Cordierite microstructure and texture in a moldanubian gneiss

The microstructure and texture in cordierites of a moldanubian gneiss from the Bohemian Massif has been analysed by transmission electron microscopy (TEM) and universal stage in order to get information on the deformation mechanisms and textural development of this rock-forming mineral. Deformation may have taken place at temperatures between about 500° C and 630° C and pressures smaller than about 3 kb. The elongated cordierite xenoblasts show a typical dislocation creep microstructure consisting of subgrain boundaries and free dislocations. The dislocations have [001], [010] and 1/2<110> Burgers vectors. [001] dislocations often have pure screw and edge character the latter type being climb-dissociated on (001). Among the dislocations reactions are common. The main subgrain boundaries observed are (010)[001], {110}[001] and (001)[010] tilt boundaries. Burgers vectors and dislocation line directions reveal (100)[001], (010)[001], (100)[010], {110} 1/2<110> and (001)1/2<110> as activated slip systems. The crystallographic preferred orientation (here referred to as texture) consists of a [001] maximum in the foliation parallel to the mineral lineation. [100] and [010] maxima are perpendicular to it within and normal to the foliation, respectively, with a girdle tendency normal to the lineation. The texture may be explained by simple shear deformation on the {hkO}[001] slip systems with preference of (010)[001].     

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.     

Simulation of screw dislocations in wadsleyite

The structure and energies of the cores of [100] and [001] screw dislocations in wadsleyite (β-Mg₂SiO₄) are calculated using a cluster-based combined elastic-atomistic method and a new parameterized interatomic potential model. For a core radius of 10 Å, core energies are found to be 2.5 and 4.4 eV/Å for the [100] and [001] dislocations, respectively. Both dislocations are associated with significant non-elastic displacement fields extending beyond the core with a radial component toward the dislocation line. The core of the [100] dislocation contains tetrahedrally coordinated magnesium, has a simple 2D structure and is spread parallel to (011) in a manner that suggests high mobility. In contrast, the core of the [001] dislocation has an extended and complex 3D structure involving the formation a large Si₆O₁₉ unit twisted around the dislocation line. This implies that movement of the [001] dislocation will be inhibited by the need to cleave Si–O bonds. These observations, combined with the anomalously low core energy of the [100] dislocation, explain the regular occurrence of [100] dislocations and very rare observation of [001] dislocations in experimentally deformed wadsleyite samples.     

Naturally decorated dislocations in olivine from peridotite xenoliths

Dislocations decorated by hematite and magnetite have been observed optically in the olivine grains of undeformed or highly annealed peridotite xenoliths from Hawaii and Baja California ( 5 × 10⁵ cm^–2). The observed structures include loops, low-angle boundaries, and structures produced by multiple cross-glide of [100] screws. Loops are almost invariably parallel to (001). Simple arrays of parallel dislocations lie predominantly in (100), (010) and (001) with dislocation lines subparallel to low-index directions. [100] screws pinned to (100) boundaries are frequently seen to bow out on (001). Preliminary electron petrography has confirmed that all dislocations are decorated.     

Analysis of dislocations in some naturally deformed plagioclase feldspars

Dislocations in intermediate plagioclase feldspars, which were deformed under granulite facies conditions, have been analysed. The study reveals extensive ductile deformation by intracrystalline slip and by twinning. Six out of the seven possible Burgers vectors were identified: \(b = \left[ {001} \right],\tfrac{1}{2}\left[ {110} \right],\tfrac{1}{2}\left[ {1\bar 10} \right],\left[ {101} \right],\tfrac{1}{2}\left[ {112} \right]and\tfrac{1}{2}\left[ {1\bar 12} \right]\) . Most, perhaps all, dislocations are dissociated by up to 200 Å. The microstructure is dominated by [001] screw dislocations, most of which appear to be dissociated in (010). The dominant slip system appears to be (010) [001]. Large grain-to-grain variations in the density of free dislocations indicate that the plastic strain in individual grains depended upon the Schmid factor for (010) [001]. The microstructure suggests that the rate-controlling step for high-temperature creep of plagioclase is cross-slip of extended [001] screw dislocations. The rheological contrast between feldspar and quartz is partly due to a difference in stacking fault energy.     

