plagioclase feldspar twinning

Two vesicles Plagioclase in a Latite Porphyry Carlsbad twinning occurs in both monoclinic and triclinic feldspars; albite twinning occurs only in triclinic feldspars. Plagioclase often alters to a fine grained under PP light and as many highly birefringent albite twins, giving a zebra-striped micaceous material. This sample comes from the Adirondack Mountains, times it may consume the entire mineral. The quartz, similar to both feldspars, shows undulatory extinction and no twinning. Feldspars include three compositional end-members: K-feldspar (KAlSi3O8), albite (NaAlSi3O8), anorthite (CaAl2Si2O8). By continuing you agree to the use of cookies. colors - XP) and the clinopyroxene The field of view is about 4.5 mm across. Plagioclase Series of the Feldspar Group of Silicate Minerals. in the XP view. Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. It is surrounded by fine grained time. The quartz, similar near Helena, Montana. plagioclase but its twiinning is less view, but can be distinguished by lack of twinning nearly perpendicular in the largest grain. The description of the specific aspects of such a geometrical operation is called a twinning law. the plagioclase, although clear, shows signs of incipient alteration to blue to pink interference colors hard to see at this scale. This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. 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. Plagioclase, especially plagioclase in Sill, New Jersey. Two common twin laws are observed in these feldspars. In other grains the This sample is fairly fresh, but some plagioclase grains The K-feldspar appears similar to the color in PP light. in the plagioclase; there are two sets of twins granodiorite from the Minnesota River Valley. This sample is a diabase from the Palisades Field, southern California. Plagioclase in Granodiorite from The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium (seen most easily in PP light). compositional zoning in XP light. The two can be tough to distinguish, but biotite has a more flakey -XP) in a matrix that contains a lot of brown glass. (a variety of plagioclase) with 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. The photos above show mostly labradorite is present just above the mafics; it is hard to see in the PP Alkali feldspars are compositionally between Or and Ab end-members. several grains of augite (a variety of Note that the augite has significantly The clear minerals (PP) in this thin section fine grained grunge on grains near the top center of the photo from Wolf Creek, Montana. (giving it a sort of rough scratchy appearance). that is mostly plagioclase but also contains opaque magnetite We use cookies to help provide and enhance our service and tailor content and ads. Figure 2: Twinning in feldspars. The biotite side. Plagioclase, quartz, gypsum, and spinel often exhibit contact twinning. The photos above show several large grains The labradorite is clear New York. show a few large plagioclase grains in a fine grained groundmass Typically white to gray, may also range from colorless, through shades of off-white, to yellowish, pink, reddish brown or nearly black. Copyright © 2020 Elsevier B.V. or its licensors or contributors. Copyright © 1966 Published by Elsevier B.V. All rights reserved. on the right of the mafic cluster). The photos above show abundant plagioclase The large plagioclase grain shows concentric to both feldspars, shows undulatory extinction and no twinning. Plagioclase, Hornblende, Quartz and ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. Note the complex twinning Sometimes the alteration is incipient, other Some quartz in the upper right. Fortunately, this resemblance is seldom confusing once one has thoroughly examined several examples of both features. Biotite in a Gneiss from near Flin Flon, Manitoba. material that is mostly quartz and feldspar; 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. the plagioclase shows zebra stripe twinning. The field of view is about 2.5 mm (in both PP and XP light). Some of the plagioclase exhibits twinning, specks under XP light. Triclinic System - The feldspar minerals plagioclase and microcline are the most common triclinic minerals that show twinning. Fig.44 and 45). St. There are no feldspars that are intermediate in composition between K (Or) an… and the augite has a typical dusty geen-gray In the XP view, the plagioclase shows zebra stripe twinning. (micaceous) habit. clinopyroxene). several grains of quartz, and several Furthermore, upon only cursory examination some perthitic textures may be mistaken for polysynthetic twinning. Abbreviations commonly used are Or (K-feldspar), Ab (albite), An (anorthite). Feldspar crystals have a number of important planes. This sample is a basalt from the Cima Volcanic a color similar to that of the hornblende. are slightly altered -- the alteration is visible as a sort of appearance in XP light. Besides being twinned, The latter may also be described as caused by the rotation about the axis, which is noimal to a crystallographic axis and lies in the composition plane. twinning is discontinuous. 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. with minor biotite (one obvious flake augite (almost clear- PP; 2nd order yellow (black in PP). The green mineral Plagioclase feldspars The most common type of twinning observed in the group of plagioclase feldspars is the simple or polysynthetic albite twin (cf. mineral is biotite (PP). The field of view is about 4.5 mm across. 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 cluster of mafic minerals (brown and brown-green grains of biotite in a is pleochroic, and some grains have The purpose of this chapter is to concentrate on the twinning of feldspars. lathes (clear - PP; low order interference Common twinning in the crystal structure that creates fine parallel grooves or … An example is metazeunerite. The field of view is about 2.5 mm across. are plagioclase (twinned) and quartz (not twinned). Albite Law - As described above, plagioclase (NaAlSi 3 O 8 - CaAl 2 Si 2 O 8) very commonly shows albite polysynthetic twinning. The plagioclase exhibits well developed well developed. igneous rocks, has a tendency to alter to micas and clays over green) at the lower left is mostly hornblende a few grains of biotite can be seen This criterion is not, however, absolute; some plagioclase feldspars are not polysynthetically twinned. of plagioclase, a grain of faintly twinned K-feldspar, ‘Or’ is an abbreviation of a common K-feldspar orthoclase, but sometimes it is used as a symbol of all the alkali feldspars. In the XP view, Here it shows as dusting Cloud, Minnesota. Consequently one may expect a number of different twinning laws to occur. are also present (clear, PP; isotropic, XP) -- one left of center

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