1 Schematic diagrams of A) an edge and B) screw dislocation in an atomic lattice. 

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... grain boundary maps shows that barroisite grains in these rocks have subgrain walls (Fig. 5.13). Intraphase misorientation angle distribution analyses from all samples shows high frequencies for low angle (<10?) misorientation for amphibole ( Fig. 5.13). This would suggest that some plasticity (likely dislocation creep) is involved in the formation of the barroisite LPO. This would suggest that the barroisite LPO is formed in a possible two stages, the first involving mimicry of omphacite and glaucophane, the second involving a bit of plastic strain (dislocation creep) creating subgrain ...

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... grain boundary maps shows that barroisite grains in these rocks have subgrain walls (Fig. 5.13). Intraphase misorientation angle distribution analyses from all samples shows high frequencies for low angle (<10?) misorientation for amphibole ( Fig. 5.13). This would suggest that some plasticity (likely dislocation creep) is involved in the formation of the barroisite LPO. This would suggest that the barroisite LPO is formed in a possible two stages, the first involving mimicry of omphacite and glaucophane, the second involving a bit of plastic strain (dislocation creep) creating subgrain ...

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... omphacite crystals have well defined, elongate areas of variable extinction and birefringence ( Fig. 6.11) suggesting a polycrystalline or subgrain nature. The long axes of these elongate grains/subgrains create an omphacite shape fabric parallel to the long axis of the overall large omphacite grain. In some cases however these large elongate omphacite grains do not display much internal detail and the whole grain will go into extinction under crossed polars at the same orientation and birefringence will vary only slightly. Smaller, irregular shaped patches of omphacite occur but are less common (Fig. 6.12). As with the larger, elongate grains, these patches display variable extinction suggesting a polycrystalline or subgrain internal structure for them as well. Another similarity that the irregular omphacite grains share with the elongate grains is that occasionally these grains display little internal ...

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... omphacite crystals have well defined, elongate areas of variable extinction and birefringence ( Fig. 6.11) suggesting a polycrystalline or subgrain nature. The long axes of these elongate grains/subgrains create an omphacite shape fabric parallel to the long axis of the overall large omphacite grain. In some cases however these large elongate omphacite grains do not display much internal detail and the whole grain will go into extinction under crossed polars at the same orientation and birefringence will vary only slightly. Smaller, irregular shaped patches of omphacite occur but are less common (Fig. 6.12). As with the larger, elongate grains, these patches display variable extinction suggesting a polycrystalline or subgrain internal structure for them as well. Another similarity that the irregular omphacite grains share with the elongate grains is that occasionally these grains display little internal ...

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... the glaucophane here contains eclogite facies garnet as inclusions and some large porphyroblasts cut the fabric formed by the omphacite. Glaucophane displays two separate morphologies, the first being large (~0.5-2.5mm), elongate grains that define a shape fabric parallel to the omphacite shape fabric (Fig. 4.10). These grains can be isolated from each other or form a series of interconnected grains. The second morphology consists of large (~1-3mm), euhedral-subhedral porphyroblasts which cut the omphacite shape fabric obliquely (Fig. 4.11). Glaucophane grains commonly contain garnet, omphacite and zoisite as ...

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... the glaucophane here contains eclogite facies garnet as inclusions and some large porphyroblasts cut the fabric formed by the omphacite. Glaucophane displays two separate morphologies, the first being large (~0.5-2.5mm), elongate grains that define a shape fabric parallel to the omphacite shape fabric (Fig. 4.10). These grains can be isolated from each other or form a series of interconnected grains. The second morphology consists of large (~1-3mm), euhedral-subhedral porphyroblasts which cut the omphacite shape fabric obliquely (Fig. 4.11). Glaucophane grains commonly contain garnet, omphacite and zoisite as ...

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... bands are composed of large, elongate omphacite crystal laths that in some cases taper to a point ( Fig. 6.10). When viewed on foliation surfaces the long axes of these omphacite grains are seen to have variable orientation. Inclusions within the omphacite grains consist mainly of small crystals of garnet and ...

