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. As nouns the difference between orthoclase and plagioclase is that orthoclase is (mineralogy) potassium]] aluminum silicate, kalsi 3 [[oxygen|o 8, a common feldspar of igneous, plutonic, and metamorphic rocks orthoclase is the main feldspar of pegmatite occurrences, where it is most commonly flesh-colored orthoclase is used in the ceramic and glass industries and as a decorative gravel while plagioclase is … Orthoclase is a common constituent of most granites and other felsic igneous rocks and often forms huge crystals and masses in pegmatite. You can find more details on the difference between orthoclase and plagioclase in the below infographic. 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Appearance and habit - Microcline and orthoclase are generally anhedral; sanidine is more likely to form euhedral phenocrysts. “PlagioclaseFeldsparUSGOV” (Public Domain) via Commons Wikimedia. The key difference between orthoclase and plagioclase is that orthoclase appears in green-yellow colour, whereas Plagioclase appears in white colour. 4. The extinction angle is an optical characteristic and varies with the albite fraction (%Ab). Well shaped crystals and twins are highly sought after by mineral collectors, and the Moonstone variety is used as a gemstone. Basically, 50% rule says that Na dominant plagioclase is now albite and Ca dominant is anorthite. With the 50% rule, all of the mineral members occuring between these two are not considered valid minerals by IMA. Subgroup orthoclase and albite constitut: Orthoclase (K-feldspar) Subgroup albite and anorthite constitut: Plagioclase The mixed crystal groups between the two end joints albite (sodium feldspar), NaAlSi3O8) and anorthite (lime feldspar, CaAl2Si2O8) constitutes plagioclase that usually can be separated from alkali feldspar by distinctive twin striping on the cleaving surfaces. Madhu is a graduate in Biological Sciences with BSc (Honours) Degree and currently persuing a Masters Degree in Industrial and Environmental Chemistry. The composition of a plagioclase feldspar is typically denoted by its overall fraction of anorthite (%An) or albite (%Ab), and readily determined by measuring the plagioclase crystal's refractive index in crushed grain mounts, or its extinction angle in thin section under a polarizing microscope. 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