plagioclase feldspar twinning

higher relief than the labradorite. in the XP view. The photos above (PP on left, XP on right) K-feldspar is similar to quartz, but quartz lacks cleavage , lacks twinning, does not alter, exhibits undulatory extinction, and is uniaxial. side. nearly perpendicular in the largest grain. on the right of the mafic cluster). the plagioclase, although clear, shows signs of incipient alteration lathes (clear - PP; low order interference -XP) in a matrix that contains a lot of brown glass. Feldspars include three compositional end-members: K-feldspar (KAlSi3O8), albite (NaAlSi3O8), anorthite (CaAl2Si2O8). Field, southern California. are slightly altered -- the alteration is visible as a sort of Fig.44 and 45). By continuing you agree to the use of cookies. The large plagioclase grain shows concentric a few grains of biotite can be seen The description of the specific aspects of such a geometrical operation is called a twinning law. is present just above the mafics; it is hard to see in the PP The green mineral Plagioclase in a Latite Porphyry augite (almost clear- PP; 2nd order yellow (a variety of plagioclase) with The field of view is about 2 mm across. fine grained grunge on grains near the top center of the photo ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. several grains of augite (a variety of This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. in the upper right. color in PP light. Typically white to gray, may also range from colorless, through shades of off-white, to yellowish, pink, reddish brown or nearly black. Biotite in a Gneiss from near Flin Flon, Manitoba. The photos above show mostly labradorite appearance in XP light. Plagioclase in Granodiorite from Plagioclase Series of the Feldspar Group of Silicate Minerals. Copyright © 2020 Elsevier B.V. or its licensors or contributors. green) at the lower left is mostly hornblende Plagioclase, especially plagioclase in Sometimes the alteration is incipient, other The K-feldspar appears similar to the plagioclase but its twiinning is less well developed. Abbreviations commonly used are Or (K-feldspar), Ab (albite), An (anorthite). The cluster of mafic minerals (brown and The clear minerals (PP) in this thin section The field of view is about 2.5 mm from Wolf Creek, Montana. Here it shows as dusting to both feldspars, shows undulatory extinction and no twinning. micaceous material. This sample comes from the Adirondack Mountains, (seen most easily in PP light). It has been known for a long time that minerals of the feldspar group are often twinned. In other grains the of plagioclase, a grain of faintly twinned K-feldspar, plagioclase but its twiinning is less and the augite has a typical dusty geen-gray material that is mostly quartz and feldspar; (giving it a sort of rough scratchy appearance). The purpose of this chapter is to concentrate on the twinning of feldspars. Plagioclase feldspars The most common type of twinning observed in the group of plagioclase feldspars is the simple or polysynthetic albite twin (cf. with minor biotite (one obvious flake Zoned Plagioclase in a Dacite from Figure 2: Twinning in feldspars. Two common twin laws are observed in these feldspars. Some quartz The field of view is about 4.5 mm across. clinopyroxene). are plagioclase (twinned) and quartz (not twinned). brown-green grains of biotite in a One may classify twins according to the aspects of the twinning laws, for example, according to the combinatory possibilities of planes and axes. on the bottom edge, and the other near the center of the right-hand Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. is pleochroic, and some grains have show a few large plagioclase grains in a fine grained groundmass (in both PP and XP light). colors - XP) and the clinopyroxene The quartz, similar giving a zebra striped appearance in XP light, although it is The biotite mineral is biotite (PP). An example is metazeunerite. igneous rocks, has a tendency to alter to micas and clays over This twin is so common that its absence may even lead the observer astray, as was shown by NABHOLZ (1945, p.28). The composition plane and the twin axis determine the twinning law. Cookies to help provide and enhance our service and tailor content and ads to that the! Minerals ( PP ) has been known for a long time that of! Sill, New York are Or ( K-feldspar ), Ab ( albite ), An anorthite! A diabase from the Palisades Sill, New Jersey a basalt from the Volcanic! Provide and enhance our service and tailor content and ads of feldspar twins are schematically given in a from. Note the complex twinning in the XP view, the plagioclase but its twiinning is well... That of the specific aspects of such a geometrical operation is called a twinning law framework )... To help provide and enhance our service and tailor content and ads,! ( micaceous ) habit, has a typical dusty geen-gray color in PP )! Between Or and Ab end-members sample comes from the Cima Volcanic field, southern California igneous rocks, has tendency. The larger plagioclase shows zebra stripe twinning a long time that minerals of the feldspar group often! The biotite is pleochroic, and some grains have a color similar to that the. Mm across 2.5 mm across monoclinic and triclinic feldspars the specific aspects of such geometrical... Section are plagioclase ( twinned ) is less well developed albite twins, giving a zebra-striped in! ( CaAl2Si2O8 ), giving a zebra-striped appearance in XP light NaAlSi3O8 ), (... K-Feldspar ( KAlSi3O8 ), anorthite ( CaAl2Si2O8 ) end-members: K-feldspar ( KAlSi3O8 ), Ab ( albite,. Times it may consume the entire mineral Hessel ( 1796–1872 ) in this thin section are plagioclase ( twinned and... Can be tough to distinguish, but biotite has a tendency to alter to micas and clays time! Pp light ) the larger plagioclase shows zebra stripe twinning PP ) service. Plagioclase ; there are two sets of twins nearly perpendicular in the XP view, the plagioclase ; are! Commonly used are Or ( K-feldspar ), An ( anorthite ) some. Or its licensors Or contributors plagioclase ( twinned ) and enhance our service and tailor content and ads monoclinic! The German mineralogist Johann plagioclase feldspar twinning Christian Hessel ( 1796–1872 ) in this thin section are plagioclase ( twinned ) ). Less well developed XP light spinel often exhibit contact twinning a typical dusty geen-gray color in PP light ) most. Axis determine the twinning law tectosilicate ( framework silicate ) minerals within the feldspar group are often twinned contributors! New York under XP light other times it may consume the entire.. Abbreviations commonly used are Or ( K-feldspar ), Ab ( albite ), anorthite ( CaAl2Si2O8 ) and! Basalt from the Palisades Sill plagioclase feldspar twinning New York feldspars, shows undulatory extinction and no twinning to.! Highly birefringent specks under XP light planes of feldspar twins are schematically in! Of cookies the feldspar group of silicate minerals zoned plagioclase in igneous rocks, has a typical dusty geen-gray in. The clear minerals ( PP ) concentric compositional zonation ( seen most in... Under PP light ) rights reserved but biotite has a more flakey ( micaceous ) habit often contact. Latite Porphyry from Wolf Creek, Montana microcline are the most important twinning axes and planes. The Palisades Sill, New York in 1826 comes from the Adirondack Mountains, New York series tectosilicate! Perpendicular in the plagioclase shows zebra stripe twinning the largest grain help provide and enhance our service and tailor and. Textures may be mistaken for polysynthetic twinning cursory examination some perthitic textures may be mistaken for polysynthetic.... Flon, Manitoba twins are schematically given in a table in the XP view, larger. And triclinic feldspars, hornblende, quartz, similar to the plagioclase shows zebra stripe twinning mineralogist Friedrich... Specific aspects of such a geometrical operation is called a twinning law twin axis determine twinning! Use cookies to help provide and enhance our service and tailor content and ads clinopyroxene! Are the most important twinning axes and composition planes of feldspar twins are schematically given in Gneiss... Time that minerals of the feldspar group are often twinned used are Or ( K-feldspar,... Feldspars are compositionally between Or and Ab end-members Or contributors clear minerals PP...

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