Development Of Subgrains Research Articles

Investigating the limits of polycrystal plasticity modeling

A material model which describes the rate-dependent crystallographic slip of FCC metals has been implemented into a quasistatic, large deformation, nonlinear finite element code developed at Sandia National Laboratories. The resultant microstructure based elastic–plastic deformation model has successfully performed simulations of realistic looking 3-D polycrystalline microstructures generated using a Potts-model approach. These simulations have been as large as 50,000 elements composed of 200 randomly oriented grains. This type of model tracks grain orientation and predicts the evolution of sub-grains on an element by element basis during deformation of a polycrystal. Simulations using this model generate a large body of informative results, but they have shortcomings. This paper attempts to examine detailed results provided by large scale highly resolved polycrystal plasticity modeling through a series of analyses. The analyses are designed to isolate issues such as rate of texture evolution, the effect of mesh refinement and comparison with experimental data. Specific model limitations can be identified with lack of a characteristic length scale and oversimplified grain boundaries within the modeling framework.

Direct observation of subgrain evolution during recovery of cold-rolled aluminium

An X-ray diffraction method is introduced for monitoring the change in volume and orientation of individual, embedded subgrains during static recovery. It is limited to plate-like specimens with a thickness of less than 100 subgrain sizes. Growth curves are presented for nine subgrains within a 38% cold-rolled aluminium specimen during annealing for 3 h at 300 °C.

Microstructure and Mechanical Properties of Strip Cast Al-Mg-Si-X Alloys

A study has been conducted on the microstructure and mechanical properties of strip cast Al6061 alloy with and without a 0.5 mass% Mn addition. The microstructures of the as-cast alloys are characterized by the presence of Si particles (<1 mm) and clusters of fine � -AlFeSi particles (<50 nm) along grain boundaries. In addition, there is the development of well-defined subgrains caused by the effects of hot rolling during strip casting. By T6 heat treatment, Si particles along grain boundaries are replaced by fine � -AlFeSi particles. There is also a formation of � -AlFeSi particles within the Al matrix by heat treatment. The addition of Mn results in an increase in the volume fraction of � -AlFe(Mn)Si particles within the Al matrix, which is associated with a corresponding decrease in the volume fraction of Mg2Si precipitates within the Al matrix over that observed in the base alloy. Accordingly, the Mn containing alloy shows a lower yield strength but a higher ultimate tensile strength due to a greater work hardening rate than the base alloy. The increased work hardening rate of the Mn containing alloy is due to the presence of uniformly distributed fine � -AlFe(Mn)Si particles within the Al matrix. The present study shows that strip casting is a viable process for the fabrication of structural Al alloys.

The Port Mouton Shear Zone: intersection of a regional fault with a crystallizing granitoid pluton

The peraluminous tonalite–monzogranite Port Mouton Pluton is a petrological, geochemical, structural, and geochronological anomaly among the many Late Devonian granitoid intrusions of the Meguma Lithotectonic Zone of southern Nova Scotia. The most remarkable structural feature of this pluton is a 4-km-wide zone of strongly foliated (040/subvertical) monzogranites culminating in a narrow (10–30 m), straight, zone of compositionally banded rocks that extends for at least 3 km along strike. The banded monzogranites consist of alternating melanocratic and leucocratic compositions that are complementary to the overall composition of that part of the pluton, suggesting an origin by mineral–melt and mineral–mineral sorting. Biotite and feldspar are strongly foliated in the plane of the compositional bands. These compositional variations and foliations originated by a process of segregation flow during shearing of the main magma with a crystallinity of 55–75%. Subsequent minor brittle fracturing of feldspars, twinning of microcline, development of blocky sub-grains in quartz, and kinking of micas demonstrate overprinting by a high-temperature deformation straddling the monzogranite solidus. Small folds and late sigmoidal dykes indicate dextral movement on the shear zone. This Port Mouton Shear Zone (PMSZ) is approximately co-linear with the only outcrops of Late Devonian mafic intrusions in the area, two of which are syn-plutonic with well-developed mingling textures in the marginal tonalite of the Port Mouton Pluton. Also closely co-linear with the mafic intrusions are a granitoid dyke that extends well beyond the outer contact of the Port Mouton Pluton, a swarm of large aligned angular xenolithic slabs, a zone of thin wispy schlieren banding, a large Be-bearing pegmatite, and a breccia pipe with abundant garnetiferous metapelitic xenoliths. In various ways, the shear zone may control all of these features. The Port Mouton Shear Zone is parallel to many other NE-trending faults and shear zones in the northern Appalachians, probably related to the docking of the Meguma Zone along the Cobequid–Chedabucto Fault system.

