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1.
Subgrain boundaries in thin sheets of octachloropropane deformed at 0.7–0.8 TM on the stage of a microscope are seen to appear in the material in seven different ways. Type I boundaries show the classical evolution by polygonization of bent crystals. Type II are essentially kink boundaries, which migrate sideways during deformation to reach their present positions in the crystals. Type III develop at the sites of former grain boundaries, by reduction of misorientation of adjacent grains. Type IV and V originate by impingement of migrating grain boundaries or subgrain boundaries, respectively. Type VI propagate in their own planes behind migrating grain boundaries to which they are attached. Type VII develop statically from optically strain-free grains by a process probably otherwise similar to the Type I process. Two thirds of the boundaries are Types I or II. In view of the variety of subgrain boundary histories in OCP, interpretation of similar features in minerals ought to be undertaken cautiously. Criteria are needed for telling the different types of subgrain boundaries apart in situations where only a final view of the structure is available, as in optical and electron micrographs of rocks.  相似文献   

2.
Chemical etching of pure melt-grown forsterite crystals is reported here for the first time. Cleaved forsterite crystals of known purity, and polished sections of the same crystals have been successfully etched revealing dislocations, subgrain boundaries, inclusions and growth imperfections.  相似文献   

3.
In order to address the growth of crystals in veins, a multiphase-field model is used to capture the dynamics of crystals precipitating from a super-saturated solution. To gain a detailed understanding of the polycrystal growth phenomena in veins, we investigate the influence of various boundary conditions on crystal growth. In particular, we analyze the formation of vein microstructures resulting from the free growth of crystals as well as crack-sealing processes. We define the crystal symmetry by considering the anisotropy in surface energy to simulate crystals with flat facets and sharp corners. The resulting growth competition of crystals with different orientations is studied to deduce a consistent orientation selection rule in the free-growth regime. Using crack-sealing simulations, we correlate the grain boundary tracking behavior depending on the relative rate of crack opening, opening trajectory, initial grain size, and wall roughness. Further, we illustrate how these parameters induce the microstructural transition between blocky (crystals growing anisotropically) to fibrous morphology (isotropic) and formation of grain boundaries. The phase-field simulations of crystals in the free-growth regime (in 2D and 3D) indicate that the growth or consumption of a crystal is dependent on the orientation difference with neighboring crystals. The crack-sealing simulation results (in 2D and 3D) reveal that crystals grow isotropically and grain boundaries track the opening trajectory if the wall roughness is high, opening increments are small, and crystals touch the wall before the next crack increment starts. Further, we find that within the complete crack-seal regime, anisotropy in surface energy results in the formation of curved/oscillating grain boundaries (instead of straight) when the crack-opening velocity is increased and wall roughness is not sufficiently high. Additionally, the overall capability of phase-field method to simulate large-scale polycrystal growth in veins (in 3D) is demonstrated enumerating the main advantages of adopting the novel approach.  相似文献   

4.
A set of coupled field equations is developed for transport of liquid brine in natural rock salt. The natural rock salt consists of individual crystals brought together so that only a portion of the crystal faces or grain boundaries contribute to the hydraulically connected pore space. Transport of brine inclusions within individual crystals is considered to be thermally driven; whereas transport along crystal interfaces or grain boundaries is considered to be pressure driven. The field equations for both transport mechanisms are developed and incorporated in a finite element program. An analytical solution to a one-dimensional boundary value problem is derived and compared to the finite element solution. An application of the finite element code to radioactive waste emplacement is briefly discussed.  相似文献   

5.
Radiaxial fibrous calcite: a replacement after acicular carbonate   总被引：1，自引：0，他引：1  
Radiaxial fibrous calcite, a common cavity fill in ancient limestones, is characterized by curved twin lamellae, optic axes that converge away from the substrate and subcrystals which diverge in this same direction. The optic axes radiate about three or four axes located in positions between adjacent crystals and orientated parallel with the crystal elongation. The crystals are commonly turbid with inclusions, which may be concentrated along twin lamellae and subcrystal and inter-crystalline boundaries, or form zones parallel to the substrate. Some inclusion patterns reveal the position of former crystal faces. From a consideration of the occurrences, gross morphological characters, the fabrics of the crystals and the inclusion patterns, radiaxial fibrous calcite is interpreted as a replacement of an early diagenetic acicular cement, composed of interfering bundles of radiating crystals. It is suggested that replacement takes place by a solution-precipitation process and the migration of a fluid film through the acicular host, with replacement occurring most rapidly between bundles of acicular carbonate. The optic axis pattern of the fibrous calcite is considered to be inherited from the c-axis orientations of the host acicular crystals. Fibrous calcite intercrystalline boundaries form as fractures after replacement. Some inclusion patterns record the characters of the replaced acicular cement; others, formed of impurities reorganized during the replacement, reveal the form of the replacement front.  相似文献   

