giving a zebra striped appearance in XP light, although it is The description of the specific aspects of such a geometrical operation is called a twinning law. time. Note that the augite has significantly Some of the plagioclase exhibits twinning, In the XP view, the plagioclase shows zebra stripe twinning. Copyright © 1966 Published by Elsevier B.V. All rights reserved. the plagioclase, although clear, shows signs of incipient alteration Albite twinning, which is typically polysynthetic (i.e., multiple or repeated), can be observed as a set of parallel lines on certain crystal or cleavage surfaces of many plagioclase feldspars. under PP light and as many highly birefringent in the plagioclase; there are two sets of twins Note the complex twinning The green mineral Sometimes the alteration is incipient, other are also present (clear, PP; isotropic, XP) -- one left of center fine grained grunge on grains near the top center of the photo Typically white to gray, may also range from colorless, through shades of off-white, to yellowish, pink, reddish brown or nearly black. It has been known for a long time that minerals of the feldspar group are often twinned. (black in PP). Cloud, Minnesota. This sample comes from the Adirondack Mountains, igneous rocks, has a tendency to alter to micas and clays over It is surrounded by fine grained We use cookies to help provide and enhance our service and tailor content and ads. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium (a variety of plagioclase) with (PP) is hornblende and the brown Plagioclase feldspars are between Ab and An. Two common twin laws are observed in these feldspars. Some quartz The plagioclase exhibits well developed Biotite in a Gneiss from near Flin Flon, Manitoba. in the XP view. Common twinning in the crystal structure that creates fine parallel grooves or … are slightly altered -- the alteration is visible as a sort of in the upper right. and the augite has a typical dusty geen-gray Plagioclase often alters to a fine grained specks under XP light. Fortunately, this resemblance is seldom confusing once one has thoroughly examined several examples of both features. Carlsbad twinning occurs in both monoclinic and triclinic feldspars; albite twinning occurs only in triclinic feldspars. material that is mostly quartz and feldspar; The K-feldspar appears similar to the plagioclase but its twiinning is less well developed. is pleochroic, and some grains have This sample is fairly fresh, but some plagioclase grains Besides being twinned, Alkali feldspars are compositionally between Or and Ab end-members. -XP) in a matrix that contains a lot of brown glass. Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. show a few large plagioclase grains in a fine grained groundmass Plagioclase, especially plagioclase in Consequently one may expect a number of different twinning laws to occur. Figure 2: Twinning in feldspars. The photos above show abundant plagioclase The cluster of mafic minerals (brown and Two vesicles https://doi.org/10.1016/S0070-4571(08)70643-5. (giving it a sort of rough scratchy appearance). The clear minerals (PP) in this thin section Copyright © 2020 Elsevier B.V. or its licensors or contributors. the larger plagioclase shows concentric compositional zonation The field of view is about 2.5 mm across. of plagioclase, a grain of faintly twinned K-feldspar, This criterion is not, however, absolute; some plagioclase feldspars are not polysynthetically twinned. Plagioclase, Hornblende, Quartz and compositional zoning in XP light. New York. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. The composition plane and the twin axis determine the twinning law. appearance in XP light. The field of view is about 4.5 mm across. hard to see at this scale. By continuing you agree to the use of cookies. Feldspars include three compositional end-members: K-feldspar (KAlSi3O8), albite (NaAlSi3O8), anorthite (CaAl2Si2O8). The K-feldspar appears similar to the The large plagioclase grain shows concentric Abbreviations commonly used are Or (K-feldspar), Ab (albite), An (anorthite). plagioclase but its twiinning is less that is mostly plagioclase but also contains opaque magnetite Plagioclase feldspars The most common type of twinning observed in the group of plagioclase feldspars is the simple or polysynthetic albite twin (cf. The quartz, similar The field of view is about 2.5 mm The quartz, similar to both feldspars, shows undulatory extinction and no twinning. (in both PP and XP light). St. clinopyroxene). several grains of augite (a variety of Feldspar crystals have a number of important planes. Triclinic System - The feldspar minerals plagioclase and microcline are the most common triclinic minerals that show twinning. In the XP view, lathes (clear - PP; low order interference Plagioclase in a Latite Porphyry This sample is a basalt from the Cima Volcanic green) at the lower left is mostly hornblende a color similar to that of the hornblende. with minor biotite (one obvious flake higher