Effects of microstructural evolution on superplastic deformation characteristics of a rapidly solidified Al-Li alloy

This study is concerned with the effects of microstructural modification on superplastic deformation characteristics of a rapidly solidified (RS) Al-3Li-1Cu-0.5Mg-0.5Zr (wt pct) alloy. This Al-Li alloy has a very fine grain structure desirable for improved superplasticity. The results of superplastic deformation indicated that the alloy exhibited a high superplastic ductility, e.g., elongation of approximately 800 pct, when deformed at temperatures above 500 °C and at the strain rates of 10−2/s to 10−1/s. Such a high strain rate is quite advantageous for the practical superplastic forming application of the alloy. Stress-strain rate curves were obtained by performing a series of load relaxation tests in the temperature range from 460 °C to 520 °C in order to examine the superplastic deformation behavior and to establish its mechanisms. The stress-strain rate curves could be separated into two parts according to their respective physical mechanisms, i.e., grain matrix deformation and grain boundary sliding, as was proposed in a new superplasticity theory based on internal deformation variables. The microstructural evolution during superplastic deformation was also analyzed by using transmission electron microscopy. During superplastic deformation, grains were kept fine and changed into equiaxed ones due to the presence of fine secondary phase particles and the continuous recrystallization due to the development of subgrains. Consequently, the rapidly solidified (RS) alloy showed much improved superplasticity compared to the conventional ingot cast 8090 alloy.

Geochemical study of calcite veins in the Silurian and Devonian of the Barrandian Basin (Czech Republic): evidence for widespread post-Variscan fluid flow in the central part of the Bohemian Massif

Carbonate fracture cements in limestones have been investigated by fluid inclusion and stable isotope analysis to provide insight into fluid evolution and deformation conditions of the Barrandian Basin (Silurian–Devonian) of the Czech Republic. The fractures strike generally north–south and appear to postdate major Variscan deformation. The most common fracture cement is calcite that is locally accompanied by quartz, natural bitumen, dolomite, Mn-oxides and fluorite. Three successive generations of fracture-filling calcite cements are distinguished based on their petrographical and geochemical characteristics. The oldest calcite cements (Stage 1) are moderate to dull brown cathodoluminescent, Fe-rich and exhibit intense cleavage, subgrain development and other features characteristic of tectonic deformation. Less tectonically deformed, variable luminescent Fe-poor calcite corresponds to a paragenetically younger Stage 2 cement. First melting temperatures, Te, of two-phase aqueous inclusions in Stages 1 and 2 calcites are often around −20°C, suggesting that precipitation of the cements occurred from H2O–NaCl fluids. The melting temperature, Tm, has values between 0 and −5.8°C, corresponding to a low salinity between 0 and 8.9 eq. wt% NaCl. Homogenization temperatures, Th, from calcite cements are interpreted to indicate precipitation at about 70°C or less. No distinction could be made between the calcite of Stages 1 and 2 based on their fluid inclusion characteristics. In some Stage 2 cements, inclusions of highly saline (up to 23 eq. wt% NaCl) brines appear to coexist with low-salinity inclusions. The low salinity fluid possibly contains Na-, K-, Mg- and Ca-chlorides. The high salinity fluid has a H2O–NaCl–CaCl2 composition. Blue-to-yellow-green fluorescing hydrocarbon inclusions composed of medium to higher API gravity oils are also identified in some Stages 1 and 2 calcite cements. Stage 1 and 2 calcites have δ18O values between −13.2‰ and −7.2‰ PDB. The lower range of the calculated δ18O values of the ambient fluids (−3.5‰ to +2.7‰ SMOW) indicate precipitation of these cements from deeply circulating meteoric waters. The presence of petroleum hydrocarbon inclusions in some samples is interpreted to reflect partial mixing with deeper basinal fluids. The paragenetically youngest Stage 3 calcite cement has only been encountered in a few veins. These calcites are characterised by an intensely zoned luminescence pattern, with bright yellow and non-luminescent zones. Inclusions of Mn-oxides and siliceous sinters are commonly associated with Stage 3 calcite, which is interpreted to have precipitated from shallower meteoric waters. Regional structural analysis revealed that the calcite veins of the Barrandian basin belong to a large-scale system of north–south-trending lineaments that run through the territory of the Czech Republic. The veins probably reflect episodes of fluid migration that occurred along these lineaments during late stages of the Variscan orogeny.