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... to the C2/c structure the length of the burgers vector remains the same but extra energy is required by the formation of stacking faults. Thus b=?<-110> dislocations will be less mobile in P2/n omphacite than in C2/c omphacite. Fig. 1.9 Schematic diagram of the omphacite structure viewed parallel to the c direction. In C2/c omphacite space group the grey arrow indicates the perfect dislocation b=1/2<-110>. In P2/n omphacite space group the grey arrow shows the doubled length of the perfect dislocation to ...

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... in the foliation are also present (Fig 4.21) as is a quartz ...

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... types of dislocations exist, edge and screw types. They both form a dislocation line, a linear defect, through the crystal lattice. Edge dislocations form the edge of an extra layer of atoms within the crystal lattice and move perpendicular to the dislocation line ( Fig. 1.1A). Screw dislocations form a line along which the crystal lattice jumps one lattice point and moves parallel to the dislocation line ( Fig. ...

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... types of dislocations exist, edge and screw types. They both form a dislocation line, a linear defect, through the crystal lattice. Edge dislocations form the edge of an extra layer of atoms within the crystal lattice and move perpendicular to the dislocation line ( Fig. 1.1A). Screw dislocations form a line along which the crystal lattice jumps one lattice point and moves parallel to the dislocation line ( Fig. ...

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... area of the Zermatt-Saas Unit examined in this study is located at the northern end of Val de Gressoney on Punta Telcio (Fig. 4.1). Punta Telcio is an excellent exposure of the Zermatt-Saas Unit containing a number of lithologies common to ophiolitic bodies. A small mapped area (430m x 210m), centred on Lago Blu, of the Zermatt-Saas Unit on Punta Telcio (Fig. 4.2) displays four lithologies common to the unit; serpentinite, quartz-garnet schist, eclogite and amphibolite (metabasics). ...

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... also suggest that the asymmetry of some of the LPOs may indicate non-coaxial strain during deformation. A study done by Ulrich and Mainprice (2005) emulates these results, with extension tests producing strong L-type fabrics, compression tests giving strong S-type fabrics and pure shear and simple shear experiments giving LS types ( Fig. 1.10). They explain how, by increasing the CRSS by one order of magnitude for the ?<110> {110} system, there is a significant reduction in its activity but this does not produce a change in the LPO of omphacite from S to L- type. This suggests that the activity of this slip system does not have a significant effect on LPO development in omphacite. Fig. 1.10 From Ulrich and Mainprice (2005). Omphacite LPO patterns generated from compression and extension VPSC models. XY plane is horizontal, compression is parallel to Z and extension is parallel to X. Minimum and maximum densities are marked on the bottom left and bottom right of each pole figure ...

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... also suggest that the asymmetry of some of the LPOs may indicate non-coaxial strain during deformation. A study done by Ulrich and Mainprice (2005) emulates these results, with extension tests producing strong L-type fabrics, compression tests giving strong S-type fabrics and pure shear and simple shear experiments giving LS types ( Fig. 1.10). They explain how, by increasing the CRSS by one order of magnitude for the ?<110> {110} system, there is a significant reduction in its activity but this does not produce a change in the LPO of omphacite from S to L- type. This suggests that the activity of this slip system does not have a significant effect on LPO development in omphacite. Fig. 1.10 From Ulrich and Mainprice (2005). Omphacite LPO patterns generated from compression and extension VPSC models. XY plane is horizontal, compression is parallel to Z and extension is parallel to X. Minimum and maximum densities are marked on the bottom left and bottom right of each pole figure ...

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... rock also contains augen, composed of green serpentine and black magnetite (Fig. 4.19), and the long axes of these augen align with the foliation. Augen serpentine has a mesh texture (<100?m-200?m), local concentrations of chrysotile, long (~500?m-1.5mm), narrow grains of serpentine/brucite and black grains of magnetite ...

