Experimental deformation of sintered albite above and below the order-disorder transition

Abstract

Purified albite powder (44-53 μm) has been sintered to form an albite polycrystal suitable for deformation studies close to the melting temperature. Experiments have been carried out in Griggs solid medium deformation apparatus at 800, 1020 °C and 700 MPa pressure in a dehydrating pyrophyllite confining medium at constant strain rates of 10^?4, 10^?5, 10^?6 and 10^?7/s. At 800 °C the samples were brittle-ductile whereas at 1020 °C they were ductile with a rheology well described by a power law with a stress exponent of 3. The transition from brittle-ductile to ductile also coincided with the order-disorder or low-high albite transition, as indicated by the marked increase in mechanical twinning on the albite law at high temperature. At 1020 °C high dislocation densities (10^?10 - 10^?11/cm² and mechanical twinning characterised the original high albite grains, whereas fine recrystallised grains ( < 5 цm) had low dislocation densities (10⁷ - 10⁸/cm²) and often contained polysynthetic albite and pericline (M-type) twins. It is suggested that the recrystallized grains were monalbite (monoclinic) under test conditions which have inverted to high albite (triclinic) and in so doing produced M-twins, and that the recrystallization mechanism involved grain-boundary bulging to nucleate new high angle boundaries. The implications of the order-disorder transition for twinning and grain boundary migration are discussed and it is suggested that the data cannot be simply extrapolated to natural deformation in the low albite field.     

Low-Temperature Plasticity of Naturally Deformed Calcite Rocks

Optical, cathodoluminescence and transmission electron microscope (TEM) analyses were conducted on four groups of calcite fault rocks, a cataclastic limestone, cataclastic coarse-grained marbles from two fault zones, and a fractured mylonite. These fault rocks show similar microstructural characteristics and give clues to similar processes of rock deformation. They are characterized by the structural contrast between macroscopic cataclastic (brittle) and microscopic mylonitic (ductile) microstructures. Intragranular deformation microstructures (i.e. deformation twins, kink bands and microfractures) are well preserved in the deformed grains in clasts or in primary rocks. The matrix materials are of extremely fine grains with diffusive features. Dislocation microstructures for co-existing brittle deformation and crystalline plasticity were revealed using TEM. Tangled dislocations are often preserved at the cores of highly deformed clasts, while dislocation walls form in the transitions to the fine-grained     

Texture development of recrystallised quartz polycrystals unravelled by orientation and misorientation characteristics

The development of microstructures and textures (i.e. crystallographic preferred orientations) during recrystallisation of naturally deformed quartz polycrystals has been studied via electron diffraction techniques in the scanning electron microscope. In the investigated sample series of quartz-rich rocks originating from different deformation regimes, the microstructural and textural changes in quartz have been significantly influenced by dynamic recrystallisation. Based on microstructural observations paired with orientation and misorientation analyses down to the scale of grains and subgrains, criteria could be established which characterise the dominant recrystallisation process and its influence on texture development. It is shown that the texture development during dynamic recrystallisation is controlled by a differential activation of slip systems in grains of ‘soft’ and ‘hard’ orientations. The analyses provide further evidence that specific grain orientations are preferred during crystal plastic deformation, recrystallisation and grain growth. The influence of twinning after the Dauphiné law was also investigated. Observations of a progressive reduction in the population of Dauphiné-twin boundaries during recrystallisation and a penetrative deformation in both hosts and twins indicate a generation prior to deformation and recrystallisation. A mechanical origin for twinning and possible influence on texture development was therefore discarded.     

An electron microscopic study of some twinning and exsolution textures in microcline amazonites

An electron microscopic study of the twinning and exsolution textures in one hydrothermal and two pegmatitic amazonites from southern Norway is reported. From observations of three types of interfaces: between twins, between domains twinned on different laws and between Na- and K-feldspar domains, perthitic textures involving low albite lenses up to 10 μm long in a twinned microcline matrix are shown to result from exsolution from the triclinic phase. Complex twinning on albite and pericline laws is suggested to be the result of strains induced by exsolution. Other textures, involving the late introduction of Na at low temperature in a pegmatite and the occurrence of Pb-rich and plagioclase crystallites in a hydrothermal amazonite, are described.     