The effect of aluminum and water on the development of deformation fabrics of orthopyroxene

The effect of alumina and water solubility on the development of fabric in orthopyroxene in response to simple shear deformation has been investigated at a pressure of 1.5 GPa and a temperature of 1,100 °C using the D-DIA apparatus. The microstructure observations at these conditions indicate that dislocation glide is the dominant deformation mechanism. In MgSiO₃ enstatite and hydrous aluminous enstatite, partial dislocations bounding the stacking faults in [001] glide parallel to the (100) (or) the (100) [001] slip system. Electron backscattered diffraction analysis of anhydrous aluminous enstatite, however, indicates operation of the (010) [001] slip system, and microstructure analysis indicates dislocation movement involving [001] on both (100) and {210} planes. The strong covalent bonding induced by the occupation of M1 and T2 sites by Al could have restricted the glide on (100), activating slip on {210}. The resulting seismic anisotropies (~2 %) in orthopyroxene are weaker compared to olivine (~9.5 %), and reduced anisotropy can be expected if orthopyroxene coexists with olivine. Weak anisotropy observed in stable cratonic regions can be explained by the relatively high abundance of orthopyroxene in these rocks.     

Characterization of the glide planes of the [001] screw dislocations in olivine using electron tomography

A San Carlos olivine polycrystal has been deformed under uppermost mantle conditions, by compression at 900 °C, at a strain rate of 1.1 × 10^?5 s^?1, under a confining pressure of 300 MPa, using the Paterson press. Transmission electron tomography of dislocations has been performed by scanning transmission electron microscopy, by conventional transmission electron microscopy using the weak-beam dark-field technique, associated with precession or not, in order to determine the glide planes of [001] screw dislocations. This recent technique is the most suitable one since most [001] dislocations exhibit straight screw segments due to the high lattice friction on this character at low temperature. We find that [001] dislocations glide in (100), (010) and {110} as already reported, but also more unexpectedly in {120} and {130}. We show that at 900 °C, [001] {110} glide is dominant in polycrystals. We have, however, noted and characterized numerous cross-slip events in the specimen.     

Quantitative characterization of plastic deformation of zircon and geological implications

The deformation-related microstructure of an Indian Ocean zircon hosted in a gabbro deformed at amphibolite grade has been quantified by electron backscatter diffraction. Orientation mapping reveals progressive variations in intragrain crystallographic orientations that accommodate 20° of misorientation in the zircon crystal. These variations are manifested by discrete low-angle (<4°) boundaries that separate domains recording no resolvable orientation variation. The progressive nature of orientation change is documented by crystallographic pole figures which show systematic small circle distributions, and disorientation axes associated with 0.5–4° disorientation angles, which lie parallel to rational low index crystallographic axes. In the most distorted part of the grain (area A), this is the [100] crystal direction. A quaternion analysis of orientation correlations confirms the [100] rotation axis inferred by stereographic inspection, and reveals subtle orientation variations related to the local boundary structure. Microstructural characteristics and orientation data are consistent with the low-angle boundaries having a tilt boundary geometry with dislocation line [100]. This tilt boundary is most likely to have formed by accumulation of edge dislocations associated with a 〈001〉{100} slip system. Analysis of the energy associated with these dislocations suggest they are energetically more favorable than TEM verified 〈010〉{100} slip. Analysis of minor boundaries in area A indicates deformation by either (001) edge, or [100](100) and [001](100) screw dislocations. In other parts of the grain, cross slip on (111), and (112) planes seems likely. These data provide the first detailed microstructural analysis of naturally deformed zircon and indicate ductile crystal-plastic deformation of zircon by the formation and migration of dislocations into low-angle boundaries. Minimum estimates of dislocation density in the low-angle boundaries are of the order of ∼3.10¹⁰ cm⁻². This value is sufficiently high to have a marked effect on the geochemical behavior of zircon, via enhanced bulk diffusion and increased dissolution rates. Therefore, crystal plasticity in zircon may have significant implications for the interpretation of radiometric ages, isotopic discordance and trace element mobility during high-grade metamorphism and melting of the crust.     

TEM investigation of the crystal microstructures in a quartz-coesite assemblage of the western alps

Atransmission electron microscope (TEM) study of quartz-coesite inclusions in garnet in crustal rocks from the Western Alps is presented. Coesite shows a low dislocation density (<10⁷ cm^?2), and quartz a higher density of defects, Brasil twins (10⁴ cm^?1) and dislocations (10⁸ cm^?2). It is concluded that coesite has been not or only slightly plastically deformed and that the yield strength of coesite is higher than that of quartz. The large scale deformation implications are briefly discussed. TEM observations show no systematic topotactic relationship between the two polymorphs and their boundaries have a scalloped morphology which suggests that growth of quartz from coesite was controlled by a diffusion process.     

