They are importantly dominant minerals in most igneous rock. In basaltic rocks it is normally gray to black. The one thing about plagioclase that's usually different is that its broken faces—its cleavage planes—almost … They are particularly common in igneous and metamorphic rocks, where they often form the bulk of the rock’s volume. Plagioclase minerals are typically white or gray, hard blocky crystal masses that exhibit cleavage in two directions that are nearly at right angles. … They are major constituents in a wide range of intrusive and extrusive igneous rocks including granite, diorite, gabbro, rhyolite, andesite, and basalt. This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium It is usually easy to distinguish plagioclase feldspars because their two planes of cleavage intersect at 90-degree angles, and their cleavage faces often display striations. All feldspar minerals have two directions of perfect cleavage. 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 of sedimentary rocks. These properties make plagioclase feldspars relatively easy to identify with a hand lens in coarse-grained igneous and metamorphic rocks. Plagioclase in granitic rocks is normally white, pink, or red in color. For rockhounds without laboratories, it's enough to be able to tell the two main types of feldspar, plagioclase (PLADGE-yo-clays) feldspar and alkali feldspar . … Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more properly known as the plagioclase feldspar series. Plagioclase feldspar group minerals are the most common rock-forming minerals. Description and Identifying Characteristics ‘Feldspar’ is derived from the German word feld, which means ‘field’, and spar, which means ‘crystal’. Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group.
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