6.
Evidence for partial melting along subgrain boundaries in quartz and plagioclase is documented for rocks from the Lost Creek Gneiss of the Llano Uplift, central Texas, the Wet Mountains of central Colorado, and the Albany-Fraser Orogen, southwestern Australia. Domains of quartz or plagioclase crystals along subgrain boundaries are preferentially involved in partial melting over unstrained domains of these minerals. Material along subgrain boundaries in quartz and plagioclase has the same morphology as melt pseudomorphs present along grain boundaries and is commonly laterally continuous with this former grain boundary melt, indicating the material along subgrain boundaries can also be categorized as a melt pseudomorph. Subgrain boundaries consist of arrays of dislocations within a crystal lattice, and unlike fractures would not act as conduits for melt migration. Instead, the presence of former melt along subgrain boundaries requires that partial melting occurred in these locations because it is kinetically more favorable for melting reactions to occur there. Preferential melting in high strain locations may be attributed to strain energy, which provides a minor energetic contribution to the reaction and leads to preferential melting in locations with weakened bonds, and/or the presence of small quantities of water associated with dislocations, which may enhance diffusion rates or locally lower the temperature needed for partial melting.  相似文献   

7.
Optical investigations of Brewster fringes in natural amethyst crystals have indicated that the fringes are Brazil twin boundaries consisting of McLaren and Pitkethly type zig-zag structures and closed parallelograms. The two types of Brazil twin boundaries appear alternately in the successive fringes and in different orientations in one fringe. The Brewster fringes develop by a process of incorporation of many Brazil twin lamellae, when growth conditions are stabilized after this process. If perturbation occurs in growth conditions, distinct growth banding with and without Brazil twin lamellae will develop, and not Brewster fringes.  相似文献   

8.
A new chemical etching technique has been developed to study defect structures in terrestrial and lunar olivine crystals. Dislocations, low-angle dislocation boundaries, dislocation arrays and pile-ups have been observed on the (010, (100), and (001) faces of both polished and cleaved naturally deformed single crystal olivine, on various orientations of crystals in dunite, and in an experimentally deformed olivine crystal. Etching of Apollo 12 igneous rocks revealed dislocations and radiation damage tracks in lunar olivine. Dislocation etching of the (001) face of olivine is reported for the first time in this paper.  相似文献   

9.
Single crystals of olivine were grown at 0.1 MPa total pressure in a floating-zone image furnace. Composition of crystals grown ranged from 67 mol% forsterite to 90 mol% forsterite. Improvements in the crystal growth technique allowed growth of large crystals (0.005 m diameter, 0.070 m length) with very low dislocation density, no visible bubbles, and no cracks. The most significant improvement was the addition of a platinum-wound resistance heater internal to the image furnace. This heater provided the dual improvements of better control over oxygen fugacity from a CO/CO2 gas mix and alteration of the crystal growth interface resulting in a significant reduction in crystal defects. No subgrain boundaries have been observed in crystals growth here and dislocation densities on the order of 108-109 m-2 have been achieved. Iron concentration is nearly uniform across the diameter (0.005 m) of crystals and varies approximately 5 mol% along the crystal length (0.040 to 0.050 m).  相似文献   

10.
Gold decoration and platinum-carbon-shadowing techniques of electron microscopy have been applied to study the surface microtopographs of noncleavage (001) faces of porphyroblastic white micas and sericite crystals collected from the chlorite, biotite, and garnet zones in the Shiragayama area in the Sanbagawa metamorphic terrain, Shikoku, Japan. Two different types of surface microtopograph have been observed; parallel step system and saw-tooth step system. The former is interpreted as representing the surface microtopographs formed either by growth or under near equilibrium conditions, the latter either by dissolution or under rigorous kinetic conditions. The former has been observed on porphyroblastic white micas occurring in the middle portion of each metamorphic zone and on all sericite crystals, the latter on most sericites and on porphyroblastic white micas occurring along the boundaries of the neighboring metamorphic zones. It is suggested that the observed variations in the surface microtopographic characteristics are due to Ostwald ripening during a kind of sintering process in which interstitial water plays an essential role, and that the drastic changes along the metamorphic zone boundaries are due to dehydration reactions. Retrogressive metamorphism is considered to have an almost negligible effect, if any, upon the surface microtopographs.  相似文献   

