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Migmatite is a composite rock found in medium and high-grade metamorphic environments, commonly within Precambrian cratonic blocks. It consists of two or more constituents often layered repetitively: one layer is an older metamorphic rock that was reconstituted subsequently by partial melting ("paleosome"), while the alternate layer has a ...
In most cases the area is a thin section or a polished slab. The objects of interest vary between subdisciplines and can for example be quartz or feldspar grains in sedimentology , any type of mineral in petrology or different taxonomic groups in paleontology .
In structural geology, ultramylonite is a kind of mylonite defined by modal percentage of matrix grains [clarify] more than 90%. [4] Ultramylonite is often hard, dark, cherty to flinty in appearance and sometimes resemble pseudotachylite and obsidian. In reverse, ultramylonite-like rocks are sometimes "deformed pseudotachylyte". [5] [6] [7] [8]
In optical mineralogy and petrography, a thin section (or petrographic thin section) is a thin slice of a rock or mineral sample, prepared in a laboratory, for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a diamond saw and ground optically flat.
Migmatite is a gneiss consisting of two or more distinct rock types, one of which has the appearance of an ordinary gneiss (the mesosome), and another of which has the appearance of an intrusive rock such pegmatite, aplite, or granite the (leucosome). The rock may also contain a melanosome of mafic rock complementary to the leucosome. [11]
A cross-polarized thin section image of a garnet-mica-schist from Salangen Municipality, Norway showing the strong strain fabric of schists. The black crystal is garnet, the pink-orange-yellow colored strands are muscovite mica, and the brown crystals are biotite mica.
In a thin section, the observed length and width may not necessarily be the true length and width of a microcrack in three dimensions. [1] [7] The aspect ratio is the ratio of width to length. [1] It is generally10 −3 to 10 −5. [1] The crack length increases with increasing maximum applied stress, resulting in a decrease in the aspect ratio ...
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