Experimental deformation of a synthetic dunite at high temperature and pressure. II. Transmission electron microscopy

We have performed detailed transmission electron microscope on most of the deformed synthetic dunite specimens prepared in the study by Zeuch and Green (1984). We have identified three basic types of sub-boundaries, simple tilt walls in (100) and (001). composed by b = [100] and b = [001] edge dislocations, respectively, and twist boundaries in (010) composed of b = [100] and b = [001] screws. We have also observed more complex, asymmetric lilt boundaries in (100) and (001). Like the (010) twist boundaries, these asymmetric tilt walls are common only at the highest temperatures and lowest strain rates. Subgrain development is extensive at the higher temperatures and lower strain rates, and subgrains are composed of the above-mentioned three types of sub-boundaries; edge components in (100) and (001) ire “knitted” to screw components in (010) as described by Kirby and Wegner (1978) for naturally deformed olivine. In many areas of the samples which we studied, subgrain development is not observed, but parallel arrays of tilt boundaries of one type or the other are present. At higher temperatures and lower strain rates. “(100) organization” (Durham et al., 1977) is common; this structure consists of parallel arrays of (100) tilt boundaries with b = [100] screws connecting the sub-boundaries. At lower temperatures we have observed an analogous arrangement of (001) sub-boundaries and b = [001] screws, which we refer to as “(001) organization”. Under all experimental conditions, dislocations with b = [100] and b = [001] are present in approximately equal numbers. However, the two types of dislocations also have distinctly different geometries under all test conditions. We suggest that the transition from slip parallel to [001] to slip parallel to [100] with increasing temperature, which has been reported in earlier studies may also depend upon water content. The substructures which we observe are virtually identical to those seen in many naturally deformed peridolites. and we conclude that the mechanisms involved in both natural and laboratory deformation of olivine polycrystals are similar. On the other hand, the substructures reported here are very different from those observed in experimentally deformed olivine single crystals. It seems likely that these substructural differences reflect fundamental differences in the behavior oh single crystals and polycrystals. which are in turn reflected in different measured creep strengths.     

Mechanical twinning in diopside Ca(Mg,Fe)Si2O6: Structural mechanism and associated crystal defects

Diopside twins mechanically on two planes, (100) and (001), and the associated macroscopic twinning strains are identical (Raleigh and Talbot, 1967). An analysis based on crystal structural arguments predicts that both twin mechanisms involve shearing of the (100) octahedral layers (containing Ca²⁺, Mg²⁺ and Fe²⁺ ions) by a magnitude of c/2. Small adjustments or shuffles occur in the adjacent layers containing the [SiO₄]^4? tetrahedral chains. While the (100) twins are conventional with shear parallel to the composition plane, this analysis predicts that (001) twins form by a mechanism closely related to kinking. A polycrystalline diopside specimen was compressed 8% at a temperature of 400° C, a pressure of 16 kilobars, and a compressive strain rate of about 10^?4/s. Transmission electron microscopy on this specimen has revealed four basic lamellar features:

1. (100) mechanical twin lamellae;
2. (100) glide bands containing unit dislocations;
3. (001) twin lamellae;
4. (101) lamellar features, not as yet identified.

The (001) twins often contain remnant (100) lamellae of untwinned host. Twinning dislocations occur in these (100) lamellae and in the (001) twin boundaries with very high densities. Diffraction contrast experiments indicate that the twinning dislocations associated with both twin laws glide on (100) with Burgers vector b=X [001] where X is probably equal to 1/2 on the basis of the structural analysis. Parallels are drawn between mechanical twinning in clinopyroxenes and clinoamphiboles. The exclusive natural occurrence of basal twins in shock-loaded clinopyroxenes and of analogous ( \(\bar 1\) 01) twins in clinoamphiboles is given a simple explanation in terms of the relative difficulty of the “kinking” mechanism as compared to direct glide parallel to the composition plane.     

