Atomic resolution microscopy of carbonates. Interpretation of contrast

The atomic resolution microscopy (ARM) at the National Center for Electron Microscopy, Lawrence Berkeley Laboratory, Berkeley, California has been used to image structural features in rhombohedral carbonates. The resolution of the microscope is better than 1.6 Å, but beam damage presently limits the resolution of some of our images to slightly better than 2.6 Å. More details can be extracted through image processing. We were able to interpret contrast in through focus series of “ideal” dolomite by comparing processed images with multibeam contrast calculations. Fair agreement was obtained for focus and thickness variations both of which display great changes. Even for ideal dolomite, the matching is not straight-forward, due to minor orientation variations, the presence of and amorphous overlayer, and surface roughness induced by ion beam thinning, etc. We also find good agreement for calcian δ-dolomite with a cation distribution model which assumes a periodic substitution of alternating Mg layers by Ca. Some atomic resolution examples are shown for coherent calcite-dolomite intergrowths and δ-dolomite domains in dolomite.     

Lunar ilmenite (Refinement of the crystal structure)

The crystal structure of ilmenites from Mare Tranquillitatis and terrestrial basaltic rocks have been refined from X-ray data using least squares methods. The two structures are virtuallly identical both in structural parameters and relative degree of order. Occupation of Fe and Ti sites is highly ordered. The metal ions have low apparent charges (near 0.3) and formal oxidation states of Fe (II) and Ti (IV).     

Petrofabrics and seismic properties of garnet peridotite from the UHP Sulu terrane (China): Implications for olivine deformation mechanism in a cold and dry subducting continental slab

Lattice-preferred orientations (LPO) of olivine, diopside, enstatite and garnet from the Zhimafang garnet peridotite body in the Sulu ultrahigh-pressure (UHP) metamorphic terrane (China) were measured using the electron backscatter diffraction (EBSD) technique. The peridotite was captured from a mantle wedge immediately adjacent the subducted Yangtze slab and then experienced the UHP metamorphism at 750–950 °C and 4–7 GPa. The olivine LPO is characterized by the [001] axis close to the stretching lineation and the (100) plane subparallel to the foliation, indicating the prevailing of (100) [001] slip. Enstatite LPO displays the dominance of (100) [001] slip. Diopside developed complex LPO patterns that are difficult to explain using a single slip system of (100) [001]. Garnet is almost randomly oriented due to its low volume fractions, cubic symmetry and the presence of numerous slip systems. Calculated seismic properties of the peridotite yield a maximum P-wave velocity normal to the foliation and a minimum along the foliation, with anisotropy up to 8% in strongly sheared samples. The S-wave velocity pattern is complex but the fast polarization plane generally normal to the foliation. The inferred shear sense from the olivine LPO is top-to-SE, in contrary to exhumation-induced top-to-NW thrusting recorded in the quartz LPO, implying that the olivine LPO formed at early UHP metamorphic conditions. The olivine crystals have relatively low water contents (141–475 H/10⁶ Si), indicating a fluid-deficient environment for the LPO formation. The present study suggests that a combination of low temperature and UHP plays a much more important role than the water content to promote the activation of (100) [001] slip in olivine.     

Das reaktivierte Grundgebirge der Zentralalpen

Zusammenfassung Die in den Lepontinischen Alpen aufgeschlossene Tiefenzone liegt noch im mesothermalen Raum und erreicht die Katazone nur lokal. Ihre wesentlichen Merkmale wurden durch Kristallisationsphänomene geprägt, welche die alpine Orogenese begleiteten und überdauerten. Nur die insubrischen Störungen sind jünger als diese regionale Kristallisation, während der aus mobilisiertem altem und aus jungem Material hervorgegangene granitische Kerne diapirartig aufstiegen und zum Teil eingefaltet wurden. An ihrem Rande entstanden Migmatitzonen; auch die Hüllgesteine sedimentären Ursprungs fielen teilweise der Anatexis anheim.An Hand der Paragenese Plagioklas-Calcit wird gezeigt, daß in Bündnerschiefern und Silikatmarmoren die Isograde der Metamorphose die Stoffgrenzen spitzwinklig schneiden, und daß im Tessin ein hochmetamorpher Bereich abgegrenzt werden kann. Der Wärmedom der Lepontinischen Alpen zeigt zugleich eine besondere, bogenförmige Anordnung des planaren und linearen Parallelgefüges, die nicht durch die hangenden großen Überschiebungsdecken diktiert sein kann.Mineralparagenesen und Struktur der Tiefenzone sind auf endogene Vorgänge im Unterbau der Alpen zurückzuführen. Das varistische Gebirge ist in der Tiefe verschwunden und alpidisches Kristallin daraus entstanden.

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The depth-zone disclosed in the Lepontine Alps consists mainly of mesothermal, locally only of catazonal state. Crystallization-events accompagnied the Alpine deformation and were still active in postorogenic times. Only the Insubric faults are younger; mobilized and intrusive granitic bodies developed and have been deformed also. Their margins display migmatizing, locally even anatexis of neighbouring portions. Plagioclase-calcite associations show that the isogrades of metamorphism (Bündner Schiefer and silicate-marbles) seem to trespass material boundaries at acute angles. The thermic dome of Lepontine Alps displays an arcuate arrangement of planar and linear structures or fabrics independent of the huge nappes in their roof regions. Mineral associations and fabrics of the depth-zones are due to endogene events beneath the Alps. A pre-existent Hercynian orogen disappeared in depth and has been transformed into Alpine basement-units.