11.
Chemical etching of dislocations has been studied in natural and synthetic quartz single crystals, in deformed synthetic quartz and in naturally and experimentally deformed quartzites. The ability of different etchants to produce polished or preferentially etched surfaces on quartz is described. Dislocation etching was achieved on all crystal planes examined by using a saturated solution of ammonium bifluoride as the etchant. Appropriate etching times were determined for etching quartzites for grain size, subgrain boundaries, deformation lamellae, dislocations and twins. Growth and polished surfaces of synthetic single crystal quartz were similarly etched and dislocation etch pits, characteristic of various orientations were found. The use of ammonium bifluoride proved to be expecially advantageous for the basal plane, producing a polished surface with etch pits, suitable for dislocation etch pit counting. “Double” etch pits have been found on Dauphiné twin boundaries on the basal plane and the first order prism, using this etchant. Slip lines and deformation bands were suitably etched on deformed synthetic crystal surfaces for identification of the slip planes. Other acidic etchants have been explored and their application to the study of deformation structures in quartz crystals is discussed.  相似文献   

12.
The shape, pattern and crystallographic orientation of grain boundaries represent important characteristics of crystalline material and contain information about its deformation and annealing history. The present study includes measurements of grain boundaries from experimentally annealed analogue material as well as natural foam texture of quartz and plagioclase. The main subject is the relation between the development of a foam texture and the crystallographic orientation of its grain boundaries and their geometry. (1) During annealing, grain sizes stabilize at certain values. On a statistical basis, these values can be applied as a geothermometer. (2) On the light-microscope scale, the grain boundaries in foam textures commonly consist of two or several planar facets. They are preferentially oriented along specific crystallographic planes, namely in relation to both neighbouring crystals; for quartz they tend to be rhombohedral. (3) Even highly misoriented facets and dihedral angles largely deviating from the 'equilibrium angle' of 120° may be stable over long periods of annealing. (4) Parts of single boundaries may migrate, whereas other parts are stationary during annealing. The results of the present study suggest that the anisotropy of surface energy has a considerable influence on the development of foam textures and that modelling of texture development should include the influence of the crystallographic orientation of grain boundaries.  相似文献   

13.
We performed high strain (up to 47 %) axial compression experiments on natural quartz single crystals with added rutile powder (TiO2) and ~0.2 wt% H2O to investigate the effects of deformation on the titanium-in-quartz (TitaniQ) geothermobarometer. One of the objectives was to study the relationships between different deformation mechanisms and incorporation of Ti into recrystallized quartz grains. Experiments were performed in a Griggs-type solid-medium deformation apparatus at confining pressures of 1.0–1.5 GPa and temperatures of 800–1,000 °C, at constant strain rates of 1 × 10?6 or 1 × 10?7 s?1. Mobility of Ti in the fluid phase and saturation of rutile at grain boundaries during the deformation experiments are indicated by precipitation of secondary rutile in cracks and along the grain boundaries of newly recrystallized quartz grains. Microstructural analysis by light and scanning electron microscopy (the latter including electron backscatter diffraction mapping of grain misorientations) shows that the strongly deformed quartz single crystals contain a wide variety of deformation microstructures and shows evidence for subgrain rotation (SGR) and grain boundary migration recrystallization (GBMR). In addition, substantial grain growth occurred in annealing experiments after deformation. The GBMR and grain growth are evidence of moving grain boundaries, a microstructure favored by high temperatures. Electron microprobe analysis shows no significant increase in Ti content in recrystallized quartz grains formed by SGR or by GBMR, nor in grains grown by annealing. This result indicates that neither SGR nor moving grain boundaries during GBMR and grain growth are adequate processes to facilitate re-equilibration of the Ti content in experimentally deformed quartz crystals at the investigated conditions. More generally, our results suggest that exchange of Ti in quartz at low H2O contents (which may be realistic for natural deformation conditions) is still not fully understood. Thus, the application of the TitaniQ geothermobarometer to deformed metamorphic rocks at low fluid contents may not be as straightforward as previously thought and requires further research.  相似文献   

