The abundance of K-spar gives the rock a predominant pink to reddish hue; peppered with minor amounts of black minerals. close examination reveals carlsbad twinning in some crystals. In thin section plagioclase commonly shows the characteristic albite polysynthetic twinning. Hornblende and biotite are the commonest mafic minerals, however, muscovite is also frequently encountered. Felted 'flow' texture made up of sanidine laths, known as trachytic texture. Large orthoclase (alkali feldspar) crystal with string perthite visible even in PPL. This twinning is the most characteristic identifying feature of plagioclase and makes its identification easy when present. Characteristic "tartan" twinning. All images copyright GEOSEC SLIDES 2013 ~ Please feel free to link to this page ~. In thin section or grain mount the general lack of pleochroism and the twinning are good indicators. Distinctive perthite, best viewed near extinction. Biotite, metaluminous granite, showing a close-up of one crystal. In thin section alkali feldspar grains often … It is not possible to distinguish between the different feldspars in PPL view. 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). St Hilaire, Canada. (1997) Compositional and microtextural zoning in alkali feldspars from the Shap granite and its geochemical implications. This was first shown by the german mineralogist johann friedrich christian hessel in 1826. Alkali Feldspar  (Click PPL image to show labels). The amateur has a few clues. Orthoclase in a monzonite, Hill of Johnston, Aberdeenshire. 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. ... Orthoclase in granite. This rock is about 97% Sanidine. This means that biotite in standard thin sections rarely goes completely extinct. Journal of the Geological Society of London: 154: 183-188. You can find the explanation on the alkali feldspar page. Plagioclase feldspar thin section. It is a dense rock with a phaneritic texture. 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. In general keys to identifying k feldspar are its lack of color its low birefringence and its twinning. 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. Feldspar thin section. (a combination of albite and pericline twinning). Carlsbad twinning differentiates the orthoclase from the albite twinning in the plagioclase, which also shows some alteration to sericite. Plagioclase is a series of tectosilicate minerals within the feldspar group. The orthoclase shows faint perthitic texture. To link to this object paste this link in email im or document to embed this object paste this html in website. This causes confusion, one is a rock type and the other a texture. Plagioclase feldspar thin section view description. Trachyte lavas would be expected to be more viscous than basalts. Small 250x250 max. 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. Can be confused with quartz if there is no twinning however the optic axis figure will resolve this. 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. Lee, M.R., Parsons, I. Kendrick Lamar Section 80 Free Download Zip. Home rocks and minerals plagioclase feldspar thin section reference url share. Sanidine in a trachyte. Straight because of its triclinic symmetry link to this object paste this html in website not always trachytic. 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