Plagioclase thin section photomicrographs are taken in plane-polarized light and cross-polarized light, and can easily be recognized under the microscope. ... Plagioclase is present in the form of black … triclinic (+,-) Relief - low. Deer, W. A., Howie, R. A., & Zussman, J. The extinction angle is an optical characteristic and varies with the albite fraction (%Ab). There are several phenocrysts of brown to green pyroxene, magnetite, and plagioclase. In thin section or grain mount the general lack of pleochroism and the twinning are good indicators. Plagioclase feldspar thin section.The series ranges from albite to anorthite endmembers with respective compositions naalsi3o8 to caal2si2o8 where sodium and calcium atoms can substitute for each other in the mineral s crystal lattice structure. This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. Colour and Pleochroism. Plagioclase is a major constituent mineral in the Earth's crust, and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks. The extinction angle is an optical characteristic and varies with the albite fraction (%Ab). 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. Cleavage. In thin section the plagioclase crystals show a distinctive cream coloured clouding, probably due to oriented inclusions of spinel (Poldervaart and Gilkey, 1954). Its name comes from Ancient Greek πλάγιος (plágios, "oblique") + κλᾰ́σῐς (klásis, "fracture"), in reference to its two cleavage angles. Birefringence - 0.012-0.013. There are several named plagioclase feldspars that fall between albite and anorthite in the series. Mineral. I-50 Ateration to fine-grained material typically occurs along fractures, twin planes, or cleavages. 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. Colorless in thin section; Grains may appear cloudy due to alteration. Cleavages intersect at 93 °, usually not observed in thin section. Home Atlas of … In thin section, plagioclase commonly shows the characteristic albite polysynthetic twinning. Colour - colourless. (2013). The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure. Hekla volcano, Iceland. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or 'record-groove' effect. The following table shows their compositio… In the solid solution series, the plagioclase refractive index varies from slightly lower than quartz to somewhat above it. An introduction to the rock-forming minerals (pp. This twinning is the most characteristic identifying feature of plagioclase, and makes its … The process of creating a thin section is a blend of artistry, technology and science. Interference colours - 1st order To be updated with all the latest minerals and rocks under the microscope. Plagioclase Feldspar (thin section) Type. Some common rock types as seen under the microscope. These are photomicrographs - very thin slices of rock, seen in plane-polarised light, or between crossed polarisers, when the colours seen are produced by interference of light. Klein, Cornelis and Cornelius S. Hurbut, Jr.; Johann Friedrich Christian Hessel (1796–1872), rock in the highlands of the Earth's moon, https://en.wikipedia.org/w/index.php?title=Plagioclase&oldid=982515290, Short description is different from Wikidata, Articles containing Ancient Greek (to 1453)-language text, Creative Commons Attribution-ShareAlike License, Biaxial (+) albite, biaxial (-) anorthite, This page was last edited on 8 October 2020, at 16:50. Plane/cross-polarized light, field width is 3 mm. Pyroxene (wholly igneous) relicts are augite with a composition ranging from Wo 43 En 44 Fs 13 to Wo 45 En 39 Fs 17 . The intermediate members of the plagioclase group are very similar to each other and normally cannot be distinguished except by their optical properties.