Plagioclase twin laws and fabrics in three specimens of anorthosite

Twinning patterns and petrofabrics of plagioclases are examined in three specimens of anorthosite from the Bushveld Complex, the Quebec Massif, and the Fiskenaesset Complex. Their plagioclases have petrographical characteristics exhibiting their different petrogeneses.In an anorthosite from the Bushveld Complex, plagioclase grains are twinned after the albite-Carlsbad, pericline, albite and Carlsbad laws. Frequency percentage of the albite-Carlsbad and Carlsbad laws reaches 43% Plagioclase grains in the adcumulate layers are developed with their composition plane (010) subparallel to the cumulate plane, whereas those in the heteradcumulate layers are developed with their composition plane (010) subperpendicular to the cumulate plane.In an equigranular anorthosite from the Quebec Massif, plagioclase grains are polysynthetically twinned after the albite and pericline laws with rare examples of the albite-Carlsbad and Carlsbad laws. Frequency percentage of the latter two laws is only 1% together. Some regularities are recognized in the petrofabrics of c-axis and (010) plane.In a calcic anorthosite from the Fiskenaesset Complex, plagioclase grains are polysynthetically twinned, exclusively according to the pericline law or a combination of pericline and albite laws. The pericline law is predominant and reaches 64% and this twinning pattern cleaarly differs from that of the former two anorthosites.     

Chemical etching of deformation sub-structures in quartz

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.     

Microstructures and flow mechanisms in regional metamorphic rocks of Japan

A number of microstructural features indicate a difference in the dominant deformation mechanism between the higher temperature Ryoke and the lower temperature Sambagawa and Shimanto metamorphic belts of Japan. The microstructures of metacherts containing deformed radiolaria are divided into two types: in both the Sambagawa and Shimanto belts the quartz grains are tabular while in the Ryoke belt they are equiaxed. TEM studies of these metacherts revealed that the tabular grains contain abundant subboundaries consisting of large numbers of network dislocations and bowe-out dislocations, while the equiaxed grains contain no subboundaries and have low densities of dislocations which are not bowed-out. There is a corresponding difference in the textures (lattice preferred orientation of quartz): the Ryoke metacherts display randomly distributed c-axes of quartz, while the Sambagawa and Shimanto metacherts show conspicuous crossed girdle patterns with some asymmetry. There is a third difference between these regions: in the metacherts of the Ryoke metamorphic belt, the strain magnitudes determined from deformed radiolaria increase with increasing volume fraction of mica in the same metamorphic P and T conditions, while in the Sambagawa and the Shimanto metamorphic cherts the strain magnitudes decrease with increasing the mica fraction.These microstructures, textures, and rheological behaviours of quartz-mica rocks suggest a change of deformation mechanism between the lower temperature Sambagawa and Shimanto, and the higher temperature Ryoke metamorphic belts. Since random fabrics of c-axes of quartz are inconsistent with lattice rotation due to dislocation glide, the Ryoke metacherts may have deformed by pressure-solution.     

Strain partitioning and deformation mode analysis of the normal faults at Red Mountain, Birmingham, Alabama