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... is the most common garnet species and generally contains large amounts of both the pyrope and spessartine molecule and may also have significant grossular components. Grossular incorporates a significant amount of the andradite molecule with which it can form a continuous series. Also despite a grossular-hydrogrossular series there is no evidence that it is typical of thermal metamorphism as it contains significant amounts of water (Deer et al. 1992). Fig. 1.11 Relative proportions of end-member garnet molecules for garnet in eclogites and related rock types. Dotted lines show average garnet compositions in; 1 -amphibolites, 2 -charnockites and granulites, 3 -eclogites in gneisses or migmatitic terrain, 4 -eclogites associated with kimberlite pipes, 5 -eclogites within ultramafic ...

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... final theory for production of the observed omphacite microstructure may be that as omphacite first began to develop it was in fact subjected to some deformation and subgrains developed as a result of plastic deformation processes ( Fig. ...

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... addition omphacite from these eclogites contains strong evidence for the action of diffusion creep in the form of preferential chemical zoning patterns and overgrowth of broken grains. As observed in omphacite grains from sample S6.8 and S6.13 aegirine/jadeite rich omphacite zone into more diopsidic omphacite often displaying quite sharp chemical boundaries. The preferential patterns observed (zoning occurs along only the long axes of omphacite grains) (Fig. 4.35, Fig. 4.37) and overgrowth of broken grains (Fig. 4.41, Fig. 4.42) suggests the action of a diffusive mass transfer process, likely coble creep or pressure solution. We rule out Nabarro-Herring creep as the acting mechanism, as lattice diffusion through omphacite grains would likely result in more diffuse chemical zoning rather than the sharp zoning patterns ...

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... garnet is of the cubic crystal system ( Fig. 1.12A), with axes lengths; a = b = c and angles between axes; ? = ? = ? = 90?. Garnet also has a body centred cubic lattice arrangement (space group Ia3d). The unit cell of garnet contains eight X 3 Y 2 (SiO 4 ) 3 formula units ( Fig. 1.12B). The structure has alternating SiO 4 tetrahedra and YO 6 octahedra which share corners to form a 3D network. Within these exist "cavities" which contain the X ions. Garnets are most often found in the dodecahedral habit but also commonly form a deltoidal icositetrahedon. It is important to keep in mind that all garnet species are isomechanical, meaning that all species behave the same way mechanically ( Karato et al., 1995). Fig. 1.12 A -Diagram of the cubic crystal system garnet belongs to, B - diagrammatical structure of garnet showing the distorted "cubes" of oxygen atoms that form around the X ...

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... garnet is of the cubic crystal system ( Fig. 1.12A), with axes lengths; a = b = c and angles between axes; ? = ? = ? = 90?. Garnet also has a body centred cubic lattice arrangement (space group Ia3d). The unit cell of garnet contains eight X 3 Y 2 (SiO 4 ) 3 formula units ( Fig. 1.12B). The structure has alternating SiO 4 tetrahedra and YO 6 octahedra which share corners to form a 3D network. Within these exist "cavities" which contain the X ions. Garnets are most often found in the dodecahedral habit but also commonly form a deltoidal icositetrahedon. It is important to keep in mind that all garnet species are isomechanical, meaning that all species behave the same way mechanically ( Karato et al., 1995). Fig. 1.12 A -Diagram of the cubic crystal system garnet belongs to, B - diagrammatical structure of garnet showing the distorted "cubes" of oxygen atoms that form around the X ...

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... garnet is of the cubic crystal system ( Fig. 1.12A), with axes lengths; a = b = c and angles between axes; ? = ? = ? = 90?. Garnet also has a body centred cubic lattice arrangement (space group Ia3d). The unit cell of garnet contains eight X 3 Y 2 (SiO 4 ) 3 formula units ( Fig. 1.12B). The structure has alternating SiO 4 tetrahedra and YO 6 octahedra which share corners to form a 3D network. Within these exist "cavities" which contain the X ions. Garnets are most often found in the dodecahedral habit but also commonly form a deltoidal icositetrahedon. It is important to keep in mind that all garnet species are isomechanical, meaning that all species behave the same way mechanically ( Karato et al., 1995). Fig. 1.12 A -Diagram of the cubic crystal system garnet belongs to, B - diagrammatical structure of garnet showing the distorted "cubes" of oxygen atoms that form around the X ...

