Key optical features of amphibole in thin section: Green (more rarely Brown, Colourless or Blue) ... Pyroxene (Two cleavages at 90 o, higher extinction angle, typically paler coloured) Common alteration product: Chlorite (pale green). minerals in the structurally related pyroxene, amphibole, and mica groups. Most apparent, in hand specimens, is that amphiboles form oblique cleavage planes (at around 120 degrees), whereas pyroxenes have cleavage angles of approximately 90 degrees. Pyroxene thin section. Orthopyroxene high in mg is common in gabbro and peridotite groups. Lower to middle second order colors. The scientific study of the origin, history, and structure of the Earth. The rock illustrated in these images contains both clinopyroxene and orthopyroxene, and the significantly higher Cr 2 O 3 content of the clinopyroxene in this sample gives it a pale green colour in plane polarized light, whereas the Cr-poor orthopyroxene is colourless (A). Thin Section Of A Gabbro Showing Plagioclase And Augite Xpl Graphisme. Biaxial 2v 25 70. The fine grained gray rock in this image is the host basalt. Thin section 2. Hornblende PPL properties. Amphiboles like horneblend tend to have a more splintery fracture/cleavage than pyroxenes. This paper is part of a larger program . Colourless pale green pale brown two cleavages intersect at 90 o moderate relief moderate order interference colours equant crystal shape common minerals that might be confused with pyroxene and occur in similar rocks. Again darker colors corresponding to higher fe content. The content of amphibole and plagioclase together is mostly higher than 90%, and may be as low as 75%. Required fields are marked *. Key optical features of pyroxene in thin section. Low birefringence first order colors. Tephrite from latium italy. 1. Geochemistry With a mineral that is too small for visual identification, the first step is making a thin section of the rock and determining the mineral composition. Although aluminium substitutes extensively for silicon in silicates consisting of feldspars and amphiboles the substitution occurs only to a confined extent in most pyroxenes. Pyroxene looks like it belongs to Sauron. Sub ophitic texture with clinopyroxene and plagioclase altered to clay xpl. Also found as anhedral to strongly irregular grains, which in some cases … The miscibility of ca poor pigeonite or orthopyroxene and ca rich augite or clinopyroxene reduces with lowering temperature and each may exolve the other. Press J to jump to the feed. Loch borrolan carbonatite. Commonly replaces: Clinopyroxene. Relief: High positive Habit/Form: Commonly found as slender prismatic to bladed crystals with a diamon-shaped cross section showing the amphibole cleavage at 56° and 124°. Amphiboles are also specifically less dense than the corresponding pyroxenes. Also, if your hand samples have other minerals, you can distinguish between the two by likely assemblage in a particular rock type. Xpl image 10x field of view 2mm hourglass zoning in a pyroxene. Another indicator may be color. Pyroxene in basal section and isotropic laucite crystals in a basanite. Inosilicates are a form of silicate minerals. The key difference between pyroxene and amphibole is that the pyroxene is a form of inosilicate, which contains single chains of SiO3 tetrahedra whereas the amphibole is a form of inosilicate, which contains double chain SiO4 tetrahedra. Cookies help us deliver our Services. Pyroxenes have two distinctive planes of cleavage with intersecting angles of about 87 and 93. 1) The modal compositions of amphibolites show that most of them contain more than 50% of amphibole, but those with 50 to 30% are not unusual. New comments cannot be posted and votes cannot be cast. Any tips on how to tell the two apart? The xenolith is dominated by green peridot olivine together with black orthopyroxene and spinel crystals and rare grass green diopside grains. There are two ways. Plane light colorless grey pale green pale brown or brownish green. Diopside has a green hue and would be blocky, as opposed to the green amphibole actinolite. August 17, 2018 Posted by Madhu. Press question mark to learn the rest of the keyboard shortcuts. Your email address will not be published. Can be weakly pleochroic. Perpendicular to their cleavage planes pyroxenes have nearly square cross sections which together with the cleavage directions are diagnostic properties. (Ex mineralogy TA here). Pyroxene cleavage fragments have square shaped cross sections with cleavage faces meeting at nearly right angles 87o and 93o. Hornblende #2 thin section (hFOV 2mm) GET 360° LIVE INTERACTION. Clinopyroxene oikocryst with ophitic texture in plagioclase altered to clay xpl. Save my name, email, and website in this browser for the next time I comment. Although aluminium substitutes extensively for silicon in silicates consisting of feldspars and amphiboles the substitution occurs only to a confined extent in most pyroxenes. This isn't always the case, but usually horneblend specifically looks like a bunch of clumped up needles where something like augite looks blockier. 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