The names of plagioclase are arbitrary and indicate a specific compositional range. The different charge of these ions is compensated by the contemporaneous substitution of Si 4+ by Al 3+ (i.e. The solid-solution between albite and anorthite is possible, since Na + and Ca 2+ have similar ionic radius (Na + = 0.97 Å Ca 2+ = 1.00 Å). Anorthite is also triclinic, but its structure is more ordered compared to albite, with atoms of Si and Al alternating with strict regularity. The structure of albite, representing also the sodic end-member of the alkali feldspars, is triclinic. The plagioclase series represents a solid-solution between albite (Na) and anorthite (Ca). Likewise alkali feldspars, plagioclase may contain traces of Fe 2+, Fe 3+, Ba, Ti, Mg, Sr, and Mn. They show a network of (Si,Al)O 4 tetrahedrons linked to one another (sharing oxygens) in all directions, distorted by large cation sites that may contain Na +, Ca 2+, and some K +. Plagioclase feldspars are framework silicates with a structure similar to that of alkali feldspars. The name plagioclase derives from the Greek πλάγιος (plágios, “oblique”) + κλᾰ́σῐς (klásis, “fracture”), in reference to its two cleavage systems. Plagioclases are fundamental constituents of the Earth’s crust and also of the anorthosite crust of the Moon (the highlands). Plagioclase feldspars are important Na- and Ca-bearing aluminosilicates that occur in most igneous rocks, as well as in many metamorphic and sedimentary rocks.
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