Plastic deformation of wadsleyite: I. High-pressure deformation in compression

 We have studied the plastic deformation of Mg₂SiO₄ wadsleyite polycrystals. Wadsleyite was synthesized from a forsterite powder in a multianvil apparatus. It was then recovered and placed in a second multianvil assembly designed to induce plastic deformation by compression between two hard alumina pistons. After the deformation experiment, the microstructures are characterized by transmission electron microscopy (TEM) and large-angle convergent beam electron diffraction (LACBED). Deformation experiments have been carried out at 15–19 GPa and at temperatures ranging from room temperature to 1800–2000 °C. Five different dislocation types have been identified by LACBED: [100], 1/2〈111〉, [010], 〈101〉 and [001]. The [001] dislocations result from dislocation reactions and not from activation of a slip system. The [010] dislocations are activated under high stresses at the beginning of the experiments and further relax by decomposition into 1/2〈111〉 dislocations or by dissociation into four 1/4[010] partial dislocations. The following slip systems have been identified: 1/2〈111〉{101}, [100](010), [100](001), [100]{011}, [100]{021}, [010](001), [010]{101} and 〈101〉(010). Received: 15 July 2002 / Accepted: 14 February 2003 Acknowledgements High-pressure experiments were performed at the Bayerisches Geoinstitut under the EU IHP – Access to Research Infrastructures Programme (Contract no. HPRI-1999-CT-00004 to D.C. Rubie). P.C. has benefited from a Congé thématique pour recherche from the University of Lille, and would like to thank warmly all the people in Bayreuth who contributed to this work by daily assistance and discussions: Nathalie Bolfan-Casanova, Daniel Frost, Jed L. Mosenfelder and Brent Poe. The quality of the preparation of the TEM specimens by H. Schultze is greatly appreciated.     

Dislocation substructure of experimentally deformed olivine

Dunite, experimentally deformed at 800° C, exhibits predominantly pure screw dislocations parallel to [001] and dense, tangled zones of dislocations subparallel to the plane (001). Olivine single crystals experimentally deformed at 900° C, are characterized by pure screw dislocations parallel to [001] and [100], and dislocations of undetermined character with Burgers vectors [001]. All observations are consistent with deformation mechanisms deduced from optically visible features.     

Deformation and recrystallisation mechanisms in naturally deformed cordierite

Cordierite — (Mg,Fe)₂Al₄Si₅O₁₈ — occurs as porphyroclasts within metapelitic and metavolcanic rocks from the Kemiö-Orijärvi belt, SW Finland. After crystallisation the cordierites have been deformed at temperatures between 550–825° C and pressures of 3–5 kbar. Optical microscopy reveals the following deformation-induced microstructures: a bimodal size distribution between host, 0.3 to 4.0 mm, and recrystallised (new) grains, 0.1 to 0.5 mm; the intracrystalline defect-structures of host grains yield undulatory extinction, subgrains and some twinning. Recrystallised grains are optically strain free. Grain and subgrain boundaries are generally straight and parallel to crystallographic low-index planes. Orientation distribution diagrams for host and recrystallised grains yield similar fabric diagrams, i.e. [010] perpendicular to foliation -S-, [001] and [100] parallel to S and [001] parallel to lineation -L-. The fabric diagrams indicate that [001] (010) is the dominant slip system. Transmission electron microscopy reveals straight free dislocations, glide and climb loops, minor {130} and {110} microtwins, isolated nodal points and dislocation walls. Contrast analyses yield Burgers vector b = [001] being dominant and b = [100] subordinate. Climb loops consist of 〈c〉-dislocations that are dissociated in (001) planes, glide loops are defined by [100] [010] and [001] (100). The cordierite microstructures have been interpreted to be generated by dislocation creep. The dominant recrystallisation mechanism is thought to be subgrain rotation subsequently followed by minor grain or twin-band boundary migration.     

Olivine as an in situ piezometer in high pressure apparatus

The deviatoric stress produced in a large-volume, high-pressure apparatus of the girdle-anvil type has been estimated from the density of free dislocations induced in natural olivine single crystals (initial density of 2×10⁶ cm^?2). Experiments at maximum pressure P=40 kbar and temperature T=1050°C for t=1 h in NaCl cell assemblies and various P-T paths yield specimens whose dislocation densities are unchanged from this initial value, implying that the deviatoric stress was less than 140 bar. In BN cell assemblies, the recovered specimen from high P-T experiments exhibit much higher densities of dislocations (～10⁹ cm^?2) which have been produced by steady-state plastic deformation of the olivine crystals under a deviatoric stress of ～3 kbar. This value of deviatoric stress in BN has been corroborated by observations of the subgrain size and recrystallized grain size in specimens of longer run duration (3 h).     

