Albite and Anorthite are the end members of the series, with the intermediary minerals Oligoclase, Andesine, Labradorite, and Bytownite. Learn vocabulary, terms, and more with flashcards, games, and other study tools. ... A special property that shows up in some sample of plagioclase feldspar is its tendency to exhibit striations on cleavage surfaces. 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 is an important mineral in the feldspar group, and the chemical formula of this mineral is NaAlSi 3 O 8 – CaAl 2 Si 2 O 8. 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 presense of parallel striae on cleavage surfaces distinguishes plagioclase from K-feldspar. For geological purposes, feldspars are quite important. This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. Feldspar - Feldspar - Identification of specific feldspars: Alkali feldspars can often be distinguished from plagioclase feldspars because most grains of the latter exhibit albite twinning (see above Crystal structure), which is manifested by parallel lines on certain cleavage surfaces, whereas grains of alkali feldspars do not. Start studying Rock & Mineral Identification Quiz. In hand specimen, : Plagioclase” is the name of a group of feldspar minerals that form a solid solution series ranging from pure albite to pure anorthite, Minerals in this series are a homogenous mixture of albite and anorthite. Plagioclase feldspars - sodium, calcium, aluminum silicates Learn more about how to identify plagioclase. This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The characteristic appearance (ah-ven-chure-ESS-ence) of this type of albite serves readily to differentiate it from its close relative, sunstone. Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Created by. Feldspars are very important rock-forming minerals and they are covered in more detail in other geology courses. In basaltic rocks it is normally gray to black. The presence of numerous tiny flakes or platelets of an included metallic mineral, such as hematite or goethite, gives rise to a reddish or golden spangled effect that is vi… The key difference between orthoclase and plagioclase is that orthoclase appears in green-yellow colour, whereas plagioclase appears in white. These properties make plagioclase feldspars relatively easy to identify with a hand lens in coarse-grained igneous and metamorphic rocks. Can be confused with quartz if there is no twinning, however the optic axis figure will resolve this. Often a grey color in comparison to other feldspars. Magnification may be required to see striations on plagioclase cleavage surfaces. 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. However, identifying feldspar is key for identifying a granite. igneous rock, fine-grained, aphanitic, plagioclase feldspars dominate with some amphibole and pyroxene. All feldspar minerals have two directions of perfect cleavage. In many instances, it is very hard to tell apart one from another. When a particular Plagioclase Feldspar cannot be identified, it is simply called "Plagioclase" or "Plagioclase Feldspar". Match. Gravity. Striations are perfectly straight, fine, parallel lines. These properties make plagioclase feldspars relatively easy to identify with a hand lens in coarse-grained igneous and metamorphic rocks. Moreover, the crystal system of orthoclase is monoclinic while in plagioclase, it is triclinic. 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. When identifying a rock you must first identify the individual minerals that make up that rock. Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Terms in this set (46) Andesite. Plagioclase in granitic rocks is normally white, pink, or red in color. The Plagioclase series is a group of related feldspar minerals that essentially have the same formula but vary in their percentage of sodium and calcium. Plagioclase in granitic rocks is normally white, pink, or red in color. hraenendayh. Alkali feldspars - sodium, potassium, aluminum silicates Learn more about how to identify microcline Learn more about how to identify orthoclase. 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