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Plagioclase Feldspars

Plagioclase feldspars are the most common feldspar minerals because calcium is somewhat more common in the crust than potassium (3.6 and 2.8 percent of the crust, respectively). Plagioclase feldspars form a continuous solid solution between Ab and An endmembers at high temperature, but the replacement of ions needs to be coupled because of the charge difference between Na+ and Ca2+. The charge balance in maintained by substituting Al3+ for Si4+. The amount of potassium that may enter the lattice is limited because of large difference in ionic radii. Members of the plagioclase group are the most common rock-forming minerals. They are important to dominant minerals in most igneous rocks of the Earth’s crust. They are major constituents in a wide range of intrusive and extrusive igneous rocks including granite, diorite, gabbro, rhyolite, andesite, and basalt. Plagioclase minerals are important constituents of many metamorphic rocks, such as gneiss, where they can be inherited from an igneous protolith or formed during the regional metamorphism.