14.
《Journal of Structural Geology》2002,24(6-7):1125-1137
Dynamically recrystallized and sutured quartz grains from metamorphic rocks with different strain intensities and temperature conditions ranging from ca. 350°C to ca. 700°C have been studied. Universal-stage measurements on quartz–quartz high-angle grain boundaries show that they are never curved but always consist of straight segments which preferentially occupy specific crystallographic orientations in relation to both neighboring crystals. With increasing temperature the segments preferentially concentrate in a decreasing number of orientations, mainly near the rhombohedral {101̄1} planes. The crystallographic data and the observations on grain boundary geometries suggest that: (i) grain boundary orientations are strongly crystallographically controlled, (ii) this control is the main factor on the textural equilibration of quartz–quartz grain boundaries in metamorphic rocks, and (iii) grain boundaries from dynamically recrystallized quartz should be regarded as annealed and equilibrated fabrics that are stable against subsequent annealing as long as the material is not re-deformed.  相似文献   

15.
Hydrogen can be stored in the structure of nominally anhydrous minerals as point defects, and these impurities substantially modify many physical properties of Earth’s mantle minerals. However, mantle rocks are composed of mineral grains separated by grain boundaries and interphase grains boundaries. Therefore, as a potential hydrogen reservoir, grain boundaries should be given proper attention. Here, I report an experimental investigation into hydrogen diffusion through grain boundaries in polycrystalline aggregates. Sintering and diffusion experiments were performed using a gas-medium high-pressure vessel at under pressure of 300 MPa and over a temperature range of 900–1,250°C. The diffusion assembly consisted of a polycrystalline cylinder of aluminous spinel + olivine crystals with a talc cylinder as the main hydrogen source. A Ni capsule was used to buffer the oxygen fugacity at Ni–NiO. Experimental durations varied from 5 min to 5 h. The presence of hydrogen in the crystals was measured by Fourier-transform infrared spectroscopy. The calculation of the diffusion coefficients was based on the estimation of the characteristic distance. The absence or presence of hydrogen recorded by the ‘hydrogen sensor’ olivines embedded in the aggregate allows the estimation of bounds on this characteristic distance. Results presented here suggest that hydrogen effective diffusion coefficients are only one order of magnitude faster (~10−9 m2s−1 at 1,000°C) than in an olivine single crystal along the [100] axis. Resulting diffusion coefficients for hydrogen in grain boundary are four orders of magnitude faster than in a single crystal, but this diffusivity is not fast enough to affect hydrogen mobility in mantle rocks with grain sizes greater than ~1 mm. Thus, very limited chemical homogenization would occur using grain boundaries diffusion in mantle hydrous peridotite for incompatible and volatile element, such as hydrogen.  相似文献   

16.
Carbonate rocks that have suffered early near-surface dolomitization followed by extended meteoric exposure commonly undergo partial delithification, a process that results in the formation of dolomitic silts and sands, herein termed dologrus. Dologrus is interpreted to form as a result of diffuse dissolution in porous and permeable dolostones prior to burial and compaction. Such dissolution occurs at the crystal–pore-water interface, causing individual crystals to corrode along interfacial boundaries, eventually leading to delithification and the formation of sediment composed of corroded crystals. The resulting sediment grain size is likely controlled in part by the crystal size of the precursor dolostone. Loss of rock competency through the process of dologrus formation in the shallow subsurface can lead to collapse of overlying bedrock and the formation of dolines common to karstic carbonate landscapes.  相似文献   

17.
This paper reports the first example of fault mirrors developed in an unusual protolith, consisting of tourmaline crystals with interstitial goethite. The deformation mechanisms active in the fault zone have been investigated from the outcrop to the nanoscale, aiming to identify possible traces of frictional heating at seismic slip rate, as observed for other fault mirrors in different protoliths. The investigation revealed the superposition of two main deformational stages. The first was dominated by brittle processes and produced a cataclastic/ultracataclastic principal slip zone, a few mm thick; the second was associated with seismic slip and produced a sharp discontinuity (the principal slip surface) within the cataclastic/ultracataclastic zone. The mirror-like coating, a few microns thick, occurs on the principal slip surface, and is characterized by 1) absence of interstitial goethite; 2) occurrence of truncated tourmaline crystals; 3) highly variable grain size, from 200 μm to 200 nm; 4) tourmaline close packing with interlobate grain boundaries, and 5) tourmaline random crystallographic orientation.Micro and nanostructural investigations indicate the occurrence of thermally-activated processes, involving both interstitial goethite and tourmaline. In particular, close to the principal slip surface, goethite is completely decomposed, and produced an amorphous porous material, with local topotactic recrystallization of hematite. Tourmaline clasts are typically characterized by strongly lobate boundaries, indicative of reaction and partial decomposition at grain boundaries. TEM observations revealed the occurrence of tourmaline nanograins, a few tens of nm in size, characterized by rounded shape and fading amorphous boundaries, that cannot be obtained by brittle processes. Lastly, the peculiar interlobate microstructure of the mirror surface is interpreted as the result of grain boundary recrystallization processes taking place by deformation at high-T conditions. Our results show that tourmaline fault mirrors recorded localized high-T processes triggered by frictional heating and can be therefore considered as reliable traces of ancient earthquakes.  相似文献   

