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Hint: to highlight K-feldspar, switch to a medium or low magnification objective (usually 4 or 10X works well), mostly close the substage iris, and raise the stage slightly (yes, raise). In thin section or grain mount, the general lack of pleochroism and the twinning are good indicators. Albite is the Calcium rich Plagioclase, and Anorthite is the Sodium rich Plagioclase. K-feldspar (with plagioclase inclusion) in Granite and simple twinning. Also very common in metamorphic rocks, with albite being common in low-grade metamorphic rocks. Plagioclase Feldspar (thin section) View Description. Photomicrograph of plagioclase in anorthosite (BV106) in plane light. Triclinic shape contributes to interesting optical angles. Also seen in sedimentary detritus. The twinning makes for different extinction angles depending on the angle of the crystal, but favors inclined extinction. Albite tends to be more common than anorthite, with the exception of metamorphosed carbonates and amphibolite. The general chemical formula is a bit complicated (K,Na) 1-x (Ca,Ba) x (Si 3-x Al 1+x O 8).This complexity is caused by presence of both monovalent and divalent cations which require more complicated heterovalent substitution. The chemical composition of feldspars includes alcalic metals, aluminum, silica and oxygen. If in doubt, use a needle or knife to see if you can flake the mineral into thin sheets. Pure Na[AlSi 3 O 8] is albite, and pure Ca[Al 2 Si 2 O 8] is anorthite. The slide is from rock sample K-84. To link to this object, paste this link in email, IM or document To embed this object, paste this HTML in website. In thin section or grain mount, the general lack of pleochroism and the twinning are good indicators. Home Rocks and Minerals Plagioclase Feldspar (thin section) Reference URL Share . Photomicrograph of plagioclase in anorthosite (BV106) in crossed polarized light. In plane-polarized light K-feldspars may be difficult to tell from plagioclase, though plagioclase has higher refractive indexes. This rollover demonstrates extinction within the plagioclase mineral. You can tell plagioclase from potassium feldspar (hereby known as Kspar) because the cleavage planes of plagioclase have striations that show clearly in the light. The second image is the same slide rotated 45 degrees clockwise and the dark twins from the first image are now white while the white twins from the first image are now dark. The characteristic twinning of Plagioclase is also visible. Click here for solid solution phase diagram. The slide is from rock sample K-84. Iron inclusions can produce reds. The first image is in plane polarized light and no twinning is seen. p. 245-250. Chemical zoning is common - the calcic core has a larger extinction angle than the sodic rim. differentiated on the basis of bulk mineral composition, but feldspar (i.e., plagioclase and K-feldspar) mineralogy does help discriminate between regions. Raquel Blonshine ('11), Michelle Cortrite ('13), Wanda Feng ('15), Helena Tatgenhorst ('16). In a given igneous rock, the major light colored minerals will be K-feldspar, plagioclase, quartz, and/or muscovite.. Muscovite is easiest because of its silvery clear color and perfect flaky cleavage. Rock-forming minerals of metamorphic rocks, in thin section (a work in progress) 1 Olivine: forsterite Olivine: Forsterite, in marble. Colorless in thin section. Select image and move cursor over image to view twinning at extinction, then rotated 45. Greater amounts of anorthite correlate to higher refractive indices. Click here for Michel-Levy methodClick here for Carlsbad-albite method. Plagioclase ranges in composition from Na[AlSi 3 O 8] to calcium Ca[Al 2 Si 2 O 8], or sodium to calcium aluminosilicate. If in doubt, use a needle or knife to see if you can flake the mineral into thin sheets. K-feldspar, Quartz and Biotite in a Granite. 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. (001) is a perfect cleavage, and (010) is also quite good. The chemical composition of feldspars includes alcalic metals, aluminum, silica and oxygen. The twins have opposite extinction. Hint: to highlight K-feldspar, switch to a medium or low magnification objective (usually 4 or 10X works well), mostly close the substage iris, and raise the stage slightly (yes, raise). It is not a reliable indicator of composition, but can help confirm a composition determined by other techniques. Plane-polarised light, width of view 2.5 mm. Oxford University Press, New York. Plagioclase series members. This rollover demonstrates the inclined extinction of Plagioclase as the microscope stage is rotated approximately 45º. In plane-polarized light K-feldspars may be difficult to tell from plagioclase, though plagioclase has higher refractive indexes. The presense of parallel striae on cleavage surfaces distinguishes plagioclase from K-feldspar. One of the most common minerals on Earth. Can be confused with quartz if there is no twinning, however the optic axis figure will resolve this. Occurs in nearly all igneous rocks - very common in granites, rhyolites, pegmatites, and similar rocks. As this is a triclinic mineral, the indicatrix axes will not line up with the crystal axes on any regular basis, and as a result extinction is almost invariably inclined. Variation is due to composition of the mineral. The sign varies depending on composition. These can be either color and are easiest to distinguish in thin section. No pleochroism. White and grey are common in hand sample; albite tends to be lighter and anorthite darker. Maximum interference colors in thin section are usually first-order grey or white. Twinning and exsolution lamellae may make it difficult to get a 2V measurement. Can be confused with quartz if there is no twinning, however the optic axis figure will resolve this. In thin section, alkali feldspar grains often look like they’re sprinkled with dirt (PPL) or tiny confetti … In the first image, some twins are white and the opposite ones are dark because they are extinct. In a given igneous rock, the major light colored minerals will be K-feldspar, plagioclase, quartz, and/or muscovite.. Muscovite is easiest because of its silvery clear color and perfect flaky cleavage. First-order yellow colors are seen with anorthite. Often a grey color in comparison to other feldspars. The rollover is the same image under crossed polars and twinning is seen. (110) is a very poor cleavage and is not usually seen in thin section. Magnification 1.5x; field of view 7.45 mm x 5.65mm. UCL Catalogue of Minerals in Thin Section, UNC Atlas of Igneous and Metamorphic Rocks, Minerals, and Textures, Union College Igneous Rocks in Thin Section. Select image and then move cursor over image to view in crossed polarized light, where olivine and pyroxene inclusions may be seen. Add tags Comment Rate. Analyser out: Analyser in: Key optical features of K-feldspar in thin section: Colourless often cloudy Low relief Low order interference colours Polysynthetic twins (Microcline), simple twins (Sanidine & Orthoclase) Nesse, W. D. (2012) "Plagioclase" in Introduction to Mineralogy. Orthoclase is frequently known as potassium feldspar or K-spar, and is easily distinguished from the plagioclase feldspars in hand sample by its lack of striations. The presense of parallel striae on cleavage surfaces distinguishes plagioclase from K-feldspar. Plagioclase in Detail . The composition of plagioclase feldspar is typically indicated by the general anorthite (% An) or albite (% Ab) fraction and is easily determined by measuring the refractive index or peel angle within the crushed particles by measuring the refraction angle under a thin section. The presense of parallel striae on cleavage surfaces distinguishes plagioclase from K-feldspar. The biotite has a pebbly texture and micaceous cleavage.The right half of the view contains a large grain of clear to cloudy K-feldspar. Be confused with quartz if there is no twinning, however the optic axis figure will this. Calcic core has a larger extinction angle than the sodic rim K-feldspars may be seen hand sample ; tends! 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