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... area of the undeformed eclogite pod investigated using EBSD (Fig. 6.18) is mapped chemically to see if the undeformed omphacite grains displayed any chemical growth zoning or whether any chemical patterns associated with the various microstructural textures existed. EDX Al, Ca, Fe and Mg chemical maps show that there are no significant chemical variation within them and no discernable chemical zoning patterns. Lack of zonation in omphacite chemistry holds possible implications for the evolution of this area of the Sesia-Lanzo Zone and these are addressed later in the chapter. This pattern of S6.42 pole figures suggests a few single grains of omphacite are dominating the data (more than one orientation measurement per ...

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... omphacite has the structural formula M2M1T2O6. M2 eight co-ordinated sites contain Ca, Na, and (Fe, Mg), M1 octahedral sites contain Mg, Fe 2+ , Fe 3+ , Al and Cr, and T (tetrahedral) sites contain Si and Al. At high temperatures all the M1 sites in the crystal structure are the same but as temperature decreases Al and Mg arrange themselves in an alternating sequence. Due to this, Ca and Na are also arranged in a similar way to maintain local charge balance. This creates the ordered P2/n space group structure in omphacite which leads to a loss of symmetry. In this space group there are two distinct octahedral sites, one being Mg rich (M1) and the other Al rich (M11). There are also two different eight coordinated sites, the Na rich M2 site and the Ca rich M21 site ( Fig. 1.4) (Clarke andPapike, 1978, Deer et al., ...

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... omphacite has a C2/c (disordered) or P2/n (ordered) structure is dependent on temperature and as previously stated chemistry. Exsolution lamellae and antiphase domains are common microstructures observed in omphacite crystals. These microstructures are interpreted in terms of a combined system in which a field of ordered omphacite at intermediate compositions is superimposed onto an immiscibility gap between disordered omphacite end members giving broad two- phase fields between impure jadeite and omphacite and between omphacite and sodic-augite ( Fig. 1.5) (Carpenter, ...

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... 2 of this thesis. What follows here is a brief description of the geology of the Sesia-Lanzo Zone as found in the study area of Colle della Barme d"Oropa. It is located in the Monte Mars nature reserve on the mountain ridge between Val di Gressoney and Val Sesia (Fig 6.1). The area contains excellent exposure of rocks associated with the Eclogitic Micaschist Complex (EMC) sub division of the Sesia- Lanzo Zone (Chapter 2). Compagnoni, 1983). Rutile is present as an accessory mineral, occasionally rimmed by sphene. Other minerals are retrogressive in origin including green amphibole and ...

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... relationship described here would be expressed so: suggest that barroisite is not completely mimicking the omphacite grains, though this can be explained. Some of these bordering omphacite grains have the special relationship boundary with the barroisite whereas others do not. It is likely that the barroisite grain nucleated on a certain omphacite grain creating the "special relationship" misorientation and then grew in that same orientation, engulfing and destroying other omphacite of varying orientation. Occasionally other omphacite grains oriented in the same way as the "nucleation" omphacite grain are encountered and the "special relationship" misorientation boundary is achieved again. These can be seen in texture component maps of omphacite and using misorientation profiles between relative omphacite grains, showing there is low misorientation between omphacite grains across a large barroisite grain (Fig. ...

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... investigation of partially retrogressed eclogites from the Gressoney Valley area of the Zermatt-Saas unit, Western Alps ( Fig. 5.1), attempts to ascertain how much mimicry may have influenced the formation of an LPO in retrogressive barroisite and what this may reveal about the exhumation of the high pressure ...