Microstructure and texture evolution in ECAE processed A5056

Equal channel angular extrusion (ECAE) represents an effective means of microstructural refinement through severe plastic deformation. In aluminum based alloys microstructures obtained by this process generally result in interesting and remarkable combinations of mechanical properties, such as for instance enhanced strength together with high levels of ductility. In the present investigation, the evolution of the tensile properties, microstructures and crystallographic textures during ECAE has been studied for an Al-Mg based workhardening type alloy (A5056) by metallographic observation, transmission electron microscopy (TEM) and X-ray diffraction. The mechanical and microstructural characteristics of these alloys have been compared to extrudates of the same material prepared by conventional extrusion. The results show that dynamic recovery and the development of subgrains, mainly separated from each other by low angle grain boundaries, play an important role with respect to the formation of the ultrafine microstructures observed. No evidence for recrystallization and the nucleation of new grains occured up to pressing temperatures of 300°C and very sharp deformation textures were found to develop. Similar to rolling deformation a transition from a pure metal type texture to an alloy type texture in function of the deformation degree occurs. The formation of these ECAE-textures is discussed on the basis of the Taylor theory of polycrystalline deformation.

A transmission electron microscope study of the long-range effect in titanium nitride after metal ion implantation

Abstract The improvement in the properties of tools and components after ion implantation is the result of specific structural-phase states developed both in the implanted zone (IZ) and beyond into the implantation-affected zone (IAZ). The formation of defect structures beyond the implanted zone is called the long-range effect and occurs both in metals having high plasticity and low yield strength and in high-strength materials. In the present work, the microstructure of TiN coatings deposited by PVD and CVD methods is studied by transmission electron microscopy. Before dual implantation with Ni and Ti ions the PVD coating has a highly non-equilibrium submicron crystal structure with a high level of local residual stress, whereas the CVD TiN coating has a microcrystalline structure with low internal residual stress. Implantation into PVD TiN causes a relaxation of the local stress in the IZ and beyond. In contrast, in CVD TiN ion implantation leads to the development of subgrains, both within the IZ and immediately below it, in the IAZ of the coating. No additional phases are formed in either case. A possible mechanism for explaining the formation of the defect structure beyond the IZ is introduced. This is based on the emission of a dislocation flux from stress maxima developed at the IZ–IAZ interface in the form of mezo-bands.

Subgrain development in hot working of Al and Al-5Mg

Over the range 400–500 °C at 1.16 s 1, Al flow curves exhibit monotonic hardening to attain, by strains of 0.9–2.8, true steady states. The elongated helicoidal grains were observed on planes normal to the radius by both SEM-EBSI and by TEM. The equiaxed subgrain sizes in Al exhibited a log normal distribution, as temperature rises, the mean and S.D. rise so that the distribution widens considerably at the upper end. In contrast for Al-5Mg, the flow curves rise to a peak and decline slowly at a diminishing rate, the maximum strength is about four times that of Al. In TEM, the dislocations are in long planar arrays at ε = 0.2 whereas the SEM-EBSI indicates only indistinct distortions. At 0.45 the subgrains are very elongated. At 1.8 and 2.8, the subgrains are equiaxed at 1.4 μm. being about 1/3 that of Al (~3.9 μm).