18.
Thermochronometry based on radiogenic noble gases is critically dependent upon accurate knowledge of the kinetics of diffusion. With few exceptions, complex natural crystals are represented by ideal geometries such as infinite sheets, infinite cylinders, or spheres, and diffusivity is assumed to be isotropic. However, the physical boundaries of crystals generally do not conform to ideal geometries and diffusion within some crystals is known to be anisotropic. Our failure to incorporate such complexities into diffusive models leads to inaccuracies in both thermal histories and diffusion parameters calculated from fractional release data. To address these shortcomings we developed a code based on the lattice Boltzmann (LB) method to model diffusion from complex 3D geometries having isotropic, temperature-independent anisotropic, and temperature-dependent anisotropic diffusivity. In this paper we outline the theoretical basis for the LB code and highlight several advantages of this model relative to more traditional finite difference approaches. The LB code, along with existing analytical solutions for diffusion from simple geometries, is used to investigate the affect of intrinsic crystallographic features (e.g., crystal topology and diffusion anisotropy) on calculated diffusion parameters and a novel method for approximating thermal histories from crystals with complex topologies and diffusive anisotropy is presented.  相似文献   

19.
Several macroscopic physical and chemical properties, such as rheology, elasticity, or transport properties are governed by grain boundary processes. An improved understanding of the structure and evolution of grain boundaries has thus become a key challenge in geosciences and material sciences. Here, we report the structure of near Σ5 (210)/[100] grain boundaries in Y3Al5O12 (YAG), which were synthesised by the wafer direct bonding method. The produced grain boundaries were annealed at different temperatures, ranging from 673 to 1,873 K. The grain boundaries annealed at different temperatures are not distinguishable based on their flatness and apparent cohesiveness in high resolution TEM (HRTEM) micrographs, but show a considerable step in their mechanical stability at around 1,273 K, a temperature that corresponds to roughly half the melting temperature of YAG. This study further focuses on the effect of a slight misorientation of the two crystals on the grain boundary structure and we discuss if the boundary can reach a state of minimum energy configuration during annealing. Along the grain boundaries, we observed a long-range strain contrast with a periodicity of 40 nm, which has not been reported for high-angle grain boundaries so far. We conclude that this contrast is caused by faceting along the grain boundary plane, which is needed to achieve minimum energy configuration of the grain boundary plane.  相似文献   

20.
Single crystals of biotite have been shortened up to 20% in compression tests parallel to [100], [110] and [010] directions at 3 Kbar confining pressure and temperatures from 300 to 700° C, and at a strain rate of 10–4 sec–1. Thick metal constraining sleeves were used and led to a distribution of kinking throughout the crystals. The orientation of kink boundaries, angle of bending and asymmetry of the basal plane across the kink boundaries and the axes of bending were measured. A minor amount of unidentified non-basal slip must have occurred to account for the assymmetry, but basal slip predominates at all temperatures. From the axes of bending, the discrete slip directions [100], [110] and [110] for basal slip are deduced. Increase in temperature mainly leads to a simpler pattern of kinking associated with the kinks being wider and the kinking angle larger, presumably as a result of greater mobility of dislocation walls that form the kink boundaries.In his summary table, Mügge lists these axes as [010] and [130] but the latter seems to be quoted in error, and in conflict with his text, in place of [310]. Borg and Handin (1966) have quoted the [130] indices as given by Mügge in his table. In the analysis of their own observations there has been a confusion between direction indices and plane normal indices. When this is corrected, their results would also indicate [100] and [110] as active slip directions in [001] (Borg, private communication).  相似文献   

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