Crystallographic control on the development of foam textures in quartz, plagioclase and analogue material

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.     

Crystallographic orientations of high-angle grain boundaries in dynamically recrystallized quartz: First results

The crystallography and geometry of high-angle grain boundaries from dynamically recrystallized quartz have been studied. On the basis of combined electron backscatter diffraction and universal stage measurements, the complete crystallographic orientation of the grain boundaries could be calculated. The u-stage rotation of the grain boundaries to a vertical position reveals that they are never curved but always consist of straight segments. Our results show that these segments preferentially occupy rhombohedral, trapezohedral and bipyramidal orientations, i.e., orientations in a  25–50° girdle to the c-axis. A specific, albeit low, number of segments with special crystallographic orientation, with respect to a neighbouring quartz grain, often shows another special orientation with respect to the other neighbouring grain. Preferred combinations of grain boundary orientations related to both neighbouring grains are (i) low-index rhombohedral and high index trapezohedral, (ii) low-index bipyramidal and low-index trapezohedral or high-index rhombohedral, and (iii) low-index trapezohedral and low or high index trapezohedral. In certain cases, such as at triple junctions, the boundaries occupy specific trapezohedral orientations with a constant angle to the c-axis. This argues for energy isotropy of trapezohedral planes with the same angle to the quartz c-axis. In general, good match coincidence site lattice (CSL) orientations are not preferentially occupied so that most of the studied grain boundaries represent general boundaries. The formation of straight segments in special crystallographic orientations indicates the crystallographic control and implies an energy reduction of certain general boundaries.     

The interaction of migrating grain boundaries with fluid inclusions in rock analogues: the effect of wetting angle and fluid inclusion velocity

The distribution of fluids in grain boundaries, fluid inclusion morphology and kinetics have important effects on the evolution of microstructure and transport properties and should be understood for correct interpretations for studies of thermobarometry and paleorheology. We compare results of in situ annealing experiments on rock analogues in the presence of different pore fluids in transmitted light: bischofite with saturated brine, camphor with ethanol, and camphor with ethylene glycol. The solid–liquid systems vary in terms of wettability and solubility, while homologous temperatures, strain rates, annealing times, and the initial textures are similar. In agreement with earlier work and theory, we observe different types of grain boundary–fluid inclusion interaction at sufficiently low grain boundary velocity such as drag and drop, necking, and the break up into arrays of smaller inclusions. In all three systems the maximum possible velocity of a fluid inclusion being dragged by a moving grain boundary is dependent on the fluid inclusion size. We interpret this to reflect the fluid inclusion mobility, and compare the trend with theoretical models which suggest that for all three systems the rate-limiting process is bulk diffusion and not surface diffusion or solution-precipitation.     

Grain boundary dissolution porosity in quartzofeldspathic ultramylonites: Implications for permeability enhancement and weakening of mid-crustal shear zones

Quartzofeldspathic ultramylonites from the Alpine Fault Zone, one of the world's major, active plate boundary-scale fault zones have quartz crystallographic preferred orientations (CPO) and abundant low-angle (<10° misorientation) boundaries, typical microstructures for dislocation creep-dominated deformation. Geometrically necessary dislocation density estimates indicate mean dislocation densities of ∼10⁹ cm⁻². A significant proportion (∼30%) of grain boundaries (>10° misorientation) are decorated by faceted pores, commonly with uniformly-oriented pyramidal shapes. Only grain boundaries with >10° misorientation angles in polymineralic aggregates are decorated by pores. Mean grain boundary pore densities are ∼5 × 10⁸ cm⁻². Grain boundary pores are dissolution pits generated during syn-deformational transient grain boundary permeability, nucleating on dislocation traces at dilatant grain boundary interfaces. They have not been removed by subsequent grain boundary closure or annealing. Pore decoration could have led to grain boundary pinning, triggering a switch in the dominant deformation mechanism to grain boundary sliding, which is supported by evidence of CPO destruction in matrix quartz. Pore-decorated grain boundaries have significantly reduced surface area available for adhesion and cohesion, which would reduce the tensile and shear strength of grain boundaries, and hence, the bulk rock. Grain boundary decoration also significantly decreased the mean distance between pores, potentially facilitating dynamic permeability. Consequently, these microstructures provide a new explanation for strain weakening and evidence of fluid flow along grain boundaries in mylonites at mid-crustal conditions.     