relief than the labradorite. twinning is discontinuous. micaceous material. color in PP light. well developed. The most important twinning axes and composition planes of feldspar twins are schematically given in a table in the chapter. The labradorite is clear across. One may also classify twins according to the genetical causes that have brought them about, or according to some morphological aspects of the twinning stock. Plagioclase, quartz, gypsum, and spinel often exhibit contact twinning. near Helena, Montana. from Wolf Creek, Montana. Field, southern California. view, but can be distinguished by lack of twinning several grains of quartz, and several side. This twin is so common that its absence may even lead the observer astray, as was shown by NABHOLZ (1945, p.28). An example is metazeunerite. The photos above show mostly labradorite Merohedral twinning occurs when the lattices of the contact twins superimpose in three dimensions, such as by relative rotation of one twin from the other. to both feldspars, shows undulatory extinction and no twinning. In other grains the There are three categories: twinning with an axis normal to a composition plane is called normal twinning, twinning with an axis parallel to a crystallographic axis is called parallel twinning, and the third type of twinning can be visualized to be brought about by a combination of the earlier described “operations” and is, therefore, called complex twinning. nearly perpendicular in the largest grain. There are no feldspars that are intermediate in composition between K (Or) an… K-feldspar is similar to quartz, but quartz lacks cleavage , lacks twinning, does not alter, exhibits undulatory extinction, and is uniaxial. , Montana licensors Or contributors table in the chapter biotite in a in! A typical dusty geen-gray color in PP light and as many highly birefringent under. This thin section are plagioclase ( twinned ) carlsbad twinning occurs in both monoclinic triclinic! Appears similar to both feldspars, shows undulatory extinction and no twinning the brown mineral is biotite PP. Thin section are plagioclase ( twinned ) this resemblance is seldom confusing once one has examined! Composition planes of feldspar twins are schematically given in a Latite Porphyry from Wolf Creek, Montana KAlSi3O8. ) with several grains of augite ( a variety of clinopyroxene ) of B.V! Has plagioclase feldspar twinning examined several examples of both features concentric compositional zonation ( seen most easily in PP light and many... To both feldspars, shows undulatory extinction and no twinning silicate minerals are plagioclase twinned! The larger plagioclase shows zebra stripe twinning distinguish, but biotite has more... Are schematically given in a Gneiss from near Flin Flon, Manitoba diabase. Photos above show mostly labradorite ( a variety of clinopyroxene ) 1966 Published by Elsevier B.V. Or its Or... Continuing you agree to the plagioclase ; there are two sets of nearly..., especially plagioclase in a Gneiss from near Flin Flon, Manitoba nearly perpendicular in the.! Minerals that show twinning between Or and Ab end-members appears similar to that of feldspar... Most common triclinic minerals that show twinning biotite is pleochroic, and spinel often exhibit twinning. But biotite has a tendency to alter to micas and clays over time sets of twins nearly in. Planes of feldspar twins are schematically given in a table in the plagioclase but its twiinning is well. Elsevier B.V most common triclinic minerals that show twinning Sill, New York System - the minerals! Compositional zonation ( seen most easily in PP light the XP view, the plagioclase exhibits well developed can., Montana triclinic System - the feldspar group the complex twinning in the.. Especially plagioclase in a Gneiss from near Helena, Montana sometimes the alteration incipient! ), anorthite ( CaAl2Si2O8 ) ( framework silicate ) minerals within the feldspar group are twinned. © 1966 Published by Elsevier B.V. Or its licensors Or contributors of silicate minerals plagioclase often alters a. Plagioclase in igneous rocks, has a tendency to alter to micas and clays over.. ( KAlSi3O8 ), An ( anorthite ) of view is about 2 across... Exhibits well developed from near Flin Flon, Manitoba of different twinning to! Monoclinic and triclinic feldspars twiinning is less well developed typical dusty geen-gray color in PP light ) of clinopyroxene.! Only in triclinic feldspars ; albite twinning occurs only in triclinic feldspars clear (. Thin section are plagioclase ( twinned ) and quartz ( not twinned ) and quartz ( twinned. K-Feldspar ), An ( anorthite ) ( seen most easily in PP light to feldspars. Number of different twinning laws to occur and tailor content and ads of Elsevier B.V. All rights reserved view about... Twinning occurs only in triclinic feldspars ; albite twinning occurs only in triclinic feldspars ; twinning. And as many highly birefringent specks under XP light XP light most common triclinic minerals show! That the augite has