215 research outputs found
Crystallographic and Seismic Anisotropies of Calcite at Different Depths : a Study Using Quantitative Texture Analysis by Neutron Diffraction
No full textEight samples of limestones and marbles were studied by neutron diffraction to collect quantitative texture (i.e., crystallographic preferred orientations or CPO) of calcite deforming at different depths in the crust. We studied the different Texture patterns developed in shear zones at different depth and their influence on seismic anisotropies. Samples were collected in the French and Italian Alps, Apennines, and Paleozoic Sardinian basement. They are characterized by isotropic to highly anisotropic (e.g., mylonite shear zone) fabrics. Mylonite limestones occur as shear zone horizons within the Cenozoic Southern Domain in Alpine thrust-and-fold belts (Italy), the Briançonnais domain of the Western Alps (Italy-France border), the Sardinian Paleozoic back-thrusts, or in the Austroalpine intermediate units. The analyzed marbles were collected in the Carrara Marble, in the Austroalpine Units in the Central (Mortirolo) and Western Alps (Valpelline). The temperature and depth of development of fabrics vary from <100◦C, to 800◦C and depth from <10 km to about 30 km, corresponding from upper to lower crust conditions. Quantitative Texture Analysis shows different types of patterns for calcite: random to strongly textured. Textured types may be further separated in orthorhombic and monoclinic (Types A and B), based on the angle defined with the mesoscopic fabrics. Seismic anisotropies were calculated by homogenizing the single-crystal elastic tensor, using the Orientation Distribution Function calculated by Quantitative Texture Analysis. The resulting P-and S-wave anisotropies show a wide variability due to the textural types, temperature and pressure conditions, and dip of the shear planes
The 3D quantitative lattice and shape preferred orientation of a mylonitised metagranite from Monte Rosa (Western Alps): Combining neutron diffraction texture analysis and synchrotron X-ray microtomography
No full textTwo complementary 3D techniques, neutron diffraction and synchrotron X-ray microtomography (SXR-μCT), were used to compare the Shape and Lattice Preferred Orientations of a mylonitised metagranite from the Monte Rosa unit (Western Alps, Italy). The goal of using these techniques was to obtain two different orientation distribution functions. Although the two functions describe relatively independent characteristics of the rock fabric, nonetheless they also exhibit close relationships to macroscopic fabrics and may be complementarily used to quantify rock fabrics and microstructures, thereby highlighting 3D features that cannot be obtained with either technique, if used independently. We describe an approach that can be potentially useful in various disciplines, e.g., structural geology, rock mechanics, tectonics and geophysics, when a complete data set of preferred orientations and size distribution is needed.Micas display a strong orthorhombic symmetry between mesoscopic lineation and microscopic SPO and LPO, whereas quartz and feldspars are characterised by a monoclinic symmetry between mesoscopic lineation and LPO. These observations suggest a rheological decoupling between the weak phase mica layers and the stronger quartz+feldspar layers. This mechanical decoupling occurred during the Alpine subduction-collision, when the Monte Rosa unit was part of the Insubric Line system and accommodated large vertical strain
Multiscalar structural study of the ultramafic rocks of the Antrona Ophiolite (Pennine Alps)
No full textIn the Alpine nappe stack, the Antrona Ophiolite (Italian side of the Western Central Alps) is sandwiched between the overlying continental Monte Rosa Nappe (upper Penninic domain) and the underlying Camughera-Moncucco continental Unit (middle Penninic). The ophiolite sequence includes ultramafic rocks, metagabbros and mafic rocks covered by calcschists. Ultramafites constitute a huge body of serpentised peridotites including interbedded layers of gabbros, clinopyroxene-rich and amphibole-rich rocks, and chloriteschist. In spite of the Alpine tectonic and metamorphic reworking, the ultramafic portion of the Antrona Ophiolite still preserves relict textures and minerals that can be referred to the pre-Alpine or early Alpine evolution. A detailed microstructural analysis performed at polarised microscope and SEM on less serpentinised, olivine-rich samples is here presented. It is integrated with a quantitative textural analysis of Lattice Preferred Orientation (LPO) by neutron diffraction acquired on selected samples of olivine-rich samples. The results allow to infer a mantle origin for the ultramafic rocks, suggesting T conditions > 800°C for the activation of slip systems in olivine