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... mica and rutile are present in various amounts, with white mica forming long grains (~300?m -500?m) that form the same shape fabric defined by other minerals (Fig. 4.17). Rutile (~50?m -200?m) can occur in the matrix of the rock but is more common as inclusions in other minerals. It occasionally has a reaction rim of sphene (Fig. ...

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... mica and rutile are present in various amounts, with white mica forming long grains (~300?m -500?m) that form the same shape fabric defined by other minerals (Fig. 4.17). Rutile (~50?m -200?m) can occur in the matrix of the rock but is more common as inclusions in other minerals. It occasionally has a reaction rim of sphene (Fig. ...

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... into the possible crystallographic relationship between omphacite and barroisite is revealed though the D-D and P-P maps and misorientation angle distribution analyses. High frequencies of 30? misorientations are noted for <100>, >170? misorientations for <010> and <10? misorientations for <001> across omphacite/barroisite phase boundaries. When exploring a single neighbour pair set of axes misorientations, these three high frequency misorientations coincide. The high frequency misorientation signals represent a "special relationship" between the crystallography of omphacite and barroisite ( Fig. 5.11). Where barroisite has formed around omphacite due to retrogression it has taken on an orientation that has resulted in these specific misorientations with the omphacite crystal lattice. In other words it suggests that the omphacite lattice has a strong effect on the orientation of forming barroisite ...

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... second possibility may involve multiple omphacite nucleations on a grain of a protolith phase. Orientation of these omphacite nuclei may have been similar but not identical thus creating low angle subgrain boundaries between neighbouring areas of omphacite growth (Fig. 6.17B). Grains continue to grow and again, due to anisotropic omphacite grain growth, results in the elongate grains parallel to the faster growing <001> ...

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... crops out as a thin, elongate body that tracks across the length of Punta Telcio ( Reddy et al., 1999). In the area mapped for this study it forms the southern boundary. This rock is dark green/black in colour and contains a strong foliation (Fig. 4.19) and lineation (Fig. ...

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... crops out as a thin, elongate body that tracks across the length of Punta Telcio ( Reddy et al., 1999). In the area mapped for this study it forms the southern boundary. This rock is dark green/black in colour and contains a strong foliation (Fig. 4.19) and lineation (Fig. ...

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... are a few theories that may explain the formation of subgrain boundaries due to omphacite grain growth. The first involves the possibility of lineage structures or lineage boundaries in omphacite. Lineage structures are noted in other minerals such as quartz (Carstens, 1967) and pyrite and beryl (Petreus, 1978) and are crystalline growth structures that divide a crystal into separate parts that are slightly misoriented relative to each other. In quartz these lineage structures form in grains with high dislocation densities which are rearranged into low angle boundaries. These dislocations are produced where non-structural impurities are trapped during rapid grain growth. The microstructural textures and misorientation patterns described here for omphacite may represent similar growth structures for this mineral with dislocations supplied by small garnet and quartz inclusions. These lineage boundaries combined with preferential growth in the <001> direction would result in the fan like omphacite grain forms (Fig. 6.17A). In quartz lineation boundaries are noted to be sub-parallel to the c axis direction (Carstens, 1968) which is also the faster growth direction in quartz (Shelley, ...

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... unroofing of the Zermatt-Saas since 36Ma appears to be a result of erosion following uplift caused by further shortening (Reddy et al, ...

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... maps for all samples show that the interphase misorientation patterns are exactly the same as they are in P-P and D-D maps. For some areas the interphase misorientation is >10? and in others areas it is <10?. PD-PD maps are shown for sample S.6.14 ( Fig. ...

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... eclogite facies rocks studied in this thesis contain a number of potential case studies for investigating the effect of pre-existing minerals on the development of new ones. The breakdown of jadeite and quartz to albite during retrogression of the Zermatt-Saas unit provides such an example. This would require an investigation of orientation between minerals of different crystal symmetries. In the Sesia-Lanzo Zone retrogression may provide further examples of eclogite facies minerals controlling the growth of retrogressive ones. For example, retrogressive fracture fills across large, elongate omphacite grains show fibrous like growths of green amphibole (Fig. 6.11). This fracture fill texture holds the potential to have formed due to retrogressive minerals forming a preferential orientation on ...

