In Switzerland, from St. Gotthard, Ticino and In basaltic rocks it is normally gray to black. Islands, Alaska; from Grass Valley, Nevada Co., California. Plagioclase Feldspar in the Upper Midwest: Plagioclase occurs as an important constituent of igneous and metamorphic rocks throughout the Upper Midwest. The name âplagioclaseâ is frequently used instead of one of the more specific names in the table above. thin section, nα = 1.573 – 1.577 nβ = 1.580 – 1.585 nγ = 1.585 – Bytownite: An igneous rock composed almost entirely of bytownite. Many people enjoy the adularescence of moonstone, the aventurescence of sunstone, and the labradorescence of labradorite. It can be the most abundant clast in sediments Perfect cleavage, with cleavage faces intersecting at right angles and striations often present on cleavage faces. Plagioclase Feldspar. easily determined by measuring the refractive index or peel angle within the On Prince of Wales base of the triangle represents the solid solution series of plagioclase NASA image. 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. The extinction angle is an optical characteristic and varies with the albite fraction (%Ab). (2020). from Monte Somma and Vesuvius, Campania; on Mt. From Virgem da Lapa and Morro Velho, Minas Gerais, This specimen is from Crystal Bay, Minnesota, and measures about 4 inches (10 cm) across. The stone on the right is a beautiful orange 7x5 mm oval faceted stone weighing 1.01 carats. indicated by the general anorthite (% An) or albite (% Ab) fraction and is At Baveno, Piedmont, and in the P¯tschtal, Trentino-Alto Inexpensive mineral collections are available in the Geology.com Store. Plagioclase is series of framework silicate minerals in feldspar group. Plagioclase minerals are important constituents Some specimens of labradorite exhibit a schiller effect, which is a strong play of iridescent blue, green, red, orange, and yellow colors when moved under a source of incident light. occurrences include: Widespread; a few localities for good crystals are: Cite this article as: Geology Science. Moonstone is a name given to a gem material that consists of very thin, alternating layers of orthoclase (an alkali feldspar) and albite (a plagioclase feldspar). Concentrated plagioclase deposits are found in north-central Minnesota, as well as the northeast corner … Toshinyama, Tochigi Prefecture; the Zao volcano, Yamagata Prefecture; Otaru, It measures about 4 inches (10 cm) across. Vitreous. With the 50% rule, all of the mineral members occuring between these two are not considered valid minerals by IMA. The accompanying table sho… The plagioclase feldspars form a solid solution series between the end members of pure albite (NaAlSi3O8) and pure anorthite (CaAl2Si2O8). The minerals of the plagioclase series are listed in the table below along with their relative abundance of albite (Ab) and anorthite (An). On Miyakejima Island, Tokyo Prefecture; at 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. The luster is vitreous, sub-vitreous or resinous. When light enters the stone, it interacts with these thin layers to produce a phenomenon known as "adularescence" (a white-to-bluish light that floats under the surface of the stone when it is turned under a source of light). Pearly on some cleavage faces. On Mt. Some specimens of labradorite exhibit a schiller This specimen is from the Petaca District of New Mexico and measures about 4 inches (10 cm) across. Both stones are from the Spectrum Sunstone Mine near Plush, Oregon. The chemical formulas vary from Na(AlSi3O8) to Ca(Al2Si2O8). The names of the minerals in the series are arbitrarily given based upon their relative abundance of albite and anorthite. âPlagioclaseâ is the name of a group of feldspar minerals that form a solid solution series ranging from pure albite, Na(AlSi3O8), to pure anorthite, Ca(Al2Si2O8).