Plagioclase Image. Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust by weight. { © Copyright 1997 - 2020 Hershel Friedman and Minerals.net, all rights reserved. The percentages of sodium and calcium for each member listed above are an approximate value; the actual percentage can vary as much as 20 percent. Chemical Formula. Mindat.org's Discussion Groups The cleavage angle is about 90º. PROP. Our Recommended Book Selection at Amazon.com, Financial Donations and Collection Acquisitions. The color is due to optical interference on Bøggild exsolution lamellae. if (document.getElementById != null) © Jeff Weissman / Photographic Guide to Mineral Species, Comments: Plagioclase, variety kenyite, crystals in volcanic rock. Feldspar - Feldspar - Chemical composition: All the rock-forming feldspars are aluminosilicate minerals with the general formula AT4O8 in which A = potassium, sodium, or calcium (Ca); and T = silicon (Si) and aluminum (Al), with a Si:Al ratio ranging from 3:1 to 1:1. There are several plagioclase feldspars in the seriesbetween albite and anorthite. html += '\n\n\n'; The minerals of the plagioclase series are listed in the table below along with their relative abundance of albite (Ab) and anorthite (An). Location: Norway. Scale: See Photo. Group name for Na, Ca feldspars. var printWin = window.open("","printSpecial", 'toolbar=no,location=no,directories=no,status=no,menubar=no,scrollbars=yes,resizable=yes,copyhistory=yes,width=350,height=350'); Rockhounds Discussion Group on Yahoo Groups 2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal. Location: Kirinyaga (Mount Kenya), Central Province, Kenya. Link to . function printSpecial() © Jeff Weissman / Photographic Guide to Mineral Species, Comments: Translucent andesine crystal twin. Sodium calcium aluminum silicate, with a varying amount of sodium and calcium. Scale: See Photo. © Jeff Weissman / Photographic Guide to Mineral Species. © Jeff Weissman / Photographic Guide to Mineral Species, Comments: Translucent crystalline bytownite. The Plagioclase Feldspars are used in ceramics. The following table shows their compositio… Cleavage: One perfect cleavage and one good cleavage that meet at nearly 90° Hardness: 6 to 6.5 (harder than glass) Specific Gravity NCalc= 1.56 - from Gladstone-Dale relationship (K, NAME( Duda&Rejl90) PHYS. } Scale: See Photo. Location: Taquaral, Minas Gerais, Brazil. Scale: See Photo. Below is an image for each one of the members of the Plagioclase group. Scale: 47x30x7 mm. { The basic feldspar recipe is X(Al,Si) 4 O 8, where X stands for Na, K, or Ca. { printWin.document.write(html); return; From the Greek plagios - "oblique" and klao - "I cleave" in allusion to the obtuse cleavage angles of the good cleavages. _uacct = "UA-3067463-1"; Scale: 17 x 15 x 7.5 cm matrix. Original Rockhounds Discussion Group © John Betts - Fine Minerals, Comments: Opaque white crystal of anorthite with dark brown solidified lava coating. © Dan Weinrich, Comments: Crystal group of bladed albite (variety cleavelandite) crystals. Massive - Granular - Common texture observed in granite and other igneous rock. © Jeff Weissman / Photographic Guide to Mineral Species, Comments: Clear glassy crystalline oligoclase with milky quartz and muscovite. Common world wide occurrences. }, Jeff Weissman / Photographic Guide to Mineral Species, Minerals in Thin Section-University of North Carolina. else printWin.print(); When a particular Plagioclase Feldspar cannot be identified, it is simply called "Plagioclase" or "Plagioclase Feldspar". © Dave Barthelmy, Comments: Rectangular cleavage fragment of salmon-orange oligoclase (sunstone) feldspar. var headTags = document.getElementsByTagName("head"); White, Gray, Bluish white, Reddish white, Greenish white. Brittle - Generally displayed by glasses and most non-metallic minerals. } html += headTags[0].innerHTML; Comments: Colorless, lustrous blocky albite crystals to 3 cm in size with a thick dusting of micaceous green micro chlorite. Location: Acushnet Quarry, Bristol Co., Massachussetts, USA. if (document.getElementsByTagName != null) Location: Crystal Bay, Hennepin County, Minnesota, USA. © John Veevaert, Comments: Dual view of a cleavage fragment of labradorite showing the multi-colored labradorescence (schiller effect) and the same area viewed without the color. The extinction angle is an optical characteristic and varies with the albite fraction (%Ab). Plagioclase Albite. Location: Maeyama Ueda shi, Nagano, Japan. else } urchinTracker(); Print or Cut-and-Paste your Plagioclase Specimen Label here : "); } If you feel any of the content is incorrect, or if you feel we are missing vital locality information, please fill out the form below so we can update the site. Scale: See Photo. The names of the minerals in the series are arbitrarily given based upon their relative abundance of albite and anorthite. var printReadyElem = document.getElementById("printReady"); if (headTags.length > 0) There are several named plagioclase feldspars that fall between albite and anorthite in the series. The other end member, Anorthite, contains calcium and no sodium.The intermediary members are Oligoclase, Andesine, Labradorite, and Bytownite.Oligoclase and Andesine are considered by some to be a variety of Albite rather then a separate mineral. The composition of a plagioclase feldspar is typically denoted by its overall fraction of anorthite (%An) or albite (%Ab), and readily determined by measuring the plagioclase crystal's refractive index in crushed grain mounts, or its extinction angle in thin section under a polarizing microscope. The exact composition of the various feldspar minerals depends on what elements balance the oxygen, which has two bonds to fill (remember H 2 O?). If you are requesting a locality be added, please only include. members are: We strive for accurate content and locality information. { Hammers, Chisels, Loupes, Cotton-filled boxes, etc. Mineral Discussion Forum from Fabre Minerals - also available in var gAutoPrint = true; // Flag for whether or not to automatically call the print function Igneous and metamorphic rocks. The composition of plagioclase feldspar is typicallyindicated by the general anorthite (% An) or albite (% Ab) fraction and iseasily determined by measuring the refractive index or peel angle within thecrushed particles by measuring the refraction angle under a thin section. Of the Plagioclase Feldspar group, an isomorphous solid solution series.Albite is one end,. Content and locality information Bay, Hennepin County, North Carolina, USA See... 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