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In some of the Blue Ridge Mountains occurrences, an epidotized augen gneiss is present exhibiting foliation structures. The dominant green epidote in unakite rocks is the metasomatic alteration product of plagioclase feldspar, while the orthoclase and quartz crystals remain unaffected.
Augen gneiss, from the German: Augen, meaning "eyes", is a gneiss resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear-bound grains (porphyroclasts), normally feldspar, surrounded by finer grained material. The finer grained material deforms around the more resistant feldspar grains to produce this ...
These show the most well-developed schistosity, defined as the extent to which platy minerals are present and are aligned in a single direction, so that the rock easily splits into plates less than a centimeter (0.4 inches) thick. [24] Gneisses, which are more coarse grained and show thicker foliation than schists, with layers over 5mm thick. [23]
Geological map of the Hebridean terrane showing distribution of rocks of the Lewisian complex Undeformed Scourie dyke cutting Lewisian Gneiss, about 1.6 km west of Scourie Scourie dykes (now foliated amphibolites) cutting grey gneiss of the Scourie complex, both deformed during the Laxfordian tectonic event and cut by later (unfoliated) granite veins - road cutting on the A838 just north of ...
Quartzite can have a grainy, glassy, sandpaper-like surface. Quartzite is a hard, non-foliated metamorphic rock which was originally pure quartz sandstone. [1] [2] Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts.
Pore fluid present between mineral grains is an important medium through which atoms are exchanged. [4] This permits recrystallization of existing minerals or crystallization of new minerals with different crystalline structures or chemical compositions (neocrystallization). [ 1 ]
A sample of banded hornfels, formed by contact metamorphism of sandstones and shales by a granite intrusion. Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. [1]
Metamorphic minerals alter with the changing P-T condition with time without reaching complete phase equilibrium, making P-T-t path tracking possible. From 1910 Ma (i.e. 1910 million years ago) to 1840 Ma, the rock experienced an increase in P-T conditions and formed mineral garnet , which is attributed to burial and heating.