Microstructural evolution and grain boundary structure during static recrystallization in synthetic polycrystals of Sodium Chloride containing saturated brine

The effects of brine on recrystallization in halite are well known. However, properties of brine such as morphology, connectivity, diffusivity and the resulting influences on deformation mechanisms are still a matter of debate. This paper presents a microstructural study of dense, statically recrystallizing synthetic polycrystalline halite containing small amounts of brine. We used powders of two different grain size classes: <10 µm and 200–355 µm. The aggregates were compacted to brine-filled porosities less than about 2% and annealed at room temperature, without an external stress field.Coarse-grained samples undergo recrystallization manifested by the growth of large (up to 300 µm) strain-free grains into the deformed old grains. The new grains are frequently euhedral, with mobile grain boundaries moving at rates up to 6 nm/s. Their mobility is interpreted to be high due to the presence of water. Grain surfaces are smooth and the width of the water-rich zones is usually below the resolution of the SEM (less than 50 nm).The evolution of fine-grained samples starts with primary recrystallization and a reorganization of grain boundaries. After this stage, which lasts a few hours, normal grain growth effectively stops, and no significant increase of grain size is observed even after several months. Microstructural observations indicate contact healing at the grain boundaries, with dihedral angles ranging between 20 and 110°. We interpret these boundaries to be fluid-free, with the brine residing in a network of triple junction tubes. This system of triple junctions is interconnected and associated with significant permeability.While grain growth is inhibited in the fine-grained samples, after a few hours of annealing exaggerated grain growth is commonly initiated. This is manifested by the growth of large, euhedral grains replacing the fine-grained matrix. These grains also grow with low-index facets and their boundaries are also interpreted to be mobile due to the existence of a water-rich phase.Editorial Responsibility: J. Hoefs     

The equilibration of high-angle grain boundaries in dynamically recrystallized quartz: the effect of crystallography and temperature

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 {101&#x0304;1} 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.     

Temperature dependent grain boundary migration in deformed-then-annealed material: Observations from experimentally deformed synthetic rocksalt

Grain boundary migration between strained, substructured grains and newly appearing, strain free grains has been observed during static in-situ annealing of pre-deformed rocksalt in the SEM. With increasing temperature (T) the migration velocity increases and the character of grain boundary migration changes. As temperature increases there is an increase in the length of individual migrating boundary segments that move at similar rates. In addition, the frequency of migrating boundaries that form traces of a {100} boundary plane of at least one of the crystals involved increases, and moving grain boundaries between new and old grains change from highly irregular to smooth, straight boundaries. At the same time there is a decrease in the influence of the substructure of pre-existing strained grains on the grain boundary movement. Resultant microstructures reflect these changes. At  325–350 °C, the deformed-then-annealed microstructure is characterized by very irregular grain boundaries, a high abundance of 5–50 m scale remnants of old, substructured grains within new grains, giving a poikilitic microstructure. At  350–400 °C, grain boundaries often exhibit elongate embayments into the strained grains and most remnants of old, strained grains are located at former grain boundaries. At > 400 °C, grain boundaries between new and old, strained grains are straight to smoothly curved.The grain boundary velocity observations are explained by the effect of temperature on mobility coupled with local driving force variations. Additionally, at low annealing temperature, impurity (solute) drag and driving-force variations are influential, while at high temperature the anisotropy in grain boundary energy with crystallographic orientation becomes more important. Transferring the knowledge from our experiments to geological samples enables us to recognize and interpret similar microstructures in rocks, thereby making it is possible to relate microstructural characteristics to the pre-annealing and post-deformational annealing history.     

Synthetic [100] tilt grain boundaries in forsterite: 9.9 to 21.5°

Pure forsterite bicrystals were produced by direct bonding of highly polished single crystal plates. We fabricated a series of synthetic grain boundaries with tilt axis a and tilt angles ranging from 9.9 to 21.5°. Transmission Electron Microscope (TEM) investigations show low-angle grain boundaries with arrays of c-dislocations for all misorientations with tilt angles up to 21.5°. Low-angle grain boundary structures were fully developed at annealing temperatures <400°C and no structural changes were observed in the grain boundary region after further annealing at 1650°C.     

