Abstract Polycrystalline calcite was deformed to high strain at room-temperature and confining pressures of 1–4 GPa using high-pressure torsion. The high confining pressure suppresses brittle failure and allows for shear strains >100. The post-deformation microstructures show inter- and intragranular cataclastic deformation and a high density of mechanical e $$ \left\{01\overline{1}8\right\} $$ 011¯8 twins and deformation lamellae in highly strained porphyroclasts. The morphologies of the twins resemble twin morphologies that are typically associated with substantially higher deformation temperatures. Porphyroclasts oriented unfavorably for twinning frequently exhibit two types of deformation lamellae with characteristic crystallographic orientation relationships associated with calcite twins. The misorientation of the first deformation lamella type with respect to the host corresponds to the combination of one r $$ \left\{10\overline{1}4\right\} $$ 101¯4 twin operation and one specific f $$ \left\{01\overline{1}2\right\} $$ 011¯2 or e $$ \left\{01\overline{1}8\right\} $$ 011¯8 twin operation. Boundary sections of this lamella type often split into two separated segments, where one segment corresponds to an incoherent r $$ \left\{10\overline{1}4\right\} $$ 101¯4 twin boundary and the other to an f $$ \left\{01\overline{1}2\right\} $$ 011¯2 or e $$ \left\{01\overline{1}8\right\} $$ 011¯8 twin boundary. The misorientation of the second type of deformation lamellae corresponds to the combination of specific r $$ \left\{10\overline{1}4\right\} $$ 101¯4 and f $$ \left\{01\overline{1}2\right\} $$ 011¯2 twin operations. The boundary segments of this lamella type may also split into the constituent twin boundaries. Our results show that brittle failure can effectively be suppressed during room-temperature deformation of calcite to high strains if confining pressures in the GPa range are applied. At these conditions, the combination of successive twin operations produces hitherto unknown deformation lamellae.
Die vorliegende Doktorarbeit prasentiert die Ergebnisse einer Studie uber die Entwicklung von Mikrostrukturen und Texturen in mit High-Pressure Torsion zu hohen Dehnungen unter Drucken im GPa Bereich und Temperaturen zwischen Raumtemperatur und 450 °C verformtem Kalzit. Die Mikrostrukturanalyse, die mit synchrotronbasierter Rontgenlinienprofilanalyse und SEM-basierter Elektronenruckstreubeugung durchgefuhrt wurde, dokumentierte einen signifikanten Einfluss des hohen Druckes wahrend der Verformung auf das Deformationsverhalten. Der hohe Druck wahrend der Verformung fuhrt durch das Unterbinden der Nukleation und Ausbreitung von Rissen zu mikrostrukturellen Merkmalen, die ublicherweise mit hoheren Verformungstemperaturen assoziiert werden. Deformationszwillinge in Proben die bei Raumtemperatur verformt wurden, dokumentieren die signifikante Aktivitat von Zwillingsverbreiterung und Zwillingsgrenzenwanderung. Diese Phanomene sind gemas Kalzitzwillingsmorphologie-Geothermometern erst bei Temperaturen von 150 beziehungsweise 250 °C zu erwarten. Allerdings limitiert der hohe Druck wahrend der Deformation die Mobilitat von Punktdefekten und hemmt somit das Klettern von Versetzungen und Erholung. Dadurch beeinflusst die Erhohung des Drucks bei der Verformung mikrostrukturelle Parameter wie Korngrose, Versetzungsdichte oder die Grose koharent streuender Domanen in ahnlicher Weise wie eine Reduzierung der Temperatur. Weiters beeinflusst das Stabilitatsfeld eines Hochdruckpolymorphs den Rekristallisationsprozess in bei 450 °C verformten Proben. Deformation im Stabilitatsfeld von Kalzit fuhrt zu einem dynamischen Gleichgewicht der Versetzungsdichte und einer Kern-Mantel Mikrostruktur nach Schehrdehnungen von etwa 10 unabhangig von der Dehnungsrate. Im Unterschied dazu fuhrt Verformung im Stabilitatsfeld des Hochdruckpolymorphs zur Einstellung des dynamischen Gleichgewichts erst bei signifikant hoheren Dehnungen und zu einem Rekristallisationsprozess, der von der Dehnungsrate abhangt. Dabei entstehen bei hohen Dehnungsraten Kern-Mantel Mikrostrukturen, wahrend niedrige Dehnungsraten zu einer homogeneren Rekristallisation der Porphyroklasten fuhrt.
