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. They are importantly dominant minerals in most igneous rock. This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. Plagioclase feldspar group minerals are the most common rock-forming minerals. Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. They are particularly common in igneous and metamorphic rocks, where they often form the bulk of the rock’s volume. All feldspar minerals have two directions of perfect cleavage. They are major constituents in a wide range of intrusive and extrusive igneous rocks including granite, diorite, gabbro, rhyolite, andesite, and basalt. These properties make plagioclase feldspars relatively easy to identify with a hand lens in coarse-grained igneous and metamorphic rocks. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium 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. … The one thing about plagioclase that's usually different is that its broken faces—its cleavage planes—almost … Plagioclase minerals are typically white or gray, hard blocky crystal masses that exhibit cleavage in two directions that are nearly at right angles. In basaltic rocks it is normally gray to black. Description and Identifying Characteristics ‘Feldspar’ is derived from the German word feld, which means ‘field’, and spar, which means ‘crystal’. 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. 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 . … Plagioclase in granitic rocks is normally white, pink, or red in color.