Grain growth and inclusion formation in partially molten carbonate rocks

To learn more about the kinetics and mechanisms of coarsening and melt inclusion formation, we investigated the effects of melt content, viscosity, and topology on the microstructural evolution of partially molten and melt-free calcite aggregates. Synthetic marbles with eutectic melts were produced by annealing mixtures of calcite and either calcium hydroxide or lithium carbonate for up to 80 h at a confining pressure of 300 MPa and temperatures of 973-1,023 K. The melts produced in the two systems are expected to differ significantly in viscosity. Generally, coarsening rates decrease with increasing melt fraction, probably because the diffusion length across melt pockets increases. Analysis of grain shapes in the samples with about 40% melt indicated that coarsening was accommodated by agglomeration in the samples of the calcium/lithium carbonate system. In the calcium carbonate/hydroxide system, classical Ostwald ripening occurred. For melt contents about 10% and below, melt-filled pores are either dropped from or dragged along with migrating grain boundaries, depending on the pore size and the grain boundary curvature. These data can be used to constrain the conditions where fluid or melt inclusions form under natural conditions. Combining our results and previous studies illustrates a systematic relation between the grain boundary mobility in calcite aggregates and the diffusion kinetics associated with second phases residing on the grain boundaries. In particular, boundaries with no porosity are most mobile, those boundaries dragging melt-filled pores are slower, those with gas-filled pores are slower yet, and those containing solid phases are slowest or may even be motionless.     

Evidence for stable grain boundary melt films in experimentally deformed olivine-orthopyroxene rocks

The microstructure of olivine-olivine grain boundaries has been studied in experimentally deformed (1200–1227 °C, 300 MPa) partially molten olivine and olivine-orthopyroxene rocks. In-situ melting produced ∼1 vol% melt in all samples studied. Grain boundary analyses were carried out using a number of transmission electron microscopy techniques. The grain boundary chemistry in undeformed olivine-orthopyroxene starting material showed evidence for the presence of an intergranular phase along some, but not all, of the olivine-olivine boundaries. In the deformed samples, ultrathin Si-rich, Al- and Ca-bearing amorphous films have been observed along all investigated olivine-olivine grain boundaries. The chemistry of the grain boundaries, which is considered to be indicative for the presence of a thin film, was measured with energy-dispersive X-ray spectroscopy (EDX) and energy-filtering imaging. The amorphous nature of the films was confirmed with diffuse dark field imaging, Fresnel fringe imaging, and high-resolution electron microscopy. The films range in thickness from 0.6 to 3.0 nm, and EDX analyses show that the presence of Al and Ca is restricted to this ultrathin film along the grain boundaries. Because thin melt films have been observed in all the samples, they are thought to be stable features of the melt microstructure in deformed partially molten rocks. The transition from the occasional presence of films in the undeformed starting material to the general occurrence of the films in deformed materials suggests that deformation promotes the formation and distribution of the films. Alternatively, hot-pressing may be too short for films to develop along all grain boundaries. A difference in creep strength between the studied samples could not be attributed to grain boundary melt films, as these have been found in all deformed samples. However, a weakening effect of grain boundary melt films on olivine rheology could not be ruled out due to the lack of confirmed melt-film free experiments. Received: 13 April 1999 / Revised, accepted: 10 February 2000     

Seven types of subgrain boundaries in octachloropropane

Subgrain boundaries in thin sheets of octachloropropane deformed at 0.7–0.8 T_M 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.     