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... a further study of Glenelg garnets by Storey and Prior (2005) Numerical simulations of garnet LPO development in garnets using VPSC models ( Mainprice et al. 2004) and slip systems determined by TEM from experimentally and naturally deformed samples have been carried out ( Fig. 1.13). The show weak patterns, with <100> poles aligned with the main shortening ...

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... can be broadly divided into two types; large porphyroblasts (1-3 mm) with Large garnet porphyroblasts are often surrounded by large patches of quartz which may be pressure shadows. This texture is most common where garnet grains occur in clusters ( Fig. 4.16). In some areas garnet grains appear to have grown into clusters such that the morphology of them takes on a jigsaw like appearance. In some of these cases garnet grains have grown so close that only very narrow spaces remain between them causing what often look like wide fractures within a single garnet grain (Fig. ...

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... can be broadly divided into two types; large porphyroblasts (1-3 mm) with Large garnet porphyroblasts are often surrounded by large patches of quartz which may be pressure shadows. This texture is most common where garnet grains occur in clusters ( Fig. 4.16). In some areas garnet grains appear to have grown into clusters such that the morphology of them takes on a jigsaw like appearance. In some of these cases garnet grains have grown so close that only very narrow spaces remain between them causing what often look like wide fractures within a single garnet grain (Fig. ...

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... Vickers optical microscope ( Fig. 3.1) is used to investigate thin sections for simple petrographical descriptions and locating areas intended for further investigation using other techniques. A Nikon coolpix 4500 camera is attached in order to digitally capture photomicrographs of the thin sections in both plane polarised light (PPL) and crossed polarised light ...

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... (straight extinction) and clinozoisite (inclined extinction) are common in this lithology and in places display a shape fabric consistent with the one defined by the amphibole. These minerals have an average grain size of ~300-600?m. Zoisite grains also occur as large porphyroblasts (~1.5mm) that can either cut across the fabric of the amphibolite or lie parallel to it (Fig. ...

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... of lattice preferred orientations (LPO) of omphacite started with Helmstaedt et al. (1972) which resulted in a classification scheme defining two end type fabrics. These are referred to as the L-type and S-type LPOs ( Fig. ...

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... also forms reaction rims around glaucophane grains and in many places epitactical and topotactical overgrowth of amphibole on glaucophane can be seen quite clearly (Fig. 4.10, Fig. 4.11, Fig. 4.13). Amphibole grains growing around garnet and glaucophane grains and replacing glaucophane grains vary widely in size (~50?m -1mm). A second stage of amphibole growth seems to occur within the rocks whereby the first stage amphibole is itself replaced by a second stage of amphibole growth. Grain sizes of this second phase of amphibole growth are very small (<100?m) (Fig. ...

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... also forms reaction rims around glaucophane grains and in many places epitactical and topotactical overgrowth of amphibole on glaucophane can be seen quite clearly (Fig. 4.10, Fig. 4.11, Fig. 4.13). Amphibole grains growing around garnet and glaucophane grains and replacing glaucophane grains vary widely in size (~50?m -1mm). A second stage of amphibole growth seems to occur within the rocks whereby the first stage amphibole is itself replaced by a second stage of amphibole growth. Grain sizes of this second phase of amphibole growth are very small (<100?m) (Fig. ...

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... also forms reaction rims around glaucophane grains and in many places epitactical and topotactical overgrowth of amphibole on glaucophane can be seen quite clearly (Fig. 4.10, Fig. 4.11, Fig. 4.13). Amphibole grains growing around garnet and glaucophane grains and replacing glaucophane grains vary widely in size (~50?m -1mm). A second stage of amphibole growth seems to occur within the rocks whereby the first stage amphibole is itself replaced by a second stage of amphibole growth. Grain sizes of this second phase of amphibole growth are very small (<100?m) (Fig. ...

