Plagioclase
Plagioclase (/ˈplædʒ(i)əˌkleɪs, ˈpleɪdʒ-, -ˌkleɪz/ PLAJ-(ee)-ə-klayss, PLAYJ-, -klayz)[4] is a series of tectosilicate (framework silicate) minerals within the feldspar group.The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure.Plagioclase is a major constituent mineral in Earth's crust and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks.[11] The ability of albite and anorthite to form solid solutions in any proportions at elevated temperature reflects the ease with which calcium and aluminium can substitute for sodium and silicon in the plagioclase crystal structure.When a plagioclase composition is described by its anorthite mol% (such as An40 in the previous example) it is assumed that the remainder is albite, with only a minor component of potassium feldspar.[13] It is from this slightly oblique cleavage angle that plagioclase gets its name, Ancient Greek plágios (πλάγιος 'oblique') + klásis (κλάσις 'fracture').The composition can be roughly determined by specific gravity, but accurate measurement requires chemical or optical tests.This plagioclase effect causes the residual melt to be enriched in sodium and silicon and depleted in aluminium and calcium.[28] In rare cases, plagioclase shows reverse zoning, with a more calcium-rich rim on a more sodium-rich core.[44] The Mohorovičić discontinuity, which defines the boundary between the Earth's crust and the upper mantle, is thought to be the depth where feldspar disappears from the rock.[45] While plagioclase is the most important aluminium-bearing mineral in the crust, it breaks down at the high pressure of the upper mantle, with the aluminium tending to be incorporated into clinopyroxene as Tschermak's molecule (CaAl2SiO6) or in jadeite NaAlSi2O6.The resulting rock, in which fine streaks of plagioclase (lamellae) are present in potassium feldspar, is called perthite.[8] However, exsolution in the andesine to labradorite compositional range sometimes produces lamellae with thicknesses comparable to the wavelength of visible light.This acts like a diffraction grating, causing the labradorite to show the beautiful play of colors known as chatoyance.[29] In addition to its importance to geologists in classifying igneous rocks, plagioclase finds practical use as construction aggregate, as dimension stone, and in powdered form as a filler in paint, plastics, and rubber.
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