Grain boundary plane populations in minerals: the example of wet NaCl after low strain deformation

Many physical properties of rocks are sensitive to grain size and hence to the structure of grain boundaries. Depending on their properties, such as deformation and transport behaviour, boundaries may be divided into two broad types, namely special and general grain boundaries. Electron backscattered diffraction (EBSD) is used to investigate the misorientation distributions of grain boundaries and, more recently, to determine the population of grain boundary planes. Studies on metals and ceramics suggest that the grain boundary plane, rather than the misorientation, is the key parameter when defining special and general grain boundaries. In this study, the distribution of grain boundary plane orientations has been successfully determined using EBSD for a slightly deformed, synthetic NaCl material containing 22 ppm water. Boundaries showed a preference for {100} planes, which occurred with twice the frequency of a random distribution. The grain boundary plane distributions found in NaCl were largely in agreement with studies on MgO. Grain boundaries, with a coincident site lattice (CSL) misorientation, also showed a preference for {100} planes, rather than the planes of high coincident density associated with the CSL. Three main types of boundary were identified, namely {100} twist boundaries, boundaries with {100}{hkl} planes and general {hkl}{hkl} boundaries. As the properties of these three types of boundary differ, then the transport and creep properties in wet NaCl will depend on the fraction of the different boundary types found in the grain boundary population.     

A finite element formulation for brine transport in rock salt

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.     

Melt segregation in the continental crust: distribution and movement of melt in anatectic rocks

The grain‐ and outcrop‐scale distribution of melt has been mapped in anatectic rocks from regional and contact metamorphic environments and used to infer melt movement paths. At the grain scale, anatectic melt is pervasively distributed in the grain boundaries and in small pools; consequently, most melt is located parallel to the principal fabric in the rock, typically a foliation. Short, branched arrays of linked, melt‐bearing grain boundaries connect melt‐depleted parts of the matrix to diffuse zones of melt accumulation (protoleucosomes), where magmatic flow and alignment of euhedral crystals grown from the melt developed. The distribution of melt (leucosome) and residual rocks (normally melanocratic) in outcrop provides different, but complementary, information. The residual rocks show where the melt came from, and the leucosomes preserve some of the channels through which the melt moved, or sites where it pooled. Different stages of the melt segregation process are recorded in the leucosome–melanosome arrays. Regions where melting and segregation had just begun when crystallization occurred are characterized by short arrays of thin, branching leucosomes with little melanosome. A more advanced stage of melting and segregation is marked by the development of residual rocks around extensive, branched leucosome arrays, generally oriented along the foliation or melting layer. Places where melting had stopped, or slowed down, before crystallization began are marked by a high ratio of melanosome to leucosome; because most of the melt has drained away, very few leucosomes remain to mark the melt escape path — this is common in melt‐depleted granulite terranes. Many migmatites contain abundant leucosomes oriented parallel to the foliation; mostly, these represent places where foliation planes dilated and melt drained from the matrix via the branched grain boundary and larger branched melt channel (leucosome) arrays collected. Melt collected in the foliation planes was partially, or fully, expelled later, when discordant leucosomes formed. Leucosomes (or veins) oriented at high angles to the foliation/layering formed last and commonly lack melanocratic borders; hence they were not involved in draining the matrix of the melting layer. Discordant leucosomes represent the channels through which melt flowed out of the melting layer.     

Effect of faceting on pore geometry in texturally equilibrated rocks: implications for low permeability at low porosity

The pore geometry of texturally equilibrated rocks is controlled by the interfacial energy ratio between grain boundaries and solid–liquid boundaries. Faceting at pore walls, which is a common feature of pore networks in rocks, strongly affects the liquid distribution. We investigated the effects of faceting on the equilibrium pore geometries based on image analysis of several systems with various degrees of faceting and dihedral angles. The degree of faceting was assessed by the F value, which is the ratio of the flat interface length at the pore wall to the length of total interfacial boundary between solid and liquid. The F values tend to increase with increasing liquid volume fraction. Little-faceted systems show relatively homogeneous liquid distribution. Moderately-faceted systems with a higher dihedral angle (∼55°) are characterized by development of large pores surrounded by faceted walls and complementary shrinkage of triple junction tubes, whereas strongly faceted systems with a low dihedral angle show no evidence of shrinking triple junction tubes, although most pores are surrounded by faceted pore walls. The faceted systems tend to produce more facet boundaries at the pore walls due to the difference of interfacial energies between the flat and curved surfaces. In the systems with the same degree of faceting, heterogeneity of liquid distribution tends to decrease with dihedral angle. For faceting systems, the permeability of texturally equilibrated rocks with low liquid fraction would be significantly decreased by the relative reduction of triple junction volumes or by closure of channels along grain edge due to the truncation of facet walls.     