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... large, elongate omphacite grains occasionally have fractures that cross their width containing retrogressive fills of green amphibole and chlorite (Fig. ...

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... have a monoclinic crystal structure such that a ? b ? c (where a = ~9.9?, b = ~18.0?, c = ~5.3?). Angles between the axes follow the monoclinic rule, ? = ? ? ?, with ? = ? = 90? and ? = 105.5?. They have the space group C2/m (C- centred with a mirror plane). The a and c axes intersect their respective planes obliquely whereas the b axes intersects the (010) plane perpendicularly in the same fashion as omphacite ( Fig. 1.3). Good cleavage in the {110} is common in hornblendes producing 56? and 124? angles and the phase generally has a hexagonal or granular ...

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... as previously mentioned eclogite pods are undeformed and as such the microstructures apparent within them must be related to some growth process. Three separate mechanisms have been suggested for creating the omphacite microstructures observed here (Fig. 6.17) but which one of these is in operation is still uncertain. The mineral johannsenite (4(CaNaSi 2 O 6 )) and diopside are shown to be isostructural (Schaller, 1938) and lineage structures have been noted in johannsenite in the b and c directions (Freed and Peacor, 1967) possibly suggesting similar structures would not be uncommon in diopside and other pyroxenes. The formation of multiple omphacite nuclei on a grain of a protolith phase may be a possible mechanism to create the observed microstructure although as previously mentioned no textural relationships remain preserved in the eclogite pod to suggest this. And as previously mentioned the theory assuming a small amount of deformation at the start of omphacite development followed by anisotropic growth seems less ...

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... amphibole appears in a variety of places in this lithology and represents a retrogressive event experienced by the Zermatt-Saas Unit. Amphibole forms reaction rims around many of the garnet grains and is also present as inclusions within garnet porphyroblasts (Fig. ...

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... layers of "streaked eclogite" are composed of large amounts of fine grained (~250?m), euhedral-subhedral, touching garnets with quartz, omphacite, zoisite and white mica scattered randomly within the layers (Fig. 6.13). Also present in this rock are small (~250?m), isolated garnets, or isolated clusters of small ...

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... discussed in chapter 1 anisotropic growth and dissolution rates in omphacite ( Fig. 1.7) can be responsible for creating both S and L-type LPO patterns. Long grains parallel to the <001> direction suggest anisotropic growth has had a significant effect on omphacite grain development in these rocks. If it were occurring during stress on these rocks it is possible that the omphacite LPOs were created by this ...

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... is the most common lithology found in the field area. The rock is blue/green in colour, and contains dark coloured amphibole, pink garnet and pale coloured minerals. The dark amphibole forms the majority of the lithology and defines a strong shape fabric which wraps garnet grains (Fig. 4.15). Occasionally dark and pale coloured minerals occur in separate bands forming a location fabric consistent with the shape fabric (Fig 4.16). In some areas the pale minerals occur in large (~3mm) polygonal patches (Fig. ...

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... is the most common lithology found in the field area. The rock is blue/green in colour, and contains dark coloured amphibole, pink garnet and pale coloured minerals. The dark amphibole forms the majority of the lithology and defines a strong shape fabric which wraps garnet grains (Fig. 4.15). Occasionally dark and pale coloured minerals occur in separate bands forming a location fabric consistent with the shape fabric (Fig 4.16). In some areas the pale minerals occur in large (~3mm) polygonal patches (Fig. ...

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... is the most common lithology found in the field area. The rock is blue/green in colour, and contains dark coloured amphibole, pink garnet and pale coloured minerals. The dark amphibole forms the majority of the lithology and defines a strong shape fabric which wraps garnet grains (Fig. 4.15). Occasionally dark and pale coloured minerals occur in separate bands forming a location fabric consistent with the shape fabric (Fig 4.16). In some areas the pale minerals occur in large (~3mm) polygonal patches (Fig. ...

