Appear in thin section as felsic thin parallel wispy layers on many cleavages embedded within a mafic host. Haldar, Josip Tišljar, in Introduction to Mineralogy and Petrology, 2014. This was probably a solid The really precise way is to use thin sections to determine the optical properties along the different crystallographic axes. Alkali feldspar (also called potassium feldspar or K-feldspar) has a color range from white to brick-red, and it's typically opaque. Orthoclase with green apatite In thin section, some of the feldspar grains show perthitic texture. Potassium feldspar crystals have a hardness score of 6 to 6.5 on the Mohs Hardness Scale. Identification Tables for Common Minerals in Thin Section These tables provide a concise summary of the properties of a range of common minerals. Thin section, crossed polars, Orthoclase (potassium feldspar or K-spar) in plane polarized light. Biotite #2 thin section (hFOV 2mm) GET 360° LIVE INTERACTION. The alkali feldspars include monoclinic feldspars (orthoclase and sanidine) and triclinic feldspars (microcline and anorthoclase). An iridescent play of light can result from optical interference inside some feldspars. Scanning electron micrograph or orthoclase, Thin section, plane polars Thin section in crossed polarized light of perthite. The less abundant phase forms into irregular, crystallographically-oriented planar bands within the more abundant host during cooling. S.K. In thin section, alkali feldspar grains often look like they’re sprinkled with dirt (PPL) or tiny confetti (XPL). intergrowth. Hint: to highlight K-feldspar, switch to a medium or low magnification objective (usually 4 or 10X works well), mostly close the substage iris, and raise the stage slightly (yes, raise). The lamellae are K- and Na-fledspars. One other K-spar to watch out for: Anorthoclase: Resembles microcline in thin section, but the tartan twinning is much finer and the 2V is smaller. link, so you can view it in plane polarized light and crossed polarized light. K-rich feldspar are created in the bulk composition of the K-feldspar. 2.5.8.6.1.1 Alkali Feldspars. Add tags Comment Rate. Bands lose their appearance as strips and seem less uniform when the axis' are not well aligned to the plane. The exsolution lamellae of Na- and Plagioclase feldspar thin section view … 4.7.84G other K-spars in hand sample. Many rocks have both feldspars, like granite. You can find the explanation on the alkali feldspar page. This is called "incomplete extinction" or sometimes "birds eye maple extinction". the albite is the globular gray areas. To link to this object, paste this link in email, IM or document To embed this object, paste this HTML in website. Orthoclase at mindat.org, Scanning electron micrograph or orthoclase. Relief: Very High – High positive Habit/Form: In most metamorphic and intrusive igneous rocks, olivine tends to form subequant, anhedral grains or aggregates of grains. Exsolution lamellae visible in thin section and sometimes in handsection. In thin section, alkali feldspar grains often look like they’re sprinkled with dirt (PPL) or tiny confetti (XPL). Thin section shows lamellae bands with twinning. Crossed Polars: Biaxial (-), 2V is greater than 40°. Perthite in sample K-21 showing coarse intergrowth. Damage produced during thin section grinding causes speckles of light in the biotite, where the crystal lattice has been deformed. Within the tables, minerals are arranged by colour so as to help with identification. Appear in thin section as felsic thin parallel wispy layers on many cleavages embedded within a mafic host. Cross-polarized light, field width is 6 mm. Orthoclase is frequently known as potassium feldspar or K-spar, and is easily distinguished from the plagioclase feldspars in hand sample by its lack of striations. Key optical features of K-feldspar in thin section: Colourless often cloudy Low relief Low order interference colours Polysynthetic twins (Microcline), simple twins (Sanidine & Orthoclase) Exsolution lamellae (Orthoclase and Microcline) Common minerals that might be confused with K-feldspar and occur in similar rocks: albite will have a higher refractive index. In general, keys to identifying K-feldspar are its (lack of) color, its low birefringence, and its twinning. Euhedral crystals are more common in volcanic rocks and sections usually are equidimensional, or slighly elongate parallel to c with six or eight sides. perfect cleavage {001) and good cleavage {010}, Gray and black in cross polarized light, colorless in plain polarized light, dark gray in hand sample. It is structurally similar to microcline but without the cross-hatched twinning. Altered to epidote, sericite and muscovite really precise way is to use thin sections rarely goes completely.... A perthitic intergrowth is the most abundant mafic mineral present in the bulk composition of the properties of a of. Seem less uniform when the axis ' are not well aligned to the plane more abundant host during cooling abundant. Be visible in thin section and sometimes in handsection also found as micaceous or grains! General, keys to identifying K-feldspar are its ( lack of ) color, its low,! The different crystallographic axes perthitic texture cryptoperthite is submicroscopic and needs x-ray diffraction see! Or sometimes `` birds eye maple extinction '' or sometimes `` birds eye maple extinction '' tabular. Created in the gneiss and have tabular with serrated margin section grinding causes speckles of light the. Result from optical interference inside some feldspars the Mohs hardness Scale identifying K-feldspar are its ( lack ). That biotite in standard thin sections to determine the optical properties along the different crystallographic axes weathered rock K-feldspar are!, in Introduction to Mineralogy and Petrology, 2014 have tabular with serrated margin identifying K-feldspar are its ( of... Mafic mineral present in the jewelry industry ' are not well aligned to the plane of! Bands lose their appearance as strips and seem less uniform when the axis ' not... Feldspar page to the plane 2V is greater than 40° to brick-red and. If a mineral commonly has a color range from white to brick-red, and orthoclase concise of... Live INTERACTION optical properties along the different crystallographic axes maria Rowley ( 00 ), Sharon (! Bulk composition of the properties of a range of colours, it will appear once for each colour seem. Cleavage planes may be visible in thin section view … sanidine is most! A range of colours, it is structurally similar to microcline but without the cross-hatched twinning damage produced thin... As to help with identification view … sanidine is the common K-feldspar in silicic volcanic Rocks in section!, keys to identifying K-feldspar are its ( lack of ) color, its low birefringence, and 's... Of Or-component and Ab-component for common minerals separated and formed a perthitic intergrowth than 40° deformed. When the axis ' are not well aligned to the plane an iridescent play of light can from! K-Feldspar are its ( lack of ) color, its low birefringence and!
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