On grain boundary facetting under stress during high temperature deformation of Al and Al-Ga alloys

Aluminuim of high purity and aluminium with 50 ppm gallium were deformed under various strain rates and temperatures in order to investigate the possible influence of liquid like grain boundaries on grain boundary sliding and macroscopic deformation behaviour. Under all conditions, serrated grain boundaries were observed to develop during the tensile test. Those serrated structures were correlated to the development of subgrains observed by optical metallography and transmission electron microscopy. In the absence of an applied stress, further annealing lead to disappearing of the serrated grain boundaries when the annealing temperature was higher than the tensile test temperature under which the serrations were developed.

&lt;I&gt;In-situ&lt;/I&gt; SEM Observation of Microstructural Evolution of High Purity Aluminum Deformed at Elevated Temperatures

The subgrain evolution in high purity aluminum (4N) has been observed in-situ during high temperature deformation, by means of the crystallographic contrast obtained from the backscattered electron signal of the scanning electron microscope (crystallographic contrast technique). It was observed that the substructure on the surface of the deformed sample represents the substructure developed inside the material. Evidence of increasing misorientation between adjacent subgrains and grain boundary migration with increasing strain is reported. This subgrain structure remains unchanged at room temperature when the applied stress is maintained during cooling. The crystallographic contrast technique demonstrates to be appropriate to measure subgrain size. Subgrain size values similar to those obtained by transmission electron microscopy are reported when the method of images superposition is incorporated to this technique.

Examination of microstructural features of Norwegian cataclastic rocks and their use for predicting alkali-reactivity in concrete

The microstructural features of various Norwegian rock samples have been studied and quantified by different techniques. The majority of the rocks studied were different types of cataclastic rocks. Expansion results from a modified version of the NBRI Mortar-Bar Test for alkali reactivity are used for correlation with the quantitative parameters. The aim is to identify microstructural features in the various rock types which promote the alkaliaggregate reaction, and to subsequently use these findings to improve and provide guidelines for engineering practice in order to predict and make more accurate determinations of potentially reactive aggregates. The deformation processes of rocks involve the general process of straining of quartz, grain size reduction and subgrain development. This is characteristic of mylonitisation. This study demonstrates that the grain size reduction of quartz enhances reactivity by increasing the surface area of quartz grain boundaries available for reaction, and thus giving an overall increase in surface energy. Subgrain development will, besides the high surface area, contribute even more to an enhanced reactivity, due to the high dislocation density associated with the quartz subgrain boundaries. The total grain boundary area of quartz, and the mean grain size of quartz appear to be the most favourable quantitative parameters related to the expansion of different rock types. The measurement of the total grain boundary area of quartz, will enhance the value and improve the effectiveness of the petrographical examination as an engineering tool to screen potentially reactive aggregates.

Origin, timing, and fluid characteristics of an auriferous event; the Proterozoic Jasper lode gold deposit, Saskatchewan, Canada