Dislocations in olivine indented at low temperatures

Microhardness experiments have been performed on faces of olivine single crystals oriented at 45° to the [100] and [001] axis. Experiments have been performed between 20°C and 900°C in order to follow the evolution of the slip systems and the evolution of the nature of dislocations with temperature. Slip systems were identified using interferential contrast, decoration and transmission electron microscopy (TEM). Although the two possible Burgers vectors [001] and [100] are acted upon symmetrically, the only activated slip systems are (100) [001] and {110} [001]. The latter system is less developed and appears only at 600°C and above. Dislocations are mainly [001] screws, which are always very straight. Microcracks are observed together with dislocations. Above 600°C there is a clear relationship between both as a narrow plastic zone is developed around the cracks.     

Dislocation dissociation in CaGeO3 perovskite

Dissociated dislocations have been observed for the first time by transmission electron microscopy in the perovskite-structure compound CaGeO₃. Dislocations with Burgers vectors \(\left[ {1\bar 10} \right]\) and [001] (in pseudo-cubic index) are dissociated into collinear partials on the (110) plane: $$\left[ {1\bar 10} \right] = {1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}\left[ {1\bar 10} \right] + {1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}\left[ {1\bar 10} \right]$$ and [001] = 1/2[001] + 1/2[001]. The partials react to form octagonal extended nodes. The stacking fault ribbons with displacement vector \(\left[ {1\bar 10} \right]\) have a width of 350 A, which corresponds to a stacking fault energy of 35 erg/cm² (or mJ/m²).     

Plastic deformation of orthoenstatite and the ortho- to high-pressure clinoenstatite transition: a metadynamics simulation study

Atomic-scale mechanisms of plastic deformation in orthoenstatite, MgSiO₃ are studied by computer simulation methods. The combined use of metadynamics and molecular dynamics allows a direct observation of the structural changes during the creation of stacking faults in the (100) plane. A sequence of slip deformations in two different (100) planes at P = 15 GPa and T = 1,000 K reveals a probable transformation mechanism for the ortho- to high-pressure clinopyroxene transition. Each of the observed slips consists of at least four partial deformations crossing high-energy intermediate structures. In agreement with experimental studies, both (100)[010] and (100)[001] slip systems are activated in the deformation process. The observation of a dominant (100)[001] single slip system in pyroxenes may be related to the fact that high-energy intermediate dislocations with (100)[010] component are not stable on geological or experimental timescales.     

Crystal bending,subgrain boundary development,and recrystallization in orthopyroxene during granulite-facies deformation

A prominent feature of a granulite-facies shear zone from the Hidaka Main Zone (Japan) is the folding of orthopyroxene (opx) porphyroclasts. Dislocation density estimated by transmission electron microscope (TEM) and chemical etching in homogeneously folded domains is too low to account for the amplitude of crystallographic bending, leading us to propose a model similar to “flexural slip” folding, where folded layers are micrometer-wide opx layers between thin planar clinopyroxene (cpx) exsolutions. Extension (compression) in the extrados (intrados) of the folded layer is accommodated by dislocations at the cpx–opx interfaces. Alternatively to distributed deformation, crystal bending also localizes in grain boundaries (GBs), mostly oriented close to the (001) plane and with various misorientation angles but misorientation axes consistently close to the b-axis. For misorientation up to a few degrees, GBs were imaged as tilt walls composed of regularly spaced (100)[001] dislocations. For misorientation angles of 7°, individual dislocations are no longer visible, but high-resolution TEM (HRTEM) observation showed the partial continuity of opx tetrahedral chains through the boundary. For 21° misorientation, the two adjacent crystals are completely separated by an incoherent boundary. In spite of these atomic-scale variations, all GBs share orientation and rotation axis, suggesting a continuous process of misorientation by symmetric incorporation of (100)[001] dislocations. In addition to the dominant GBs perpendicular to the (100) plane, boundaries at low angle with (100) planes are also present, incorporating dislocations with a component of Burgers vector along the a-axis. The two kinds of boundaries combine to delimit subgrains, which progressively rotate with respect to host grains around the b-axis, eventually leading to recrystallization of large porphyroclasts.