Plagioclase is a series of tectosilicate minerals within the feldspar group. The series ranges from albite to anorthite endmembers where sodium and calcium atoms can substitute for each other in the mineral s crystal. In thin section or grain mount the general lack of pleochroism and the twinning are good indicators. Anorthite ca 90 100 na 10 0 from greek for not straight because of its triclinic symmetry. Hornblende and biotite are the commonest mafic minerals, however, muscovite is also frequently encountered. Often a grey color in comparison to other feldspars. 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). Bytownite ca 70 90 na 30 10 locality at bytown now the city of ottawa canada. Trachytes do not always show trachytic texture, and other rocks, eg basalts, do sometimes show trachytic texture. This was first shown by the german mineralogist johann friedrich christian hessel in 1826. Feldspar thin section. Plagioclase is series of framework silicate minerals in feldspar group. You can find the explanation on the alkali feldspar page. Plagioclase feldspar thin section. Granular texture, Microcline in a syenite. Earlier workers interpreted the Lottah Granite as a metasomatised differentiate of the Poimena Granite. Distinctive perthite, best viewed near extinction. The abundance of K-spar gives the rock a predominant pink to reddish hue; peppered with minor amounts of black minerals. Characteristic "tartan" twinning. Felted 'flow' texture made up of sanidine laths, known as trachytic texture. Sanidine in a trachyte lava, possibly from a lava dome. 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. 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. This twinning is the most characteristic identifying feature of plagioclase and makes its identification easy when present. close examination reveals carlsbad twinning in some crystals. 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. Sanidine in a trachyte. 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. Large orthoclase (alkali feldspar) crystal with string perthite visible even in PPL. In general keys to identifying k feldspar are its lack of color its low birefringence and its twinning. Can be confused with quartz if there is no twinning however the optic axis figure will resolve this. Plagioclase is a continuous series of solid solutions known as the plagioclase feldspar series rather than a specific mineral with a particular chemical composition. (a combination of albite and pericline twinning). (1997) Compositional and microtextural zoning in alkali feldspars from the Shap granite and its geochemical implications. Kendrick Lamar Section 80 Free Download Zip. Add tags comment rate. Olivine Visual Identification Assistant for thin section microscope slides ... (Click PPL image to show labels) Orthoclase in a granite from Fionnphort, Isle of Mull. Lee, M.R., Parsons, I. It is not possible to distinguish between the different feldspars in PPL view. Central Depression, (Malpais) Fuerteventura. Alkali Feldspar (Click PPL image to show labels). St Hilaire, Canada. In thin section alkali feldspar grains often … Journal of the Geological Society of London: 154: 183-188. The amateur has a few clues. This rock is about 97% Sanidine. The presense of parallel striae on cleavage surfaces distinguishes plagioclase from k feldspar. To link to this object paste this link in email im or document to embed this object paste this html in website. Plagioclase feldspar thin section view description. ... Orthoclase in granite. This causes confusion, one is a rock type and the other a texture. Alkali feldspar granite, some varieties of which are called 'red granite', is a felsic igneous rock and a type of granite rich in the mineral potassium feldspar (K-spar). Orthoclase in monzonite. Microcline in a scapolite syenite from Mt. Biotite, metaluminous granite, showing a close-up of one crystal. The orthoclase shows faint perthitic texture. 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. Carlsbad twinning differentiates the orthoclase from the albite twinning in the plagioclase, which also shows some alteration to sericite. In thin section plagioclase commonly shows the characteristic albite polysynthetic twinning. Small 250x250 max. Trachyte lavas would be expected to be more viscous than basalts. This means that biotite in standard thin sections rarely goes completely extinct. Home rocks and minerals plagioclase feldspar thin section reference url share. Damage produced during thin section grinding causes speckles of light in the biotite, where the crystal lattice has been deformed. Orthoclase in a monzonite, Hill of Johnston, Aberdeenshire. In thin section or grain mount the general lack of pleochroism and the twinning are good indicators. All images copyright GEOSEC SLIDES 2013 ~ Please feel free to link to this page ~. 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.