In a low-temperature environment, the thin-section scale rock-deformation mode is primarily a function of confining pressure and total strain at geological strain rates. A deformation mode diagram is constructed from published experimental data by plotting the deformation mode on a graph of total strain versus the confining pressure. Four deformation modes are shown on the diagram: extensional fracturing, mesoscopic faulting, incipient faulting, and uniform flow. By determining the total strain and the deformation mode of a naturally deformed sample, the confining pressure and hence the depth at which the rock was deformed can be evaluated. The method is applied to normal faults exposed on the gently dipping southeast limb of the Birmingham anticlinorium in the Red Mountain expressway cut in Birmingham, Alabama. Samples of the Ordovician Chickamauga Limestone within and adjacent to the faults contain brittle structures, including mesoscopic faults and veins, and ductile deformation features including calcite twins, intergranular and transgranular pressure solution, and deformed burrows. During compaction, a vertical shortening of about 45 to 80% in shale is indicated by deformed burrows and relative compaction of shale to burrows, about 6% in limestone by stylolites. The normal faults formed after the Ordovician rocks were consolidated because the faults and associated veins truncate the deformed burrows and stylolites, which truncate the calcite cement. A total strain of 2.0% was caused by mesoscopic faults during normal faulting. A later homogenous deformation, indicated by the calcite twins in veins, cement and fossil fragments, has its major principal shortening strain in the dip direction at a low angle (about 22°) to bedding. The strain magnitude is about 2.6%. By locating the observed data on the deformation mode diagram, it is found that the normal faulting characterized by brittle deformation occurred under low confining pressure (< 18 MPa) at shallow depth (< 800 m), and the homogenous horizontal compression characterized by uniform flow occurred under higher confining pressure (at least 60 MPa) at greater depth (> 2.5 km).     

The effect of ductile deformation on the kinetics and mechanisms of the aragonite-calcite transformation

Abstract The effect of ductile deformation (dislocation creep) on the kinetics of the aragonite-calcite transformation has been studied at 1 atm (330° C and 360° C) and 900-1500 MPa (500° C) using undeformed and either previously or simultaneously deformed samples (500° C and a strain rate of 10^-6 s). Deformation enhances the rate of the transformation of calcite to aragonite, but decreases the rate of transformation of aragonite to calcite. The difference results from a dependence of transformation rate on grain size, coupled with a difference in the accommodation mechanisms, climb versus recry-stallization, of these minerals during dislocation creep. Dislocation climb is relatively easy in calcite and thus plastic strain results in high dislocation densities without significant grain size reduction. The rate of transformation to aragonite is enhanced primarily because of the increase in nucleation sites at dislocations and subgrain boundaries. In aragonite, on the other hand, dislocation climb is difficult and thus plastic strain produces extensive dynamic recry-stallization resulting in a substantial grain size reduction. The transformation of aragonite is inhibited because the increase in calcite nucleation sites at dislocations and/or new grain boundaries is more than offset by the inability of calcite to grow across high angle grain boundaries. Thus the net effect of ductile deformation by dislocation creep on the kinetics of polymorphic phase transformations depends on the details of the accommodation mechanism.     

Post-deformational annealing of calcite rocks

The evolution of microstructure and crystallographic preferred orientation (CPO) during post-deformational annealing was studied on three calcite rock types differing in purity and grain size: Carrara marble (98% calcite, mean grain size of 115 μm), Solnhofen limestone (96%, 5 μm) and synthetic calcite aggregates (99%, 7 μm). Samples were first deformed in torsion at 727 °C at a shear strain rate of 3 × 10^{− 4} s^{− 1} to a shear strain of 5 and subsequently heat-treated at 727 °C for various durations between 0 and 24 h. Microstructures and CPOs were analysed by optical microscopy, image analysis and electron backscatter diffraction (EBSD).All rock types deformed in the dislocation creep field at the same applied conditions, but their microstructures and CPOs after deformation and after annealing differed depending on starting grain size and material composition. In Carrara marble and in the synthetic calcite aggregate, a strong CPO developed during deformation accompanied by dynamic recrystallisation with significant changes in grain size. During annealing, widespread grain growth and subtle changes of CPO occurred, and equilibrated foam microstructures were approached after long annealing times. The CPO is the only feature in annealed samples indicating an earlier deformation phase, although it is not always identical to the CPO formed during deformation. In the more impure Solnhofen limestone, secondary phases on grain boundaries suppressed grain boundary mobility and prevented both the formation of a recrystallisation CPO during deformation and grain size modification during deformation and annealing.     