Brittle plus plastic deformation of gypsum aggregates experimentally deformed in torsion to high strains: Quantitative microstructural and texture analysis from optical and diffraction data
No full textThis contribution presents a quantitative microstructural analysis of a polycrystalline aggregate of gypsum, deformed in torsion (T = 70-90 °C) at γ (shear strain) ranging from 0 to 4.82. Quantitative microstructural analysis is used to compare the evolution of microstructures observed by optical microscope with those obtained from analysis of X-ray and neutron diffraction data. This analysis shows that during experimental deformation, gypsum accommodated strain by brittle and plastic deformation mechanisms, developing Riedel-like microfaults with plastic foliations and crystallographic preferred orientation (CPO). The relations of microstructures show that with increasing strain, the Riedel systems start from R planes with an angle of ≈30° to the Imposed Shear Plane. This angle decreases (5°-15°) when strain increases, and Y planes develop. Quantitative texture analysis (QTA) shows that S-foliations start developing at low γ and maintain their orientation up to high y, and that the most active slip system is the (010) along normal to (100) and the [001]-axis. Shape preferred orientation (SPO) of gypsum does not coincide with the theoretical orientation as it does not decrease with increasing strain. This discrepancy is explained by the role of the brittle shear planes that impose a back rotation to gypsum. No brittle to plastic transition occurs. But both plastic and brittle structures contemporaneously accommodate and localize strain
High-pressure phases and transitions of the layered alkaline earth nitridosilicates SrSiN2 and BaSiN2
No full textWe investigate the high-pressure phase diagram of SrSiN2 and BaSiN2 with density-functional calculation. Searching a manifold of possible candidate structures, we propose new structural modifications of SrSiN2 and BaSiN2 attainable in high-pressure experiments. The monoclinic ground state of SrSiN2 transforms at 3 GPa into an orthorhombic BaSiN2 type. At 14 GPa a CaSiN2-type structure becomes the most stable configuration of SrSiN2. A hitherto unknown Pbcm modification is adopted at 85 GPa and, finally, at 131 GPa a LiFeO2-type structure. The higher homologue BaSiN2 transforms to a CaSiN2 type at 41 GPa and further to a Pbcm modification at 105 GPa. Both systems follow the pressure-coordination rule: the coordination environment of Si increases from tetrahedral through trigonal bipyramidal to octahedral. Some high-pressure phases are related in structure through simple group–subgroup mechanisms, indicating displacive phase transformations with low activation barriers
A density functional study of the high-pressure chemistry of MSiN2(M = Be,Mg, Ca)
No full textNormal pressure modifications and tentative high-pressure phases of the nitridosilicates MSiN2 with M = Be, Mg, or Ca have been thoroughly studied by density functional methods. At ambient pressure, BeSiN2 and MgSiN2 exhibit an ordered wurtzite variant derived from idealized filled β-cristobalite by a C1-type distortion. At ambient pressure, the structure of CaSiN2 can also be derived from idealized filled β-cristobalite by a different type of distortion (D1-type). Energy–volume calculations for all three compounds reveal transition into an NaCl superstructure under pressure, affording sixfold coordination for Si. At 76 GPa BeSiN2 forms an LiFeO2-type structure, corresponding to the stable ambient-pressure modification of LiFeO2, while MgSiN2 and CaSiN2 adopt an LiFeO2-type structure, corresponding to a metastable modification (24 and 60 GPa, respectively). For both BeSiN2 and CaSiN2 intermediate phases appear (for BeSiN2 a chalcopyrite-type structure and for CaSiN2 a CaGeN2-type structure). These two tetragonal intermediate structures are closely related, differing mainly in their c/a ratio. As a consequence, chalcopyrite-type structures exhibit tetrahedral coordination for both cations (M and Si), whereas in CaGeN2-type structures one