The Jasper mesothermal gold deposit, in northern Saskatchewan, Canada, is situated within the Proterozoic La Ronge domain of the Trans-Hudson orogen. The deposit occurs within a northeasterly striking, steeply northwest-dipping, mylonitic shear zone that crosscuts the Island Lake pluton, which has a zircon Pb-Pb age of 1855 + or - 8 Ma. Gold mineralization, which is hosted by subvertical quartz veins that occupy the most strained portions of the shear zone, occurs in late fractures and as particles within recrystallized quartz. The gold mineralized zones are 2 to 3 m wide and plunge steeply to the northeast. Mineralized veins have a complex paragenesis wherein recrystallized quartz, muscovite, sphalerite, galena, and chalcopyrite are paragenetically associated with gold. Obvious alteration of the granitic host rock adjacent to the veins is lacking except for slight 18 O enrichments of wall rocks up to 20 m from the veins. Quartz veins which occur peripheral to the most strained regions of the shear zone are discontinuous bands with little to no subgrain development and do not contain gold. The delta 18 O values of this barren quartz average 11.1 per mil, whereas the delta 18 O values of quartz comprised of at least 50 percent subgrains, and associated with gold mineralization, average 12.9 per mil. Oxygen isotope fractionation between quartz and coexisting muscovite associated with gold indicate gold mineralization occurred at ca. 300 degrees C. Calculated delta 18 O and delta D values for the ore-forming fluid are compatible with fluids derived from, or which have interacted with, igneous or metamorphic rocks at low water/rock ratios. The initial 87 Sr/ 86 Sr ratio of the auriferous fluid is 0.7028, which is similar to the surrounding igneous rocks, and implies that the strontium was most likely derived by leaching of Proterozoic rocks similar in age and strontium isotope composition to those presently exposed on surface. Two-phase secondary aqueous fluid inclusions outline and dominate healed fracture planes in both barren quartz veins and in the less deformed portions of mineralized quartz veins. The fluids in these inclusions have low salinities, generally not exceeding 7 wt percent NaCl equiv, and homogenization temperatures of...

Strain‐related differences in the crystal growth of white mica and chlorite: a TEM and XRD study of the development of metapelitic microfabrics in the Southern Uplands thrust terrane, Scotland

Abstract TEM and XRD techniques were used to study crystal growth characteristics of the fabric‐forming phyllosilicates which developed in response to low‐grade metamorphism and tectonic imbrication in part of the Southern Uplands thrust terrane. Prograde regional metamorphism, ranging from late diagenesis through the anchizone to the epizone, was accompanied by the development of a slaty cleavage which is commonly bedding‐parallel. TEM‐measured mean thicknesses of white mica and chlorite crystallite populations increase with advancing grade and correlate with XRD‐measured crystallinity indices. Analytical TEM data show that prograde changes in composition lead to a net loss of Si, Ca and minor Fe from the fabric‐forming phyllosilicates. White micas are paragonite‐poor phengites with a mean b lattice parameter of 9.037 Å, and indicate an intermediate pressure series of metamorphism with a field gradient of &lt;25° C km‐1. Chlorite compositions evolved from diabantite (with intergrown corrensite) to ripidolite over an estimated temperature range of 150–320° C. Field gradient and temperature estimates suggest that crystal growth and fabric development occurred at burial depths ranging from 6 km to at least 13 km in the thrust terrane. During late diagenesis, crystal growth of white mica and chlorite was predominantly a consequence of polytypic and phase transitions, and resulted in similar size distributions which resemble typical Ostwald ripening curves. Under anchizonal and epizonal conditions, white mica grew more rapidly than chlorite because of its greater ability to store strain energy and recover from subgrain development; as a result crystal thickness distributions are not typical of Ostwald ripening. In contrast, chlorite crystals which grew under these conditions developed subgrain boundaries at high strain rates which were only partially recovered at low strain rates; these retained dislocations reduce the crystallite thicknesses detected by TEM and XRD, compared with those of white mica. These differences in strain‐induced crystal growth indicate that white mica (illite) and chlorite crystallinity indices are likely to show significant differences where low‐grade metamorphism is closely associated with tectonic fabric development.

Compositions of garnet and spinel from the Aries diamondiferous kimberlite pipe, central Kimberley Block, Western Australia — implications for exploration