Mechanical twinning and slip in experimentally deformed plagioclases

Mechanical albite and pericline twins were induced in seven feldspars, (Na, Ca) (Al, Si)₄O₈, in triaxial compression tests at 800° C, 8 to 10 kb confining pressure. Two were initially in a high temperature structural state (disordered) (An₁, An₅₉). Three (An₃₂, An₃₉, and An₅₃) were slightly disordered, and two (An₇₆ and An₉₅) had the transitional and primitive anorthite structure, respectively. No microscopic twinning was detected in comparable tests on ordered albite and oligoclase (An₁, An₂₀), feldspars in the peristerite range (An₁₁ An₁₄) or in a low structural state labradorite (An₅₁). Other deformational features include lamellae 60 to 80° from the c axis and (010) slip in the opposite sense for (010) twinning.Work performed under the auspices of the U. S. Atomic Energy Commission.     

Deformation mechanisms in experimentally deformed plagioclase feldspars

Optical and transmission electron microscopy have been used to study the microstructures in a series of plagioclase feldspars which had been experimentally deformed in compression. The observations show that deformation takes place by three mechanisms: (1) brittle fracture, (2) twinning and (3) slip due to the generation and motion of dislocations. Optical “deformation lamellae” are shown to be due to bunches of microfractures and to walls of tangled dislocations. Twins and fractures are often intimately associated and dislocations are often generated at fracture steps or voids. Moving dislocations apparently always generate a strip of fault in the slip plane. This, together with structural considerations as well as the visibility of dislocations (under various diffracting conditions) and the orientation of the applied stress, has made it possible to determine the slip systems which have operated in the deformed specimens.     

Fabrics of deformed sulphides

Determination of preferred orientations in sulphide ores is an important facet in the analysis of the deformation and metamorphism in ore deposits. The methodology and problems of texture determinations in deformed sulphides are briefly reviewed. Deformation mechanisms and texture development in the common sulphide minerals are summarised. Axi-symmetric experimental deformation of galena, sphalerite and chalcopyrite produces similar pole figures with the (110) planes aligned normal to the compression axis. Deformation textures in naturally deformed sulphides however are best preserved in monomineralic ores which have undergone simple shear deformation. These textures can be correlated with the microstructures found in the sulphides. Computer simulations of deformation textures arising from dislocation mechanisms can be used to interpret the textures but the results to date are preliminary. These theoretical models emphasize the importance of the symmetry of the kinematic axes of the deformation, the operative glide systems and the strain history and the deformation path. Little is known, however, of the influence of static and dynamic recrystallisation on sulphide preferred orientations.     

Dynamic recrystallization and chemical evolution of clinoamphibole from Senja,Norway

Clinoamphibole from a mylonitic amphibolite exhibits microstructures characteristic of dynamic recrystallization, including porphyroclasts in a finer grained matrix of needle-shaped amphibole. The matrix amphibole defines an LS fabric and porphyroclasts have core and mantle structures with a core containing undulose to patchy extinction and (100) deformation twinning surrounded by a mantle of recrystallized grains. In addition intragranular grains also occur within the cores. TEM analyses of the porphyroclasts reveal that they contain a wide variety of lattice defects including high densities (5 × 10⁸cm^–2) of free dislocations and dislocation arrays, dissociated dislocations, stacking faults, and (100) micro-twins. TEM also shows that matrix grains and intragranular grains have relatively low defect densities, and that the intragranular new grains occur at localities in the porphyroclasts characterized by high densities of dislocations. These observations along with the chemical and orientation relationships between the recrystallized grains and porphyroclasts indicate that the new grains may have formed by heterogeneous nucleation and that further growth probably occurred by both strain assisted and chemically induced grain boundary migration or liquid film migration. This recrystallization event is interpreted to be synkinematic based on the fact that no recrystallization textures are present in the matrix grains and that the matrix grains define an LS fabric. However, the low defect densities in the matrix grains and the lack of intracrystalline strain in other phases indicate that post-kinematic recovery processes were active.     