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Résumé La zone profonde dégagée dans les Alpes lépontiennes se trouve encore dans un domaine mésothermal et n'atteint la catazone que localement. Ses caractéristiques essentielles ont été marquées par des phénomènes de cristallisation qui ont accompagné l'orogenèse alpine et ont perduré après elle. Seules les dislocations insubriques sont plus récentes que cette cristallisation régionale, tandis que des noyaux granitiques provenant d'un matériau ancien mobilisé, comme aussi d'un matériau plus jeune, sont montées par voie diapirique et ont été en partie incorporés dans le plissement. Sur leur bordure sont apparues des zones migmatitiques; de même une partie des roches de l'enveloppe sédimentaire ont été soumises à l'anatexie.Sur la base de la paragenèse plagioclase-calcite, l'auteur montre que dans les Schistes lustrés et dans les marbres à silicates, les isogrades du métamorphisme recoupent à angle aigu la disposition des matériaux et qu'on peut, dans le Tessin, délimiter un domaine à degré de métamorphisme élevé. Le dôme thermique des Alpes lépontiennes montre en même temps une disposition particulière en forme d'arc des textures planaires et linéaires qui ne peut pas être dictée par les grandes nappes de charriage sus-jacentes. Les paragenèses minérales et la structure des zones profondes doivent être rapportées à des processus endogènes se situant dans la partie inférieure des Alpes. La chaîne varisque a disparu en profondeur et le massif cristallin alpin en est issu.

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Junge Bewegungen und Piedmontflächen im Raum von Agadir

Zusammenfassung Die vorliegende Arbeit schildert junge Bewegungen, die zwischen dem Villafranchien und dem Sicilien I eingetreten sind. Zwischen diesen Bewegungen kam es auch zur Ausbildung von Piedmontflächen in der weiteren Umgebung, vor allen Dingen in der Ebene des Sous, und südlich im Antiatlas.     

Basal slip and texture development in calcite: new results from torsion experiments

The deformation behavior of calcite has been of longstanding interest. Through experiments on single crystals, deformation mechanisms were established such as mechanical twinning on in the positive sense and slip on and both in the negative sense. More recently it was observed that at higher temperatures slip in both senses becomes active and, based on slip line analysis, it was suggested that slip may occur. So far there had been no direct evidence for basal slip, which is the dominant system in dolomite. With new torsion experiments on calcite single crystals at 900 K and transmission electron microscopy, this study identifies slip unambiguously by direct imaging of dislocations and diffraction contrast analysis. Including this slip system in polycrystal plasticity simulations, enigmatic texture patterns observed in compression and torsion of calcite rocks at high temperature can now be explained, resolving a long-standing puzzle.     

In situ TEM observations of plastic deformation in quartz crystals

With in situ nanocompression experiments in a transmission electron microscope, we investigated plastic deformation in natural quartz crystals and observed both dislocation plasticity as well as mechanical twinning. Through this experimental method, we are able to provide direct evidence of Dauphiné twin nucleation and could measure the intrinsic twinning stress. The twinning phenomena appear to include a memory effect, where the same twin can reappear upon successive loading and unloading events. The data provide insight into this twin generation mechanism and can be used as a benchmark for the use of twins in quartz for paleopiezometry. Together, the observation of room-temperature dislocation plasticity and reversible twinning adds new insight into the extensive field of quartz plasticity and demonstrates the usefulness of small-scale testing techniques for mineral physics.     

Intergrowth of andesine and labradorite in marbles of the Central Alps

Crystallographic intergrowth of An 34 andesine and An 66 labradorite which can be seen with a petrographic microscope in sections cut perpendicular to [001] has been observed in amphibolite facies calcsilicate rocks of the Central Alps. The texture indicates that both plagioclases grew simultaneously in equilibrium, andesine mainly nucleating on faces (1¯10), (110), (100) and (130) of labradorite. The chemical composition measured on the U-stage and confirmed by microprobe establishes the endmembers of the andesine-labradorite miscibility gap. As shown by X-ray and electron diffraction both endmembers have the same intermediate plagioclase structure with periodic antiphase boundaries parallel to 10¯3 and spaced 25–30 Å which is characteristic of the structure observed in andesine from igneous rocks. Two different types of domains have been observed; one is in contrast with e-reflections and the other with strong a-reflections.     

Modulated structures in calcian dolomite: A study by electron microscopy

Calcian dolomite from the Devonian Lost Burro formation has been investigated with electron microscopy techniques. Electron diffraction shows evidence for “c” and “d” type reflections which may occur independently and are indicative of ordered superstructures. High resolution electron microscopy combined with selected area optical diffraction is the basis for models to explain the superstructures in calcian dolomite. It is proposed that “c” reflections are due to ordered substitution of Mg by Ca in basal cation layers. “d” reflections result when the rhombohedral stacking of basal layers is interrupted by intercalation of additional Ca layers. During electron irradiation at 1 MeV the Mg-Ca distribution becomes disordered and the crystal structure attains calcite symmetry. The arrangement of CO₃ groups remains ordered.