Granular texture, Microcline in a syenite. In thin section or grain mount the general lack of pleochroism and the twinning are good indicators. Plagioclase feldspar thin section view description. This twinning is the most characteristic identifying feature of plagioclase and makes its identification easy when present. In thin section or grain mount the general lack of pleochroism and the twinning are good indicators. Trachytes do not always show trachytic texture, and other rocks, eg basalts, do sometimes show trachytic texture. All images copyright GEOSEC SLIDES 2013 ~ Please feel free to link to this page ~. 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. In thin section alkali feldspar grains often … Olivine Visual Identification Assistant for thin section microscope slides ... (Click PPL image to show labels) Orthoclase in a granite from Fionnphort, Isle of Mull. Bytownite ca 70 90 na 30 10 locality at bytown now the city of ottawa canada. Biotite, metaluminous granite, showing a close-up of one crystal. Add tags comment rate. (1997) Compositional and microtextural zoning in alkali feldspars from the Shap granite and its geochemical implications. Alkali fedspar syenite Syenite, from Latin "Lapis Syenitis" (lapis = stone), from Syene (an ancient city of southern Egypt), is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts (5%).The feldspar component of syenite is predominantly alkaline in character (usually orthoclase). You can find the explanation on the alkali feldspar page. St Hilaire, Canada. Plagioclase feldspar thin section. Alkali Feldspar Granite Granite is a medium-tocoarse-grained acid igneous rock with essential quartz (>20%) and feldspar, where alkali feldspar constitutes between 100 and 35% of the feldspars, and minor mafic minerals. Plagioclase is series of framework silicate minerals in feldspar group. Felted 'flow' texture made up of sanidine laths, known as trachytic texture. Can be confused with quartz if there is no twinning however the optic axis figure will resolve this. This causes confusion, one is a rock type and the other a texture. This was first shown by the german mineralogist johann friedrich christian hessel in 1826. Often a grey color in comparison to other feldspars. Earlier workers interpreted the Lottah Granite as a metasomatised differentiate of the Poimena Granite. Plagioclase is a continuous series of solid solutions known as the plagioclase feldspar series rather than a specific mineral with a particular chemical composition. Anorthite ca 90 100 na 10 0 from greek for not straight because of its triclinic symmetry. Carlsbad twinning differentiates the orthoclase from the albite twinning in the plagioclase, which also shows some alteration to sericite. Plagioclase feldspar thin section.The series ranges from albite to anorthite endmembers with respective compositions naalsi3o8 to caal2si2o8 where sodium and calcium atoms can substitute for each other in the mineral s crystal lattice structure. Sanidine in a trachyte. Small 250x250 max. It is not possible to distinguish between the different feldspars in PPL view. (a combination of albite and pericline twinning). Orthoclase in a monzonite, Hill of Johnston, Aberdeenshire. In general keys to identifying k feldspar are its lack of color its low birefringence and its twinning. This means that biotite in standard thin sections rarely goes completely extinct. Kendrick Lamar Section 80 Free Download Zip. To link to this object paste this link in email im or document to embed this object paste this html in website. In thin section plagioclase commonly shows the characteristic albite polysynthetic twinning. Plagioclase is a series of tectosilicate minerals within the feldspar group. Lee, M.R., Parsons, I. The series ranges from albite to anorthite endmembers where sodium and calcium atoms can substitute for each other in the mineral s crystal. Large orthoclase (alkali feldspar) crystal with string perthite visible even in PPL. Feldspar thin section. The Lottah Granite is a composite pluton of tin-mineralised, strongly peraluminous alkali-feldspar granite which intrudes the Poimena Granite, a major component of the mid-Devonian Blue Tier Batholith of northeastern Tasmania. Microcline in a scapolite syenite from Mt. Alkali feldspar (orthoclase) - Grains typically 2-4mm long, subhedral to anhedral, dirty looking due to alteration.Many display a microperthite texture.. Plagioclase feldspar - Grains typically 1-4mm long, subhedral, characteristic lamellar twinning.Many of the grains look “dirty” under plain polarised light, and have high birefringence speckles under cross-polarised light. close examination reveals carlsbad twinning in some crystals. The series ranges from albite to anorthite endmembers with respective compositions naalsi3o8 to caal2si2o8 where sodium and calcium atoms can substitute for each other in the mineral s crystal lattice structure. Alkali Feldspar  (Click PPL image to show labels). Journal of the Geological Society of London: 154: 183-188. Orthoclase in monzonite. The abundance of K-spar gives the rock a predominant pink to reddish hue; peppered with minor amounts of black minerals. ... Orthoclase in granite. Distinctive perthite, best viewed near extinction. Trachyte lavas would be expected to be more viscous than basalts. The amateur has a few clues. This rock is about 97% Sanidine. 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