P-T path development derived from shearband boudin microstructure

This work focuses on the development of a regional P-T-path from the Malpica-Lamego Ductile Shear Zone, NW Portugal, based on the microstructures of shearband boudins evolved during progressive simple shear. The combination of microstructural analysis, fluid inclusion studies, crystallographic preffered orientation and fractal geometry analyses, allows to link several stages in the internal evolution of the boudin to regional P-T conditions. The boudinage process is initiated under differential stress after the original layer achieved sufficient viscosity contrast relative to the surrounding matrix. Two main transformations occur simultaneously: i) change in the external shape with continuous evolution from tabular rigid body to sigmoidal asymmetric morphology (shearband boudin) and ii) localized dynamic recrystallization in the sharp-tips of the structure (acute edge of shearband boudin), and along the boudin's margin and grain boundaries. Smaller recrystallized grains, particularly in the sharp-tip domains, accommodate most of the external strain, and larger relict grains are preserved in the centre. Dynamic recrystallization under constant strain rates and strain partitioning inside the boudins is indicated by fractal geometry based on grain boundary and grain area analysis. Progressive deformation leads to the generation of structural and textural heterogeneous domains inside the boudins, and is recorded by quartz c-axis orientation analysis and fluid inclusion studies. The last deformation episode shows the final formation of the blunt-tip domain and internal secondary shear planes. The regional P-T path begins with the crystallization of andalusite after an internal shearband boudin dilation event and ends with quartz dynamic recrystallization on boudin tips. The main deformation stage (310/315 Ma) led to reactivation of internal secondary shear zones with sillimanite crystallization.     

Recognising the crystallographic signature of recrystallisation processes in deformed rocks: a study of experimentally deformed rocksalt

The microstructural development of synthetic rocksalt experimentally deformed at 100–200°C can be dominated either by grain boundary migration recrystallisation or by subgrain rotation recrystallisation, depending on water content. Samples taken from both regimes have been analysed using automated electron backscatter diffraction in order to collect crystallographic orientation and misorientation data. The frequency distribution of boundary misorientations, the boundary hierarchy characteristics and the nature of any crystallographic preferred orientation (CPO) have been used to determine the crystallographic signature of both recrystallisation processes. Dominant subgrain rotation recrystallisation results in many low to medium angle (4–20°) boundaries, a strong CPO and a continuous boundary hierarchy. Dominant grain boundary migration recrystallisation results in few low or medium angle boundaries, and a discrete boundary hierarchy. The causes of these differences and the potential application of crystallographic signatures to the study of naturally deformed rocks are discussed.     

Thin amorphous films (1–2 nm) at olivine grain boundaries in mantle xenoliths from San Carlos,Arizona

 Olivine grain boundaries and phase boundaries in xenoliths from San Carlos have been investigated by high-resolution transmission electron microscopy (HREM) and analytical electron microscopy (AEM). Thin amorphous intergranular layers with variable width (1–2 nm) were detected along olivine grain boundaries. The Al₂O₃, TiO₂ and CaO concentrations of the amorphous layers increase with increasing width of the layer. The composition of the amorphous intergranular layers depends on the interface type – grain or phase boundary. Morphology, amorphous state and chemical composition of the intergranular layer suggest the presence of a melt film at olivine grain boundaries. Since the composition of the amorphous phase strongly depends on the type of interface, the melt must have been generated at the grain boundary. Also, the melt chemistry is different from the composition of partial melts produced from possible hydrous phases, such as phlogopite or amphibole, and from the host basanite. The mobility of very thin melt films is assumed to be very limited due to the strong interface forces between the melt and the grain boundary. It is concluded that grain boundary melting occurred at the interfaces due to decompression during uplift. The melt wetted olivine grain boundaries as well as olivine-opx phase boundaries. The thin amorphous layers formed melt microsystems. Mixing of melts from different microsystems is suggested to occur in wider melt films, melt veins or melt pockets thus creating a magmatic melt that could be extracted from its source. Received: 6 November 1995 / Accepted: 24 January 1996     

Synthetic near Σ5 (210)/[100] grain boundary in YAG fabricated by direct bonding: structure and stability

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.