You can find the explanation on the alkali feldspar page. Anorthoclase is the only alkali feldspar that is not K-feldspar. Working on polishing my samples for reflective light microscopy. Plagioclase feldspar thin section view description. Unless it’s heavily altered, you’ll still be able to identify the underlying feldspar by rotating the stage and looking for its wispy lamellae. An iridescent play of light can result from optical interference inside some feldspars. The plagioclase has polysynthetic twinning.The muscovite is pleochroic under PPL and has clinopyroxene embayed in the core. In thin section, alkali feldspar grains often appear to be sprinkled with dirt (PPL) or tiny confetti (XPL). The amateur has a few clues. Feldspar thin section. Home Rocks and Minerals Plagioclase Feldspar (thin section) Reference URL Share . Albite is considered both a plagioclase and alkali feldspar. Your email address will not be published. Opaques (oxides) also present. Large orthoclase (alkali feldspar) crystal with string perthite visible even in PPL. White, grayish, reddish; colorless in thin section: 2V: ... Moonstone is a name given to a gem material that consists of very thin, alternating layers of orthoclase (an alkali feldspar) and albite (a plagioclase feldspar). Alkali Feldspar (Click PPL image to show labels) Orthoclase in a granite from Fionnphort, Isle of Mull. Plagioclase feldspars may be present in small quantities, less than 10%. The feldspar component of syenite is predominantly alkaline in character (usually orthoclase). Clinopyroxene is present because the melt reacted with the clinopyroxene and create muscovite. Feldspars weather readily to a fine-grained white mica called sericite. In thin section, plagioclase feldspar grains often look like they’re sprinkled with dirt (PPL) or tiny confetti (XPL). Therefore, we see this low grade process which wasn’t preserved, yet a whole different metamorphic event. The plagioclase has To link to this object paste this link in email im or document to embed this object paste this html in website. The alkali feldspar grains are large and take up a decent amount of the slide. var sc_project=12387068; Alkali Feldspar crystals are large in this thin section. Anorthite weathers quite readily, so it’s not terribly abundant on Earth. Alkali feldspars. Add tags Comment Rate. To differentiate between the muscovite and the biotite pleochroism comes into effect. There are several plagioclase feldspars in the seriesbetween albite and anorthite. The perthite is mainly one grain that is large and is about 11mm. What would really help the viewers would be a photo of the rock illustrated in the thin sections. The perthite is mainly one grain that is large and is about 11mm. Re: you latest post: How is that low grade process preserved in a gniess? It is sodic in composition, optically triclinic and characterized by a twinning similar to microcline but on a smaller scale. Moonstone is a name given to a gem material that consists of very thin alternating layers of orthoclase an alkali feldspar and albite a plagioclase feldspar. In thin section microcline orthoclase and sanidine are distinguished by their twinning optical sign and 2vo. Add tags comment rate. Thedeflection angle is an optical characteristic and varies according to thealbite fraction (Ab). Feldspar poikiloblasts with inclusions of quartz and oxides are prominent throughout much of the sample. Clinopyroxene and orthopyroxene with few anhedral plagioclase crystals in interstitial space. Plagioclase Feldspar (thin section) View Description. The quartzy matrix in some areas is intergrown at the edge of the alkali feldspar crystal faces. I was an Optical Microscopy teaching assistant in grad school 35 years ago. Leucite is also present in a minimal quantity. Anorthite is the calcic end member of the plagioclase feldspar series. var sc_security="b06e0aa2"; [CDATA[ I’m impressed at your work. Thin-section Bands of interlocking quartz grains and some feldspar (plagioclase and alkali, lamellar twinning and alteration ), alternate with bands composed … D1 had a direct effect on the foliation (S1) of minerals that already crystallized. Major or essential minerals are alkali feldspar with less amount of plagioclase, quartz or feldspathiod. The rock is a gabbronorite due to the presence of both orthopyroxene and clinopyroxene. Composition includes: biotite, muscovite, chlorite, feldspars, oxides, and quartz. Another metamorphic event occured after this gneiss formed. ... Alkali Feldspar group . This sample is a pyroxenite from Hogen Camp Mine in the Hudson Highlands region of New York. This rock is predominantly composed of quartz, alkali feldspar, and plagioclase. The plagioclase in this thin section has polysynthetic twinning. In general keys to identifying k feldspar are its lack of color its low birefringence and its twinning. The leucite is embayed in the alkali feldspar. Alkali feldspars are grouped into two types: those containing potassium in combination with sodium, aluminum, or silicon; and those where potassium is replaced by barium. Most poikiloblasts occurred synkinematic to the 1st deformation event (D1) and rotation occurred. 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K feldspar in thin section rarely has well developed cleavage but two cleavage planes may be visible in some crystals. Mike // Undergrad Geology major at SUNY Oswego // Focus in Petrology/Mineralogy. S-tectonite with muscovite and quartz in thin section, This rock is predominantly composed of quartz, alkali feldspar, and plagioclase. This happened when the alkali feldspar started to crystalize. The really precise way is to use thin sections to determine the optical properties along the different crystallographic axes. The silica content was too great and the leucite reacted with the melt to form alkali feldspar. Syenites are usually either peralkaline with high proportions of alkali elements relative to aluminum, or peraluminous with a higher concentration of aluminum relative to alkali elements (K, Na, Ca). To link to this object, paste this link in email, IM or document To embed this object, paste this HTML in website. It is distinguished from alkali feldspar by lack of perthitic intergrowths, from plagioclase by lack of polysynthetic twinning, from both by parallel extinction on cleavage and uniaxial negative interference figure, from apatite by its much lower relief, and from quartz by its cleavage, common alteration, and uniaxial negative interference figure. The area was once a lead producer of iron ore and magnetite for the east coast. The rock formed due to an eruption of gaseous, low viscous magma which resulted in vesicles throughout much of the rock. Section 2 Complex Patterns Of Inheritance, Section 13050 Of The California Probate Code. The alkali feldspar have subhedral crystals. 25. Small 250×250 max. Feldspar - Feldspar - Identification of specific feldspars: Alkali feldspars can often be distinguished from plagioclase feldspars because most grains of the latter exhibit albite twinning (see above Crystal structure), which is manifested by parallel lines on certain cleavage surfaces, whereas grains of alkali feldspars do not.
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