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.

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[@geoffgeoff143]

Why does some Labrorite form clear crystals?

[@morakamalebati7698]

The main reason why not all labradorites exhibit labradorescence (i.e., some are clear) is because of lamellar separation and composition. The requirement for lamellar separation is a very slow cooling of the rock containing the plagioclase. This slow cooling will enable Ca, Na, Si, and Al ions to diffuse through the plagioclase and produce the lamellar separation. The lamellar separation only occurs in plagioclases of a certain composition; those of calcic labradorite (50-70% anorthite and bytownite with an anorthite content of approximately 70-90%. Therefore, not all labradorites exhibit labradorescence (they might not have the correct composition and or/ they cooled too quickly), and not all plagioclases that exhibit labradorescence are labradorites (they may be bytownite).

[@ninentek5142]

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Will improve. Thank you for your input

[@Bloodknok]

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[@4integrity]

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