Notes on Potassium Feldspars

15 Sept. 2005 Potassium Feldspars KAlSi3O8 There are four varieties of K-feldspar:Microcline, Orthoclase, Sanidine, and Adularia. Microcline is optically triclinic and is characterized by a distinctive crosshatched twin pattern, often referred to as tartan twinning (see fig 15-18). Orthoclase is optically monoclinic K-spar with a 2V greater than 40°. Sanidine is also optically monoclinic but has a much lower 2V (0-40°). Adularia is a low-temperature hydrothermally produced mineral. Feldspar textures: A variety of intergrowth and exsolution textures can develop in the feldspars. At high temperatures the alkali feldspars show continuous solid solution between NaAlSi3O8 and KAlSi3O8. At lower temperatures, solid solution is limited to minor substitution near each end member. As a result, when alkali feldspars cool the intermediate members of the solid solution series are thermodynamically more stable as nearly pure sodic and potassic end-member phases. Solid-state exsolution of one feldspar by another will occur along crystallographic planes and ranges in scale from submicroscopic to macroscopic. Perthite consists of lamellae of albite in an orthoclase or microcline host, while Antiperthite is lamellae of K-spar in an albite host (see fig 15-15, top). When quartz is involved the relationships are more complicated and not well understood. Myrmekite is a quartz-plagioclase intergrowth relatively common in acid and intermediate rocks (fig 15-15 middle). Quartz is commonly found as stringers or vermicules of random orientation within a host plagioclase crystal. Myrmekitic texture is most common along boundaries between plagioclase and orthoclase or microcline grains. Granophyre is a textural term 15 Sept. 2005 referring to an intergrowth of alkali feldspar and quartz (fig 15-15 bottom). It may be wormy or vermicular, or may occur in regular geometric patterns. Granophyric texture is an example of a eutectic texture, resulting from the simultaneous precipitation of quartz and feldspar from a melt. Identification: Microcline: Hand Sample: Cannot be reliably distinguished from orthoclase or sanidine in hand sample. However, the presence of visible exsolution lamellae is almost always indicative of microcline. Thin section: Plane Light: Low negative relief. Colorless but may appear dusty or cloudy. Crossed Polars: Biaxial (-), 2V 65-88°. Crosshatch or tartan twinning is common and diagnostic. Birefringance is low, producing first order greys. Sanidine: Hand Sample: Cannot be distinguished from other K-spars in hand sample. Thin Section: Plane Light: Low negative relief. Colorless but may be cloudy. Crossed polars: Biaxial (-), 2V 0-40°. First order grey interference colors. Sanidine will resemble quartz but is biaxial. Orthoclase: Hand Sample: Cannot be distinguished from other K-spars in hand sample. Thin Section: Plane Light: Low negative relief. Colorless but may be cloudy. Crossed Polars: Biaxial (-), 2V is greater than 40°. 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. Anorthoclase only occurs in Na-rich felsic volcanics and hypabbysal intrusives.