied in transmitted light and under the scanning electron microscope. Both high-pressure silica phases were identified in situ by laser-Raman spectroscopy. They formed from silica melt as well as by solid-state transformation. In weakly shocked rocks (stage I), fine-grained stishovite (≤1.8 μm) occurs in thin pseudotachylite veins of quartz-rich rocks, where it obviously nucleated from high-pressure frictional melts. Generally no stishovite was found in planar deformation features (PDFs) within grains of rock-forming quartz. The single exception is a highly shocked quartz grain, trapped between a pseudotachylite vein and a large ilmenite grain, in which stishovite occurs within two sets of lamellae. It is assumed that in this case the small stishovite grains formed by the interplay of conductive heating and shock reverberation. In strongly shocked rocks (stages Ib–III, above ∼30 GPa), grains of former quartz typically contain abundant and variably sized stishovite (<6 μm) embedded within a dense amorphous silica phase in the interstices between PDFs. The formation of transparent diaplectic glass in adjacent domains results from the breakdown of stishovite and the transformation of the dense amorphous phase and PDFs to diaplectic glass in the solid state. Coesite formed during unloading occurs in two textural varieties. Granular micrometre-sized coesite occurs embedded in silica melt glass along former fractures and grain boundaries. These former high-pressure melt pockets are surrounded by diaplectic glass or by domains consisting of microcrystalline coesite and earlier formed stishovite. The latter is mostly replaced by amorphous silica.  相似文献   

9.
尹锋  陈鸣 《岩石学报》2022,38(3):901-912
撞击角砾岩是陨石撞击过程形成的特有岩石种类,是研究撞击成坑过程、陨石坑定年、矿物岩石冲击变质的理想对象。岫岩陨石坑是一个直径1800m的简单陨石坑,坑内有大量松散堆积的撞击角砾岩。本研究通过光学显微镜、费氏台、电子探针、X射线荧光光谱仪、电感耦合等离子质谱仪等分析测试手段,主要研究了岫岩陨石坑撞击角砾岩的岩相学和冲击变质特征,并在此基础上讨论了撞击角砾岩的形成过程和陨石坑的形貌特征。岫岩陨石坑内产出有三种撞击角砾岩,分别是来自上部的玄武质角砾岩和复成分岩屑角砾岩,以及底部的含熔体角砾岩。组成玄武质角砾岩和复成分岩屑角砾岩的碎屑受到的冲击程度较低,仅有少量石英发育面状变形页理,指示不超过20GPa的冲击压力。而组成含熔体角砾岩的碎屑受到了很强的冲击,发育了熔融硅酸盐玻璃、石英面状变形页理、柯石英、二氧化硅玻璃、击变长石玻璃、莱氏石等冲击变质特征,指示的峰值压力超过50GPa。本研究证实了含熔体角砾岩通常产出在简单陨石坑底部,由瞬间坑的坑缘和坑壁垮塌的岩石碎屑与坑底的冲击熔体混合形成。岫岩坑的真实深度是495m,真实深度与直径的比值为0.275,符合简单陨石坑的尺寸特征。陨石坑内的撞击角砾岩中心厚度为188m,与直径之比为0.104,略低于其它简单坑,可能是受丘陵地貌影响导致改造阶段垮塌到坑内的岩石角砾偏少。  相似文献   

10.
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   

11.
Clasts of shocked garnet-sillimanite gneisses comprise a minor fraction of the allochthonous breccia at the Haughton impact structure. Refractive indices of the diaplectic and fused components of the gneisses, and reduced specific gravity indicate shock pressures from 35 to 55±5 GPa and effective post-shock temperatures from 500° to 1,000° C in a suite of selected samples.Sillimanites remain birefringent but display several effects of shock metamorphism. Shock-produced planar features and planar fractures are highly developed; optic axial angle (2V y ) increases from near normal (26°) to over 80° within a sample; there is a reduction in optical relief and a development of a pale brown colouring which generally deepens in shade as shock level increases. There is no unambiguous evidence, optically or from X-ray investigation, of a high-pressure Al2SiO5 polymorph or breakdown to mullite and silica. The highly shocked sillimanites have anomalous K2O contents from 0.11% to 0.92%. Potassium appears to substitute for aluminum and, to a lesser degree, for iron while retaining sillimanite stoichiometry, and the amount of substitution generally reflects increased shock level. The source of the contributed potassium is the coexisting shock-fused feldspar glass. The glass of each sample is derived primarily from melted alkali feldspar with a minor and varied admixture from the breakdown of mafic minerals. The glasses are depleted in K2O, although Na2O is unaffected, and the extent of depletion can be correlated with the increased K2O content of the associated sillimanite. The incorporation of potassium in shocked sillimanites is a function of both degree of shock deformation and availability of potassium from other coexisting shocked phases. It is speculated that the brown colouration is a function of ferrous iron content and may reflect post-crater thermal history rather than shock level.Contribution from the Earth Physics Branch No. 951  相似文献   