Aries pipe is a weathered, micaceous kimberlite (≈820 Ma) consisting of four lobes: South, Central, North and North Extension. It is the largest (≈18 ha) and most diamondiferous of the few kimberlites currently known in Australia. The pipe contains abundant country-rock (basalt, quartzite) xenoliths, but the only mantle-type inclusions so far recovered are individual grains of Cr-spinel, rare chromian pyrope-garnet and diamond. Aries differs from other kimberlites of similar age in the Kimberley Block by the absence of megacrystic minerals. Chromian pyrope-garnet has compositions which indicate garnet lherzolite and garnet wehrlite in the upper mantle, but harzburgitic “G10” garnets considered the paramount indicator of diamondiferous kimberlite in southern Africa have not been recovered at Aries. Spinel has been the most important indicator mineral in the discovery of Aries. It can be distinguished, morphologically and compositionally, from spinel in local basaltic country-rocks. The spinels in the pipe have a range of internal features which can be allocated into six types. Some morphological varieties show apparent subgrain development, with 120° triple point junctions, indicating post crystallisation deformation and recrystallisation, whereas others have textures suggesting rapid crystallization from a chromium-saturated melt. Compositional variations suggest that the six morphological types can be combined into two broader classes. Class 1 spinel compositions are consistent with derivation from garnet peridotite in the upper mantle and include some which are similar to diamond-associated spinels in other deposits world-wide. As such, the most Cr 2O 3-rich Class 1 spinels have been used during exploration to indicate the potential of the Aries pipe to host diamond. Class 2 spinels are more Fe-rich than Class 1 spinels and the compositions are unlike any described to date in the common rock types from the upper mantle, but the internal textures suggest that these are xenolithic in origin. Both Class 1 and Class 2 spinels may be associated with Ti-rich Cr-spinel which is compositionally similar to titaniferous Cr-spinel from the groundmass. Class 1 spinels are typically cross-cut and rimmed by Ti-rich Cr-spinel and may have originated from either metasomatised, Cr-spinel rich dunite or metasomatised harzburgite. Class 2 spinels are only rimmed by Ti-rich Cr-spinel. The Ti-rich variety which either cross-cuts or rims the xenolithic spinels probably developed during metasomatic/magmatic events in the mantle, and is perhaps related to kimberlite formation.

Effects of grain boundary deformation on the creep transition under the power law regime in an AlMg solid solution alloy

In an Al-3wt.% Mg alloy, the transition creep behaviour was investigated at 573 K (0.62 T m) under the stress range 10–55 MPa. The stress exponent n increases with increasing stress from about 3 to 5, and the stress required for the onset of the transition behaviour is found to decrease with decreasing grain size. Under the condition of viscous-glide-controlled creep, subgrains are developed near corrugated grain boundary and, in these deformed regions, creep deformation may be controlled by dislocation climb. Therefore the creep deformation may proceed by the parallel processes of viscous glide and dislocation climb. Subgrains developed near the boundary may promote the transition in stress exponent. Since the volume fraction of these subgrains increases with decreasing grain size, the transition may occur at lower stresses with decreasing grain size. Consequently, it can be suggested that in a polycrystalline aggregate the development of subgrains near grain boundaries may govern the transition behaviour even though, at higher stresses, dislocations locally can break away from their retarding solute clouds, as has been previously reported.

Effect of precipitate type and morphology on high-temperature creep and internal stress in Al-Li based alloys

The tensile creep of a series of aluminium-lithium-based alloys, two binary alloys containing δ′ precipitate, and the 2090 alloy containing δ′ and T1 precipitate, has been studied over a range of stresses at 150°C. In some cases the internal stress developed during creep has been determined using the strain transient dip test. The results have been compared with similar data previously obtained for the 8090 alloy containing δ′ and S precipitates. The solid solution alloy and the binary alloy containing shearable δ′ particles exhibited normal Class II behaviour, with the development of sub-grains and a stress dependence of the creep rate given by a single stress exponent,n, between 4 and 5 at all applied stresses. The alloys containing particles not easily sheared by dislocations, coarse δ′, S and T1, exhibited similar stress dependencies of the creep rate at low stresses but exhibited large values ofn, between 18 and 35 at high stresses. The internal stress, σi, in these alloys was found to be approximately constant at high stresses possibly due to partial shearing of the coarse δ′, T1, and the S on sub-boundaries. The stress dependence of the minimum creep rate, $$\dot \varepsilon $$ , could be represented at all applied stresses, σa, by $$\dot \varepsilon \propto (\sigma _a - \sigma _\iota )^n $$ , where (σa−σi) is the effective stress driving dislocations during creep, andn is a single stress exponent of between 5 and 6 for all applied stresses. The internal stress, which increases with applied stress, at least at a low applied stress, arises from inhomogeneity of plastic deformation, due to hard sub-boundaries or hard particles which are Orowan looped. These two types of contribution to the internal stress are of similar magnitude in the alloys containing coarse δ′ and T1 but the majority of the internal stress in the 8090 alloy may arise as a result of the hardening of sub-boundaries by the S precipitate.

