Pseudotachylyte
Glassy, or very fine-grained, rock type / From Wikipedia, the free encyclopedia
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Pseudotachylyte (sometimes written as pseudotachylite) is an extremely fine-grained to glassy, dark, cohesive rock occurring as veins[1] that form through frictional melting and subsequent quenching during earthquakes,[2] large-scale landslides, and impacts events.[3] Chemical composition of pseudotachylyte generally reflects the local bulk chemistry, though may skew to slightly more mafic compositions due to the preferential incorporation of hydrous and ferro-magnesian minerals (mica and amphibole, respectively) into the melt phase.[4]
Pseudotachylyte was first documented by Shand in the Vredefort Impact Structure and was named due to its close resemblance to tachylyte, a basaltic glass.[5] Though pseudotachylyte is reported to have a glassy appearance, they are extremely susceptible to alteration and are thus rarely found to be entirely composed of glass.[6][7] Typically, they are completely devitrified into a very fine-grained material with quench textures such as chilled margins,[8][9] radial and concentric clusters of microcrystalites (spherulites)[10][11] or as radial overgrowths of microcrystalites on clasts,[12] as well as skeletal and spinifex microcrystalites.[6][10]