cation is tetrahedrally (Si) and one bisdisphenoidally (M) coordinated. Both structure types, chalcopyrite and CaGeN2, can also be derived from idealized filled β-cristobalite through a B1-type distortion. The group–subgroup relation of the BeSiN2/MgSiN2, the CaSiN2, the chalcopyrite, the CaGeN2 and the idealized filled β-cristobalite structure is discussed and the displacive phase transformation pathways are illustrated. The zero-pressure bulk moduli were calculated for all phases and have been found to be comparable to compounds such as α- Si3N4, CaIrO3 and Al4C3. Furthermore, the thermodynamic stability of BeSiN2, MgSiN2 and CaSiN2 against phase agglomerates of the binary nitrides M3N2 and Si3N4 under pressure are examined
Neutron and X-ray diffraction studies of the SrCr8Ga4O19 kagomè compound synthesised by the citrate route
No full textThe SrCrxGa12-xO19 (SCGO) has the magnetoplumbite structure with part of the Cr3+ (S=3/2) moments lying on the kagomè layers (12k sites) separated by Cr3+ triangular layers (4f and 2a sites). The strong geometric frustrations of the magnetic sublattices gives a high degeneracy of the ground state that is the origin of particular low temperature properties. The magnetic sublattice is diluted by a partial interchange of the Cr and Ga atoms that has been observed in all structural refinements published up to now. This dilution affects the magnetic properties and therefore it is important to investigate the Cr/Ga distribution in this system, as well as the synthesis conditions that control the structural properties.
We synthesized different samples of SCGO by solid state reaction and by citrate route. The last one is a wet method that is known to give homogeneous powders with high purity and controlled stoichiometry. For the last method we followed, by X-ray diffraction patterns, the formation of the crystalline phase starting from the amorphous precursor and calcining at different temperatures. The analysis of the X-ray spectra reveals the formation of the Cr2O3 at 600°C, and due to the high stability of this oxide, it is necessary to reach very high temperatures (like in the solid state reaction) to synthesize the SCGO. The complete crystallization of the compound is reached at 1150°C, but we obtained a very pure sample only at 1350°C.
Comparing the spectra for the samples prepared by the different methods, we observed a better crystallization in the sample from the citrate route, but no other differences are evident from the XRD spectra. We performed also high resolution neutron powder diffraction analysis on the D2B two axis diffractometer at the Institute Laue Langevin to investigate the differences in the Cr/Ga distributions on the 12k and 4f sites in order to relate them with the synthesis route
Submarine lava flow direction revealed by neutron diffraction analysis in mineral lattice orientation
No full textThe ocean crust is formed by the rising of magma from mid-ocean ridges and voluminous (1-30 km3) flows of lava away from ridge axes. However, our understanding of the emplacement kinematics of submarine lava is often limited to plan view geometries of near-axis lava. Drilled cores provide in situ access to the intact internal structure of submarine lavas. We used neutron diffraction to study off-axis lava flows drilled into the uppermost crust of ODP/IODP-Site 1256 (Cocos Plate). We provide quantitative insights into submarine lava microstructures and strong evidence for a secondary lava injection into the interior of a solidifying flow of lava along the NW-SE direction parallel to the paleo-ridge axis of the East Pacific Rise. The dynamics of lava inflow are controlled by crystal abundance and the temperature of the lava-crystal mixture rather than by local seafloor topography. We provide a description of an in situ shear within submarine lavas revealed by composite shape and lattice preferred orientations, accounting for a dominant laminar nonuniform-type flow. Key Points Method to define the direction of subaqueous lava flow First LPO and SPO analysis conducted on subaqueous basalt Evidence for inflated lava flows at ODP Site 1256 Laminar flow and evidence of vertical shear supporting inflation of the lava flow by a second episode
Characterization, interpretation and modeling of the present day oceanic crust microstructure by using innovative techniques based on neutron diffraction and X-ray microtomograph
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