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... growth, by whatever mechanism, alters the shape of the grains but not the crystal orientation. Rather it affects and creates LPO in a mineral by the selective growth of some grains and the elimination of others. Strain induced GBM rates will vary depending on the relative orientations of the two grains involved and in this way may have an effect on LPO (Poirier andGuillop?, 1979, Godard andvan Roermund, 1995). Anisotropic growth and dissolution rates of diffusive mass transfer processes will have an effect on LPO. In clinopyroxene attachment energies and face growth and dissolution rates are minimum for <010>, intermediate for <100> and maximum for <001> (Van Panhius-Sigler and Hartman, 1981). Therefore grains with <001> parallel to the tensional direction will grow faster than other grains and those with <010> parallel to the compressional direction will dissolve slower, favouring the crystal orientations compatible with omphacite LPO patterns observed in deformed eclogite (Godard and van Roermund, 1995) (Fig. 1.7). Similar studies of preferential c-axis growth in quartz veins show LPO formation by this mechanism (Cox andEtheridge, 1983, Shelley, 1983). Mauler et al. (2001). Schematic diagram of anisotropic grain dissolution and growth in omphacite and its effect on LPO. This process will favour preservation of grains in orientation A and those in orientation B will ultimately vanish. Mauler et al. (2001) suggest that a DMT mechanism is dominant in LPO formation and is the rate controlling one as opposed to dislocation creep. Their evidence is similar to that described above. Diffusion creep is grain size sensitive; with increasing grain size the length of diffusion pathways increase and potential dissolution and precipitation sites decrease, thus flux decreases. However eclogites examined in this study show that omphacite reaches grain sizes of over 7mm but still show evidence for diffusion creep. They suggest that in the Earth the domain of geological conditions in which diffusion processes are active is wider than ...

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... latter is investigated in Brenker et al. (2002) where a suite of eclogites from variable temperatures is studied. LPOs from the samples is collected and correlated to the C2/c and P2/n omphacite phase diagram using deformation temperatures and omphacite chemical compositions (Fig 1.8). The results show a strong correlation between the temperature of deformation and the resulting LPO. They also display a pattern such that S-type LPOs fall within the C2/c field whereas the L-type LPO mainly falls within the P2/n field. The authors then go on to suggest that this apparent correlation means it is the space group of omphacite that controls the mechanisms by which it deforms and through these the type of LPO that is generated. The main argument supporting this theory rises from slip system investigation in both ordered and disordered omphacite. The b = ?<110> dislocation is a perfect dislocation in the C2/c space group. Cation ordering in the P2/n space group produces a doubling of the length of the burgers vector for a perfect dislocation to b =<-110> ( Fig. 1.9). For energy reasons this will then split into two partial dislocations <-110>=?<-110> + ?<-110> and a stacking fault forms between ...

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... latter is investigated in Brenker et al. (2002) where a suite of eclogites from variable temperatures is studied. LPOs from the samples is collected and correlated to the C2/c and P2/n omphacite phase diagram using deformation temperatures and omphacite chemical compositions (Fig 1.8). The results show a strong correlation between the temperature of deformation and the resulting LPO. They also display a pattern such that S-type LPOs fall within the C2/c field whereas the L-type LPO mainly falls within the P2/n field. The authors then go on to suggest that this apparent correlation means it is the space group of omphacite that controls the mechanisms by which it deforms and through these the type of LPO that is generated. The main argument supporting this theory rises from slip system investigation in both ordered and disordered omphacite. The b = ?<110> dislocation is a perfect dislocation in the C2/c space group. Cation ordering in the P2/n space group produces a doubling of the length of the burgers vector for a perfect dislocation to b =<-110> ( Fig. 1.9). For energy reasons this will then split into two partial dislocations <-110>=?<-110> + ?<-110> and a stacking fault forms between ...