XPL, Scale: lower edge – 3.5mm. Physical Properties: Chemical formula: NaAlSi 3 O 8: Class: Tectosilicate 3D frameworks of linked tetrahedra: In the central cordierite grain, however, twinning extends full-length. Framework Silicates. The twin lamellae in most cordierite grains are discontinuous and wedge out. Plagioclase. Alex Strekeisen - I vetrini della mia fantasia. triclinic (+,-) Relief - low. Feldspar (mineral group name), from Swedish names for field (feldt or falt) and spar (spat), in reference to the spar in tilled fields overlying granite; in German the word became Feldspat. Plagioclase is series of framework silicate minerals in feldspar group. 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 … You can find the explanation on the plagioclase feldspar page. The plagioclase in this thin section has polysynthetic twinning. Birefringence - 0.012-0.013. K-ts-crd-twn-1: Polysynthetically twinned cordierite appears like plagioclase feldspar. Colour - colourless. Locality: … Orthoclase – from Greek for straight and fracture, in reference to the cleavage angle, which is 90°. In thin section or grain mount the general lack of pleochroism and the twinning are good indicators. Alkali Fesldspar - KAlSi 3 O 8 Unlike plagioclase feldspars, alkali feldspars are divided into separate minerals not only based on their chemistry, but more importantly, based on optical properties determined by a petrographic microscope. The orthopyroxene also has twinning present in most grains. The clinopyroxene is filling voids in the rock and have an anhedral structure, filling interstitial space, compared to to the plagioclase and orthopyroxene which range from subhedral to … RI - 1.527-1.590. Plagioclase feldspar thin section view description. The thin sections in plane-polarized light (PPL) and cross-polarized light (XPL) reveal an opaque mineral. In thin section, plagioclase feldspar grains often look like they’re sprinkled with dirt (PPL) or tiny confetti (XPL). Interference colours - 1st order Hematite crystals have grown in micro-fractures around and across plagioclase crystals and originated in hydrothermal alteration. 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. It is hematite (Fe 2 O 3), as shown in reflected light on p. 1 (click here to go to that page).