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Annemarie Meike 《Physics and Chemistry of Minerals》1988,16(2):148-156
The stress gradient calculated from an isotropic elastic approximation does not directly reflect the distribution of permanent deformation in a crystal of the same shape under the same conditions. However, with additional crystallographic constraints, it serves to predict locations where twinning and slip are first activated in a stressed crystal. In this study, thick-walled hollow cylinders were cored from single calcite crystals parallel to 0001 and
. One cylinder of each orientation was loaded at room temperature under one of two conditions: internal pressure (P
p
)=60 MPa, external pressure (P
c
)=100 MPa, or P
p
= 20 MPa, P
c
= 50 MPa. These conditions would produce a radial stress gradient in an isotropic elastic cylinder. Mechanical twins in the deformed calcite samples had a hexagonal distribution in the 0001 oriented cylinder and an orthorhombic distribution in the
oriented cylinder.Zones of dense r-slip dislocations were observed in the
cylinder. Calculated resolved shear stresses for r-slip in either cylinder remained far below the published critical resolved shear stress (c.r.s.s.) value. Calculated contributions from twinning back stress and anisotropy do not account for the difference between the resolved shear stress and the c.r.s.s. These results underscore the necessity of considering dislocation activiation stresses rather than c.r.s.s. in quantitative analyses of heterogeneous of deformation in single crystals. 相似文献
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International Journal of Earth Sciences - Earthquakes in the brittle upper crust induce viscoelastic flow in the lower crust and lithospheric mantle, which can persist for decades and lead to... 相似文献
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Creep tests were performed on cylindrical cold-pressed polycrystalline specimens of pure NH4NO3 and KNO3 while the specimens were simultaneously cycled through the known phase transformations. The observed deformation behaviour and the transformation strains were both qualitatively and quantitatively consistent with the Greenwood and Johnson (1965) theory of transformational plasticity, which applies to external stresses less than the yield stress of the weaker phase. A new and simpler development of this theory is given in the Appendix. The transformation strains observed in composite specimens, in which only one component transformed, were all significantly less than those observed in the pure materials. This behaviour is observed even in specimens where the volume fraction of the marginally-stronger non-transforming phase is too small to form a continuous framework in the specimen. It is suggested that the stress associated with the difference in thermal expansion of the two components, especially during a phase transformation, leads to the nucleation of a high density of tangled, essentially immobile, dislocations in the transforming component, similar to work-hardening. The relevance of these observations to the possibility of transformation plasticity as an important deformation mechanism in the Earth is discussed. 相似文献
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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. 相似文献
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The paper presents preliminary results of the use of a high resolution double-focussing, magnetic sector inductively coupled plasma-mass spectrometer (HR-ICP-MS) with ultraviolet laser ablation (LA) for the bulk analysis of geological materials fused with Li2 B4 O7 . Detection limits are based on data from precision measurements of a fused SiO2 sample of high purity, and sensitivity data (cps/μg g-1 ) obtained on the Reference Material (RM) Syenite SY-2. For many trace elements, the detection limits are better than 0.05 μg g-1 using a sample to flux weight ratio of 1:7.
Calibration curves, which are based entirely on RMs, are established for Hf, Ta, Tb, Tm and Lu. They indicate that, even at this early stage in the development of the technique, data accurate to ˜ 25% can be collected. It is concluded that the method may prove to be a valuable supplement to XRF for low level element concentration measurements; it is also very practical, as the same sample discs can be used for both XRF and LA-ICP-MS analyses. 相似文献
Calibration curves, which are based entirely on RMs, are established for Hf, Ta, Tb, Tm and Lu. They indicate that, even at this early stage in the development of the technique, data accurate to ˜ 25% can be collected. It is concluded that the method may prove to be a valuable supplement to XRF for low level element concentration measurements; it is also very practical, as the same sample discs can be used for both XRF and LA-ICP-MS analyses. 相似文献
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The Clarion-Clipperton Zone (CCZ) of the central Pacific is one of the few regions in the world’s oceans that are still lacking full coverage of reliable identifications of seafloor spreading anomalies. This is mainly due to the geometry of the magnetic lineations’ strike direction sub-parallel to the Earth’s magnetic field vector near the equator resulting in low amplitude magnetic anomalies, and the remoteness of the region which has hindered systematic surveying in the past. Following recently granted research licenses for manganese nodules in the CCZ by the International Seabed Authority, new magnetic data acquired with modern instrumentation became available which combined with older underway data make the identification of seafloor spreading anomalies possible for large parts of the CCZ and adjacent areas. The spreading rates deduced from the seafloor spreading patterns show a sharp increase at the end of Chron 21 (47.5 Ma) which corresponds to the age of the bend in the Hawaii-Emperor seamount chain and an associated plate tectonic reorganisation in the Central Pacific. An accurate map of crustal ages for the central-eastern Pacific based on our anomaly picks may provide a basis for improved plate tectonic reconstructions of the region. 相似文献
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