Combining ECCI and FIB milling techniques to prepare site-specific TEM samples for crystal defect analysis of deformed minerals at high pressure

Dislocation microstructures in experimentally deformed single-crystal pyrope-rich garnet, (Mg,Fe)₃(Al,Cr)₃Si₃O₁₂, and polycrystalline forsterite, Mg₂SiO₄, were investigated by using electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM) combined with a focused ion beam (FIB)-microsampling. In the orientation-optimized ECCI method, we successfully observed individual dislocations across subgrain boundaries in a low-atomic-number mineral, pyrope-rich garnet (averaged Z-numbers, AZs ～ 10). Dislocations in a deformed forsterite (iron-free olivine) were also visible in the ECCI. In the ECCI on the single-crystal garnet, deformation bands consisting of dislocations, unusual contrasts in stripes and inhomogeneous distributions of sub-micrometer-sized pores were found. Further site-specific TEM observation on the deformation band revealed a high density of partial dislocations and stacking fault ribbons. The site-specific characterizations from ECCI to TEM, with assistance of FIB, can provide a new approach to investigate dislocation microstructures of deformed materials at high pressure and high temperature.     

Grain and sub-grain size variations across a mylonite zone

The results are reported of a combined optical and electron microscopy study of microstructural variations across a quartz mylonite zone with increasing shear strain. The mylonite developed by recrystallization of the deformed quartz grains with increasing shear strain. It was found in a given specimen that the size of recrystallized grains and of sub-grains were always smaller in electron micrographs. The possible reasons for this are discussed. The size of both features decreased with increasing shear strain irrespective of the microscope used. However the density of unbound dislocations remained constant. A marked grain size reduction occurred in phyllosilicate rich layers. Variations in sub-grain size were observed within the relict old grains which remained at low shear strains. These are thought to reflect stress intensification adjacent to grain boundaries during deformation. The relict grains recrystallized at higher strains. Stresses were estimated from grain and sub-grain sizes and from the dislocation density. The results indicate that estimates based on grain size are unreliable if phyllosilicates inhibit the growth of grains during recrystallization, and that the dislocation densities are altered during uplift and are unlikely to give meaningful estimates. It is also concluded that the microstructures reflect stress gradients present during the formation of the mylonite, that is the initiation and propagation of the shear zone and that these were subsequently replaced by strain rate gradients.     

Electron microscope study of peridotite xenoliths in kimberlites

Dislocation structures in naturally deformed olivine from garnet peridotite xenoliths from South African kimberlites have been studied by electron microscopy. The substructure consists mainly of straight subboundaries of dislocations with Burgers vectors [001]. Most of the dislocations have both edge and screw components, and the slip planes are mainly (100). The dislocation density between the subboundaries is low.The slip planes in olivine are discussed in relation to the olivine structure. The observed dislocation structures seem to indicate that the large difference in strain rate between natural and experimental deformation will produce a difference in the slip mechanisms.The nature of the deformation lamellae visible in optical microscope is discussed.     

大别山双河地区超高压变质岩矿物超微构造的HRTEM研究

报道了大别山双河地区超高压榴辉岩和硬玉石英岩矿物超微构造及缺陷结构的高分辨透射电镜研究结果。在天然变形绿辉石晶内的自由位错、位错倾斜壁、位错环、位错网、亚晶界、堆垛层错等亚构造中 ,发现了纳米级水分子团 ,这种球形状水分子团包体的存在是导致绿辉石晶体水解弱化和塑性变形的重要因素。在榴辉岩矿物中广泛发育的层错、(10 0 )变形双晶、晶畴结构、界面与晶面的交叉滑移、晶格畸变等变形构造及缺陷结构 ,指示超高压岩石经历了快速折返。在硬玉单晶大约 5 0 0nm的微晶畴内 ,发现了C2 /c和P2 1/n两种结构 ,C2 /c结构的晶体学参数对应于硬玉 ,而P2 1/n结构的晶体学参数对应于绿辉石 ,纳米级P2 1/n出溶结构的存在 ,表明在退变质过程中 ,硬玉在纳米尺度上部分转变为绿辉石 ,并且未能达到平衡。也说明在主体岩石的抬升过程中 ,硬玉晶体伴随有复杂的非平衡退变质作用。对于大别山超高压变质岩的p T轨迹描述及其构造解释具有重要意义。     