12.
Seventeen shock-recovery experiments were performed on powder mixtures of one part (by weight) olivine (St. John's forsterite) plus two parts silica glass (pure vitreous silica) in order to characterize the physical and chemical interaction of two chemically incompatible components during shock. Powders of <45 m grain size were shocked by impact of projectiles launched from a 20 mm gun which created pressures ranging from 6.2 to 64.2 GPa (1 GPa= 10kbar).Petrographie features observed in thin section attest to mechanical and thermal metamorphism. Samples shocked to pressures from 6.2 to 39.3 GPa form compacted, mosaic, granular aggregates with fractured and strained grains. Samples shocked to pressures from 42.9 to 64.2 GPa form vesicular, mixed melts containing flow schlieren and relict olivine fragments. Petrographic disequilibrium is manifested in cataclastic textures showing deformational anisotropy and in thermal effects showing non-uniform intergranular melting. This disequilibrium is caused by an irregular pressure distribution resulting from the rapid collapse of pore spaces.The chemical composition of the shock melts are similar in each of six samples shocked to pressures of 42.9 to 64.2 GPa. Melt chemistry is bimodal in each sample. Colorless melts are 99.9% SiO2 and represent pure silica glass melts; pale to dark green melts range in composition from 47% to 64% SiO2 and represent a progressive mixture of olivine melt (41% SiO2) with silica glass melt. Surprisingly, the compositions of the colored glasses are intermediate between the composition of pure olivine and the bulk composition of the original starting material (79% SiO2) and are similar to enstatitic pyroxene compositions (50% to 57% SiO2; 33% to 37% MgO). Although bulk compositions of shocked samples are unchanged, the creation of melts with pyroxene compositions instead of bulk sample compositions may indicate that an incipient eutectic-type fusion may have occurred in small olivine-normative domains surrounding individual olivine grains. Chemical disequilibrium is evidenced by the creation of these olivine-normative melts from a quartz-normative starting compositions and by the chemical heterogeneity in the melts.  相似文献   

13.
We have carried out a Raman Spectroscopic study of single crystalline quartz samples shocked to peak pressures up to 31.4GPa. Samples shocked to above 22 GPa show shifts in peak positions consistent with the quartz being under tensile stress, and new broad bands associated with the formation of high density SiO2 glass appear in the spectra. These changes are accompanied by an increase in the lattice parameters of the quartz. Formation of the diaplectic glass could be due to a metastable melting event, or spinodal lattice collapse on attainment of a mechanical stability limit of crystalline quartz, as suggested by previous studies of pressure-induced amorphization in static pressurization experiments on SiO2 and GeO2 polymorphs.  相似文献   

14.
Samples of synthetic diaplectic anorthite glass (38 GPa shock pressure), thermal glass and synthetic anorthite crystals were investigated using infrared spectral methods at one atmosphere and high pressures (near 4 GPa). Band positions and pressure derivatives for the Si-O asymmetric modes in the region 1,300–900 cm?1 indicate that the diaplectic glass has more structural similarities with the crystalline material than with thermal glass even though the overall infrared spectral characteristics suggest a glassy state.  相似文献   

15.
The Slate Islands archipelago is believed to represent the central uplifted portion of a complex impact structure. Planar microstructures in quartz and feldspars and shock vitrification of rocks are the most common shock metamorphic features encountered. No diaplectic quartz was identified in the exposed rocks, but minor maskelynite is present. Shatter cones occur on all islands of the archipelago suggesting minimum pressures of 4 ± 2 GPa. The relative frequency of low index planar microstructures of specific, optically determined crystallographic orientations in quartz are correlated with results from shock barometric experiments to estimate peak shock pressures experienced by the exposed target rocks. In general, there is a decrease in shock pressure recorded in the target rocks from about 20–25 GPa in east-central Patterson Island to about 5–10 GPa at the western shore of this island and on Mortimer Island. The shock attenuation gradient is ∼4.5 GPa/km across this section of the island group. However, the shock attenuation has a roughly concentric plan only over the western part of the archipelago. There is no distinct shock center and there are other deviations from circularity. This is probably the result of: (1) the shock wave not having expanded from a point or spherical source because of the ∼1. 0 to 1.5 km size of the impactor; (2) differential movement of large target rock blocks during the central uplift and crater modification phases of the impact process. The orientation of planar deformation features in quartz appears to be independent of the shock wave direction suggesting that crystal structure exerts the primary control on microstructure development. Based on the results of XRD analyses, residual, post-impact temperatures were high enough to cause annealing of submicroscopic damage in shocked quartz. Received: 15 July 1997 / Accepted: 6 October 1997  相似文献   

