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Petrologists distinguish the position of a plagioclase along this series using two numbers- %Ab (percent albite) and %An (percent anorthite.) Plagioclases are triclinic and have a Moh's hardness of 6 to 6 ½. The pure sodium feldspar, albite has the formula NaAlSi3O8 and the pure calcium endmember, anorthite has the formula CaAl2Si2O2.
Feldspar
Similarly the combination of albite twinning and pericline twinning in alkali feldspar results when high temperature sanidine (monoclinic) transforms to low temperature microcline (triclinic). This type of twinning is only observed using the polarizing microscope, and results in a "tartan" twinning pattern as shown in your text book on page 231 ...
Pure feldspar, free of exsolution, is colorless. However, minor chemical substituents, inclusions, interference effects from exsolution lamellae, and radiation damage can produce color in the mineral. Chemical impurities produce the yellow color of sanidine, orthoclase and calcic plagioclase, the blue to green colors of amazonite, and the blue-green color of sodic plagioclase.
The plagioclase feldspar with an iridescent play-of-color that is often used as a gemstone. Author: Hobart M. King, Ph.D., GIA Graduate Gemologist Labradorite: Photograph of labradorite gemstones exhibiting a beautiful labradorescent play of iridescent colors.
plagioclase feldspars - Ca-Na alkali feldspars - Na-K in common usage, the terms alkali feldspar and K- feldspar are used interchangeably. Plagioclase Feldspar Physical Color.
Feldspar is the name of a group of rock-forming tectosilicate minerals that make up as much as 60% of the Earth's crust.. Feldspar forms crystals from magma in both intrusive and extrusive rocks, and they can also happen as compact minerals, as veins, and are also present in many types of metamorphic rock.Rock formed entirely of plagioclase feldspar is known as anorthosite.
Anorthite belongs to the Plagioclase Feldspar group, an isomorphous solid solution series. Albite is one end member, containing sodium and no calcium. The other end member, Anorthite, contains calcium and no sodium. The intermediary members are Oligoclase, Andesine, Labradorite, and Bytownite.Labradorite and Bytownite are considered by some to be a variety .
feldspar species is an important factor in igneous rock classification. Feldspars are also important constituents of many metamorphic rocks and are found in some sedimentary rocks, especially sandstones. Some plagioclase feldspars will have one composition in the interior of the crystal, and a gradually or sharply changing composition toward ...
Feldspar can also be chemically zoned. Plagioclase feldspar grows in igneous environments and crystals which grow early in a cooling magma contain more calcium than those formed at the end of crystallization, which are enriched in sodium. This is predicted by the Bowen's Reaction Series.
Plagioclase and Augite in a Basalt. The photos above show abundant plagioclase lathes (clear - PP; low order interference colors - XP) and the clinopyroxene augite (almost clear- PP; 2nd order yellow to blue to pink interference colors-XP) in a matrix that contains a lot of brown glass.Two vesicles are also present (clear, PP; isotropic, XP) -- one left of center on the bottom edge, .
Oligoclase belongs to the Plagioclase Feldspar group, an isomorphous solid solution series. Albite is one member, containing sodium and no calcium. The other end member, Anorthite, contains calcium and no sodium.Oligoclase is an intermediary member of this series. Oligoclase is considered by some authorities as a variety of Albite rather than a separate mineral.
Plagioclase is feldspar that contains "sodium (Na)" and "calcium (Ca)". When it contains a lot of sodium, it is called "Albite". If calcium does, it is called "Anorthite". When it is in the middle of "Albite" and "Anorthite", it becomes "Labradorite". The appearances also change greatly depending on the ingredients.
Jul 25, 2017· Most crystals have color variations such as opaque shades of white, gray, and pink. Potassium feldspar and plagioclase crystals are so similar that they may react to create crystals with thin alternating seams. This happens when hot potassium and sodium feldspars are cooled in a way that their solid solution phase ex-solves.
Apr 28, 2020· Low-temperature fluid alteration of plagioclase to K-feldspar has been reported in several igneous settings (Larsson et al., 2002; Morad et al., 2010), but the replacement of plagioclase by K ...
Rhyolite; Rhyolite is felsic igneous extrusive rock and it is a fine-grained and dominated by quartz (>20%) and alkali feldspar (>35%).Due to the high silica content, rhyolite lava is very viscous.It is often difficult to identify rhyolites without chemical analysis due to their glassy groundmasses. Many rhyolites consist mainly of glass, and are termed obsidian, or are partially devitrified ...
Oligoclase is a high sodium feldspar crystallizing in the triclinic system. The Mohs hardness is 6 to 6.5 and the specific gravity is 2.64 to 2.66. The refractive indices are: nα=1.533–1.543, nβ=1.537–1.548, and nγ=1.542–1.552. In color it is usually white, with shades of grey, green, or red.
Aug 10, 2019· Alkali feldspar (also called potassium feldspar or K-feldspar) has a color range from white to brick-red, and it's typically opaque. Many rocks have both feldspars, like granite. Cases like that are helpful for learning to tell the feldspars .
Orthoclase (K-feldspar): Porcelain luster; commonly colored pink, white, or gray. Cleavage in 2 directions at right angles may be detected by a reflection of light when specimen is rotated. Plagioclase: Usually gray or white in granite, dark-bluish color in gabbro. Striations common. 2 cleavage directions at right angle may be detected.
The most important gem member of the plagioclase group is labradorite, the chief attraction of which is its change of color that may be visible at one angle across the entire surface of the stone. This effect is created by light interference set up by thin plates of feldspar that result from repeated twinning.
Aug 10, 2019· Alkali feldspar (also called potassium feldspar or K-feldspar) has a color range from white to brick-red, and it's typically opaque. Many rocks have both feldspars, like granite. Cases like that are helpful for learning to tell the feldspars apart. The differences can be subtle and confusing.
Plagioclase definition, any of the feldspar minerals varying in composition from acidic albite, NaAlSi3O8, to basic anorthite, CaAl2Si2O8, found in most igneous rocks: shows twinning striations on good cleavage surfaces. See more.
May 30, 2020· INTRODUCTION. Plagioclase feldspar is the most abundant rock‐forming mineral in the crust of solid planetary bodies, including Earth, and represents up to 70% of the Moon's crust (e.g., Papike et al. 1991).Despite the abundance of plagioclase, studies of its response to shock metamorphism are limited in comparison to those of quartz or olivine (e.g., .
Aug 28, 2019· The key difference between orthoclase and plagioclase is that orthoclase appears in green-yellow colour, whereas plagioclase appears in white colour.. Orthoclase and plagioclase are important minerals.Both these forms are members of the feldspar group. A feldspar mineral is an abundant rock-forming mineral typically occurring as colourless or pale-coloured crystals.
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