Přesně měsic po nicive katastrofě v jihovýchodni Asii si v ni můžete přecist, co zažival Karel Gott na maledivskem atolu uprostřed Indickeho oceanu ci na co myslela těhotna Vendula Svobodova ve chvili, kdy se dozvěděla, že niciva vlna připravila o život desetitisice lidi.
The presence of hydrostatic pressure is a general crucial characteristic of severe plastic deformation methods for reaching high strains and for introducing large quantities of lattice defects, which are necessary to establish new grain boundaries. Insights into the processes occurring during deformation and the influence of hydrostatic pressure are necessary to help better understand the SPD methods. A special experimental procedure was designed to simulate the hydrostatic pressure release: High pressure torsion (HPT)-deformed microstructure changes related to the release of hydrostatic pressure after the HPT deformation of copper and nickel were studied by freezing the sample before releasing the pressure. High-resolution in-situ X-ray diffraction of the heating process was performed using synchrotron radiation in order to apply X-ray line profile analysis to analyze the pressure release. The results on copper and nickel generally indicated the influence of hydrostatic pressure on the mobility and interaction of deformation-induced defects as well as the resulting microstructure.
Abstract Micron to sub‐micron sized ferromagnetic inclusions in rock forming silicate minerals may give rise to particularly stable remanent magnetizations. When a population of inclusions have a preferred crystallographic or shape orientation in a rock, the recorded paleomagnetic direction and intensity may be biased by magnetic anisotropy. To better understand this effect, we investigated plagioclase grains from oceanic gabbro dredged from the Mid‐Atlantic Ridge at 11°–17°N. The plagioclase grains contain abundant needle and lath shaped magnetite inclusions aligned along specific directions of the plagioclase lattice. Electron back scatter diffraction and anisotropy of magnetic remanence measurements are used to correlate the orientation distribution of the magnetite inclusions in the host plagioclase that contains multiple twin types (Manebach, Carlsbad, Albite, and Pericline) with the bulk magnetic anisotropy of the inclusion‐host assembly. In non‐modified plagioclase, the anisotropy ellipsoid of magnetic remanence has oblate shapes that parallels the plagioclase (010) plane. It is suggested that recrystallization of magnetite inclusions during hydrothermal overprint shifts the relative abundance of the inclusions pertaining to the different orientation classes. We show that the maximum axis of the anisotropy ellipsoid of magnetic remanence parallels the plagioclase [001] direction, which in turn controls the recorded remanent magnetization direction. Our results are relevant for paleointensity and paleodirection determinations and for the interpretation of magnetic fabrics.
Abstract The microstructural and textural characteristics of a spinel corona that formed around a faceted corundum xenocrystal by reaction with the hosting basaltic melt in the Siebengebirge volcanic field demonstrate that the crystallographic and shape preferred orientation of spinel is influenced by the orientation of the reaction interface with respect to the corundum crystal lattice. The spinel roughly shows the common topotactic orientation relationships with corundum, where one of the $\{111\}_{Spl}$ planes is parallel to the (0001)$_{Crn}$ plane, and three of the $\{110\}_{Spl}$ planes are parallel to the $\{10\overline {1}0\}_{Crn}$ planes. In detail, there are subtle but systematic deviations from this topotactic relationship due to small rotations about the c-axis and/or an a-axis of corundum. The former is observed when the corundum c-axis is closely parallel to the interface plane, while the latter require a corundum a-axis orientation perpendicular to the interface. In this case, the preferred sense of rotation depends on the sign of the a-axis direction, irrespective of the spinel growth direction being parallel or antiparallel to this axis. Additionally, the selection of either one or both of two spinel twin variants that equally fulfill the topotactic orientation relationship depends on the orientation of the corundum-spinel interface with respect to the lattices of both the corundum and the spinel. Finally, also the grain boundary character is controlled by the interface orientation and the corundum lattice. Despite the differences between corona segments, the nature of these textures are persistent along and across each segment. We emphasize that all these microstructural and textural features are ascribed to the period of spinel growth in magmatic environment. The extent to which prominent slip planes in spinel are aligned parallel with the corundum-spinel interface seems to be of crucial importance for the nature of the spinel texture and microstructure, indicating that the activity of dislocations pertaining to these slip systems ease the accommodation of lattice misfit across the corundum-spinel interface. By comparison with experimentally grown spinel layers, we infer predominantly interface reaction controlled growth of the studied spinel corona.