16.
Plagioclase from fragments of crystalline basement rocks in breccias found in the area of the Nördlinger Ries crater displays characteristic plastic deformation and phase transition phenomena due to shock metamorphism at different pressures in the range of 100 to 1000 kilobars.These phenomena are discussed in the scope of a progressive impact metamorphism the degree of metamorphism reflecting a radial gradient of pressure and temperature diminishing outward from the point of meteorite impact.Within the lowest pressure range of about 100 to 300 kilobars (shock stage I) strong fracturing and plastic deformation such as bending of crystals, deformation bands and planar features (lamellae of lowered refractive index and of lowered or no birefringence) are to be found. The lamellae which are mostly isotropic, are interpreted as slip bands the glide planes of which are low indices planes of the plagioclase lattice such as (001), (010), (100), (1¯20), (130) and others. These slip bands are unknown from feldspar formed by normal processes within the earth's crust. Plagioclase of such a stage of deformation shows an unusual strong decrease of refraction and birefringence. Its optical properties are those of a highly disordered plagioclase. It may be called diaplectic plagioclase.Total isotropization of plagioclase is a typical feature of the pressure range from 300 to 500 kilobars (shock stage II). This glass which is called diaplectic glass differs strongly from the normal glass in physical properties and structural state. It is formed by a kind of solid state transformation without actual melting.Shock pressures in the order of 500 to 650 kilobars (shock stage III) are able to cause selective melting of plagioclase grains in a crystalline rock. Normal glasses with vesicles and streaks are formed by this process.Within the pressure range of about 650 to 1000 kilobars (shock stage IV) residual temperatures are so high that total melting of rocks occurs. Plagioclase melts are mixed inhomogenously with other silicate melts forming rock melts which can be found in suevite as flat glassy bombs. Vaporization of silicates must be expected in the upper pressure range of this shock stage.Statistical universal stage measurements on the fabric of plagioclase support theoretical considerations after which the deformation pattern of a single crystal should depend on the fabric relations to the surrounding minerals and on their physical properties. Strongly inhomogenous deformation of plagioclase minerals within the microscopic rock scale was observed because polycrystalline rocks are disorganizing a unique shock front by interaction of wave fronts at interfaces and free surfaces and perhaps by multiwave shocks. Directions of compressive and tensile stresses on a mineral are therefore changing from grain to grain.  相似文献   

17.
Patterns in the shock metamorphic transformations of rock-forming minerals (garnet, biotite, plagioclase, and K-feldspar) in quartz-feldspar-biotite-garnet schist (the Southern Urals) that were subjected to shockwave compression using planar geometry recovery ampoules were investigated. The maximum shock pressures that were attained upon a few wave circulations in a sample (stepwise shock compression) reached 26, 36, and 52 GPa. Comparison of the transformations that occurred due to stepwise shock compression with diaplectic changes of the studied minerals in nature and in the course of experiments with convergent spherical shock waves revealed significant differences in the transformations of melanocratic (garnet and biotite) minerals under different types of loading.  相似文献   

18.
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   

19.
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20.
In the suevite breccia of the Ries impact crater, Germany, glasses occur as bombs, and small particles in the groundmass. These glasses were formed from melt produced by shock fusion of crystalline basement rocks. Ejection from the crater resulted in the formation of aerodynamically shaped bombs, a few homogeneous spherules and a large mass of small glass particles which were deposited in the suevite breccia. Bombs and small particles included within chilled bottom and top layers of suevite deposits have been preserved in vitreous state, whereas glasses within the interior of the suevite devitrified, due to slower cooling rates.This paper summarizes the results of petrographical and chemical investigations of suevite glasses and their devitrification products. Conclusions are derived on origin and history of bombs and glass particles.Vitreous bombs and glass particles consist of schlieren-rich glass, mineral fragments (mainly quartz), rock fragments and vesicles. Wet chemical, trace element and microprobe analyses reveal that a primary melt was formed by shock fusion of a basement complex, consisting of about 80% biotite granite and 20% amphibolite. The, originally, more than 1800° C hot melt then incorporated shocked and desintegrated rocks of outer zones of the impact. Partial fusion of the rock debris resulted in a polyphase mixture consisting of melts, different in composition, accumulations of refractory mineral fragments and vesicles.Devitrified bombs and glass particles which are found in the interior of suevite deposits show alterations of texture and composition, due to microcrystallite growth and action of hydrothermal and weathering solutions. Incipient devitrification is indicated by brown staining of the glasses, originating, probably, by exsolution of minute magnetite particles. By optical microscopy and X-ray analysis, plagioclase and pyroxenes have been identified as main devitrification products. Shapes and textures of microcrystallites indicate fast crystal growth in a viscous and supercooled medium. Hot fluids permeating the suevite deposited microcrystalline quartz in vesicles and cracks. Later, montmorillonite was precipitated by solutions corroding the glass. Action of solutions on glasses which were weakened in coherence by devitrification resulted in oxidation of iron, leaching of iron and magnesium, and enrichment in alkalis.  相似文献   

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