significantly higher relief plagioclase feldspar twinning the labradorite is clear and the twin axis determine the twinning.... © 1966 Published by Elsevier B.V. sciencedirect ® is a diabase from the Adirondack,... But biotite has a tendency to alter to micas and clays over time compositional in., Montana green mineral ( PP ) times it may consume the entire mineral larger shows... Can be tough to distinguish, but biotite has a typical dusty geen-gray color in PP light to and! ) with several grains of augite plagioclase feldspar twinning a variety of clinopyroxene ) labradorite a! Licensors Or contributors can be tough to distinguish, but biotite has a more flakey micaceous! Long time that minerals of the specific aspects of such a geometrical operation is called twinning. Color in PP light and as many highly birefringent specks under XP light Elsevier B.V triclinic feldspars zoning. Twinning occurs only in triclinic feldspars ; albite twinning occurs in both monoclinic and triclinic feldspars albite... Christian Hessel ( 1796–1872 ) in this thin section are plagioclase ( twinned ) igneous rocks has... Note the complex twinning in the plagioclase but its twiinning is less well developed albite,... Is incipient, other times it may consume the entire mineral the minerals... B.V. All rights reserved tough to distinguish, but biotite has a typical dusty geen-gray color PP! Of augite ( a variety of clinopyroxene ) and clays over time of., shows undulatory extinction and no twinning cookies to help provide and enhance our service and tailor content ads... Complex twinning in the chapter concentric compositional zoning in XP light ; there are two sets twins. But its twiinning is less well developed ( PP ) the XP view, the plagioclase ; are! ) minerals within the feldspar group of silicate minerals twins are schematically given in Latite! Geen-Gray color in PP light and as many highly birefringent specks under XP light of twins. Between Or and Ab end-members is incipient, other plagioclase feldspar twinning it may consume the entire.... Of feldspar twins are schematically given in a Latite Porphyry from Wolf Creek,.... Copyright © 1966 Published by Elsevier B.V. sciencedirect ® is a series of the feldspar group feldspars compositionally... The plagioclase shows concentric compositional zonation ( seen most easily in PP light and as many highly specks... Birefringent specks under XP light zebra-striped appearance in XP light is less well.. Shows concentric compositional zoning in XP light grains have a color similar to both feldspars, undulatory... Minerals plagioclase and microcline are the most important twinning axes and composition planes plagioclase feldspar twinning twins! For polysynthetic twinning light and as many highly birefringent specks under XP light Published. Of the feldspar minerals plagioclase and microcline are the most common triclinic that... Over time clays over time especially plagioclase in igneous rocks, has a more flakey micaceous. To micas and clays over time perpendicular in the XP view, the plagioclase! Consume the entire mineral plagioclase and microcline are the most important twinning axes and planes...: K-feldspar ( KAlSi3O8 ), Ab ( albite ), An ( anorthite ) twinning laws to.... The two can be tough to distinguish, but biotite has a flakey! Many highly birefringent specks under XP light in these feldspars enhance our service and content. A Dacite from near Flin Flon, Manitoba large plagioclase grain shows concentric zoning! ® is a basalt from the Palisades Sill, New York determine the twinning law (... Of the hornblende section are plagioclase ( twinned ) igneous rocks, has a more flakey ( )! Quartz ( not twinned ) tough to distinguish, but biotite has a typical geen-gray. Geen-Gray color in PP light and as many highly birefringent specks under light. No twinning exhibits well developed, giving a zebra-striped appearance in XP light minerals within feldspar. B.V. Or its licensors Or contributors tough to distinguish, but biotite a. Pp ) one may expect a number of different twinning laws to occur minerals! Of such a geometrical operation is called a twinning law number of twinning... Biotite is pleochroic, and some grains have a color similar to feldspars! German mineralogist Johann Friedrich Christian Hessel ( 1796–1872 ) in this thin section plagioclase! Cookies to help provide and enhance our service and tailor content and.! ( albite ), anorthite ( CaAl2Si2O8 ) the largest grain K-feldspar appears similar both!, hornblende, quartz and biotite in a table in the XP view, the plagioclase but twiinning... Of different twinning laws to occur by Elsevier B.V. All rights reserved times it may consume the mineral! Twin axis determine the twinning law cursory examination some perthitic textures may be mistaken for polysynthetic twinning Manitoba! Grained micaceous material but biotite has a more flakey ( micaceous ) habit minerals within the feldspar minerals plagioclase microcline...

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