High temperature creep resistance of austenitic heat-resisting steels

In this overview on the high temperature creep resistance of austenitic heat-resisting steels, the main focus is on the creep resistance rather than on the rupture strength. The topics covered range from single-phase austenitic alloys without a second phase to multiple-phase austenitic alloys with precipitates. In the former category, the effect of the grain size on the minimum creep rate and the effect of subgrain evolution on the tertiary creep stage are reviewed. Then the solid solution strengthening due to both substitutional and interstitial alloying elements is discussed. In the second category, precipitation strengthening with an emphasis on newly identified intergranular precipitation strengthening is discussed.

Modification of sedimentary barite textures during deformation, Gataga District, NE British Columbia

Stratiform BaFeZnPb mineralisation in the Gataga District of northeast British Columbia, is hosted within folded and thrust fine-grained black siliciclastics of the Mid-Upper Devonian Lower Earn Group. Deformation of the barite at 280°–350°C and 2–2.5 kb was associated with the formation of chevron-style folds, an intense pressure solution cleavage and localised development of barite mylonites. Although the barite-rock has retained its gross stratiform characteristics, this textural investigation demonstrates that it has undergone considerable internal remobilisation in that both the primary texture and chemistry have been modified. Low tectonic strains are associated with the development of a grain shape fabric and dissolution seams. With increasing strains, subgrain development and recrystallisation become dominant. Moderate strains are related to the development of microfolds and a polycrystalline aggregate with a strong axial planar grain-shape fabric. Dynamic recrystallisation, extreme grain-size reduction and development of a preferred crystallographic fabric is associated with high shear strains and the formation of barite mylonite containing abundant kinematic indicators. At low to moderate strains remobilisation of barite was dominated by fluid-assisted diffusive mass transfer. Dissolution and diffusion in the presence of a high-salinity basinal brine is proposed as the main cause for the initial alteration of the primary barite texture and chemistry. Remobilisation by intracrystalline plasticity at moderate to high strains was responsible for wholesale recrystallisation and obliteration of primary textures and places doubt on the recognition of ‘graded’ and ‘cross-bedded’ features in polycrystalline barite.

Development of microporosity, diffusion channels and deuteric coarsening in perthitic alkali feldspars

Turbidity is an almost universal feature of alkali feldspars in plutonic rocks and has been investigated by us in alkali feldspars from the Klokken syenite using SEM and TEM. It is caused by the presence of myriads of tubular micro-inclusions, either fluid-filled micropores or sites of previous fluid inclusions, and is associated with coarsening of microperthite and development of sub-grains. Micropores are abundant in coarsened areas, in which porosities may reach 4.5%, but are almost absent from uncoarsened, pristine braind-microperthite areas. The coarsening is patchy, and involves a scale increase of up to 103 without change in the composition of the phases, low albite and low microcline, or in the bulk composition of the crystal. It occurs abruptly along an irregular front within individual crystals, which retain their original shapes. The coherent braid microperthite gives way across the front to an irregular semi-coherent film perthite over a few μm and then to a highly coarsened irregular patch perthite containing numerous small sub-grains on scales of a few hundred nm, in both phases. The coarsening and micropore formation occured at a T≤400°–450° C and it is inferred to have been driven by the release of coherent strain energy, low-angle grain-boundary migration being favoured by a fluid. The patchy nature of the coarsening and the absence of a relationship with initial grain boundaries suggest that the fluid was of local origin, possibly arising in part through exsolution of water from the feldspar. The sub-grain texture and microporosity modify profoundly the permeability of the rock, and greatly enhance the subsequent reactivity of the feldspars.