Dissolution kinetics of experimentally shocked silicate minerals

The effect of lattice strain on mineral dissolution rates was examined by comparing the dissolution rates of shocked and unshocked minerals. Labradorite, oligoclase and hornblende were explosively shocked at mean pressures ranging from 4 to 22 GPa. The labradorite was examined with transmission electron microscopy to estimate the density of dislocations produced by the shock-loading experiment. Subsamples of the labradorite were then thermally annealed to remove some of the dislocations, and to evaluate the importance of such thermal pre-treatment in preparing mineral surfaces for experiments. The dissolution rates of these minerals were measured in batch experiments at pH-values of 2.7 and 4.0.

Shock-loading did not produce extremely high dislocation densities in the labradorite. The density of dislocations in the unshocked labradorite is ≤ 10¹⁰ m⁻². After shocking, the density increases to 10¹²-10¹³ m⁻². The distribution of dislocations is heterogeneous, and the amount of deformation does not increase substantially with shock pressure. These results are highly atypical of shock-modified minerals, where relatively low shock pressures usually induce high ( 10¹⁵ m⁻²) densities of dislocations. Thermal annealing for 1 hr. at 900°C in a dry furnace removes many dislocations from the shocked labradorite.

The difference in observed dissolution rates between shocked and unshocked minerals appears to have a weak correlation with the increase in the density of dislocations on the mineral surface. The unshocked and shocked oligoclase and hornblende samples exhibit limited dissolution enhancement at pH 4.0. Increasing the density of dislocations by several orders of magnitude with shock-loading causes a relatively small increase in dissolution rates for these silicate minerals. These results suggest that the surface dislocations produced by the shock treatment are not the primary sites for dissolution reactions.     

Brittle-plastic deformation in initially dry rocks at fluid-present conditions: transient behaviour of feldspar at mid-crustal levels

We present detailed microstructural and chemical analyses from an initially dry anorthositic rock deformed during wet amphibolite facies conditions. Three different domains representing the microstructural variation of the deformed samples are investigated in detail in terms of fracture morphology and mode, grain characteristics and chemistry of present phases. Results show transient deformational behaviour where a close interaction between brittle, plastic and fluid-assisted deformation mechanisms can be observed. Our analysis allows us to describe the succession, interrelationships and effects of active mechanisms with progressively increasing strain in three so-called stages. In Stage 1, initial fracturing along cleavage planes promoted fluid influx that caused fragmentation and chemical reactions, producing fine-grained mineral assemblages in the fractures. Deformation twins and dislocations developed in clast pieces due to stress relaxation. Passive rotation of conjugate fracture sets and interconnection of intracrystalline fractures formed micro-shear-zones, constituting Stage 2. Microstructures and grain relationships indicate the activity and fluctuation between fracturing, dissolution-precipitation creep, grain boundary sliding and locally dislocation creep, reflecting the transient behaviour of brittle and plastic deformation mechanisms. Further rotation and widening of fractures into overall foliation parallel shear-bands (Stage 3) promoted strain partitioning into these areas through increased fluid influx, influence of fluid-assisted grain boundary sliding, phase mixing and presence of weak phases such as white mica. We suggest that local differences in fluid availability, volume fraction of weak phases produced by fluid present metamorphic reactions coupled with volume increase and local variations in stress concentration induced transient brittle-plastic behaviour. The studied shear-zone represents an example of the transformation of a rigid dry rock to a soft wet rock during deformation through syntectonic fracturing.