Metamorphic Textures

Slaty CleavageRocks with slaty cleavage will split into thin sheets along the cleavage planes. It occurs in fine grained rocks formed by low grade regional metamorphism. it can only form in rocks consisting of platy minerals such as clay minerals, chlorite and micas. at the microscopic scale, these minerals become aligned at 90 degrees to the direction of maximum pressure during metamorphism slaty cleavage may be at any angle to bedding, but is usually parallel to axial planes of the folds it cannot occur in rocks with rounded grains, such as quartz in sandstones Bedding and fossils may not be completely destroyed metamorphism, leaving traces or relict structures. Fossils may be deformed due to the high levels of compressive stress. Slates are common in North Wales and the Lake District.SchistosityFound in schists (medium grained rocks formed by regional metamoprhism), schistosity results from the alignment of flat, platy minerals, commonly muscovite mica, at 90 degrees to the direction of maximum pressure during metamorphism. Lightly coloured muscovite mica is concentrated into thin parallel bands, giving the rock a characteristic shiny appearance (micaceous sheen) where flat surfaces of mica are visible. Garnet porphyroblasts are often present and they disrupt the alignment of mica minerals. Schists are found in the Highlands of Scotland in Dalradian rocks.Gneissose Banding Found in gneisses (coarse grained rocks formed by regional metamorphism), gneissose banding is formed when light (usually quartz and feldspar) and dark coloured minerals (biotite mica and mafic minerals) are separated into bands. The mica-rich layer is foliated and the pale layer has granoblastic texture. The bands may be contorted or folded but are roughly at 90 degrees to the maximum pressure direction.Porphyroblastic TextureThis texture occurs in both regional and contact metamorphic rocks. Porphyroblasts are large crystals that grow during metamorphism and are surrounded by a finer groundmass. Metamorphic rocks that contain these large crystals are described as porphyroblastic. Garnet prophyroblasts found in schists may contain inclusions. Pyrite porphyroblasts can develop in slate, often forming clear cubic crystals.Granoblastic TextureThis is an unfoliated texture and is formed by thermal metamorphism. Pressure is not a factor in the formation of a granoblastic texture. The main characteristics are randomly orientated, equidimensional crystals usually in rocks with a few and sometimes only one mineral. Hornfels is an example of a fine grained rock with granoblastic texture. Marble and quartzite are granoblastic. Because of their medium grain size and white colour, their texture is sometimes described as sugary.

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