Science and Engineering at The University of Edinburgh |
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Section Contents
- Career outline
- Research highlights
- Current Research
- Publications
Current research: structural dislocations in perthitic K-feldspar
Structural dislocations have important roles in low temperature reactions. In K-feldspars in plutonic rocksthey form during cooling as planar microperthites exsolve. We are studying their development and properties in alkali feldspar phenocrysts from the Shap granite, Cumbria, although they are common in two-feldspar granites generally. The small difference in unit cell size between coherent albite-rich lamellae and the orthoclase-rich host leads to a build up of spontaneous structural strain across the interface. As exsolution progresses and lamellae coarsen this strain exceeds the binding forces for structural coherency and dislocations form. They form continuous loops around albite lamellae, running in two directions (parallel to Y and close to Z in the so-called Murchison Plane between ~ -801 and -601).
Once formed, dislocations represent reactive sites with high free energy that increase local dissolution rates by several orders of magnitude. Dissolution at these sites produces an intricate network of interconnected sub- µm microtubes which penetrate deep (>1cm) into the structure. Together with deuteric alteration these processes control the chemical weathering and dissolution characteristics of alkali feldspars. As one of the most abundant minerals in the Earth's crust, proper understanding of dissolution behaviour and reactivity of this mineral is important for many areas or Earth Science. The T-t formation of tiny dissolution tubes is important for argon retention and Ar-Ar thermochronology, and 18O-16O exchange. We have also suggested that these tiny tubes would have been perfect reactors for the organisation of the complex organic reactions leading to the the emergence of life on Earth.
Weathering |
Search for superfresh phenocrysts |
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In previous work on weathered material from the Shap Granite we established the presence of deep etch pits and honeycomb structures produced by a combination of deuteric coarsening and preferential dissolution of albite lamellae along dislocations.
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Recent TEM studies on samples from the Shap Granite (Parsons & Fitz Gerald, 2003) yielded the unexpected result that dislocations from apparently 'fresh' alkali feldspar from the interior of large phenocrysts are in fact tiny etch tubes, sometimes only a few nm across. This is interesting in itself as it suggests that no part of even large alkali feldspar phenocrysts may be deemed entirely unaffected by deuteric fluids. Using a combination of SEM and TEM microscopy John Fitz Gerald (ANU) and I hope to extend this observation to a variety of granitic rocks. We hope to find material with lattice scale dislocations (as opposed to etch-tubes) and to study their character and T stability using HRTEM combined with heating and cooling. Although in perfect crystal structures reactions involving diffusion always slow with decreasing temperature, the spontaneous appearance of defects decreases the effective diffusion dimension and increases the low T reactivity. This will affect not only dissolution-reprecipitation reactions, but also processes such as 40Ar and 18O exchange |
Using a combination of SEM and TEM microscopy we intend to better constrain the morphology and extent of these structures. Early conclusions suggest honeycomb networks may be traced at least fifteen millimetres in from the surface of large weathered phenocrysts. Where one phenocryst surface is exposed in outcrop, a progression of weathering and dissolution textures may be observed perpendicular to the outcrop surface. The extent of these textures confirms the importance of lattice-scale microstructures in controlling the reactivity and weathering characteristics of alkali feldspar, and produces a very high reactive surface area for dissolution, and, ultimately, a fragile surface for mechanical degradation.Heating experiments |
Other topics |
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The stability of defect microtextures, including dislocations and interfaces of perthitic lamellae, during heating, is crucial to the reactivity of feldspars during burial, and in the interpretation of 39Ar/40Ar spectra from plutonic alkali feldspars. Preliminary work on Shap feldspars (Parsons and Fitz Gerald, 2003) has shown that coherent perthite lamellae and compositional contrast in incoherent lamellae may disappear rapidly during heating above the solvus, while dislocations and features dependent on the Si, Al-O framework persist for long periods. TEM images showing what appear to be 'perthite' lamellae and twins may be of chemically homogeneous feldspar. We are studying the kinetics of the homogenisation process in detail using SEM and TEM methods. |
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