Editing Feldspar (section)

===Alkali feldspars===
Alkali feldspars are grouped into two types: those containing potassium in combination with sodium, aluminium, or silicon; and those where potassium is replaced by barium. The first of these includes:
* [[orthoclase]] ([[monoclinic]])<ref name="orthoclase">[http://www.galleries.com/minerals/silicate/orthocla/orthocla.htm "The Mineral Orthoclase"]. Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.</ref> {{chem2|KAlSi3O8}}
* [[sanidine]] ([[Monoclinic crystal system|monoclinic]])<ref name="sanidine">[http://www.galleries.com/minerals/silicate/sanidine/sanidine.htm "Sanidine Feldspar"]. Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.</ref> {{chem2|(K,Na)AlSi3O8}}
* [[microcline]] ([[triclinic]])<ref name="microcline">[http://www.galleries.com/minerals/silicate/microcli/microcli.htm "Microcline Feldspar"]. Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.</ref> {{chem2|KAlSi3O8}}
* [[anorthoclase]] ([[Triclinic crystal system|triclinic]]) {{chem2|(Na,K)AlSi3O8}}

Potassium and sodium feldspars are not perfectly [[Miscibility|miscible]] in the melt at low temperatures, therefore intermediate compositions of the alkali feldspars occur only in higher temperature environments.{{sfn|Klein|Hurlbut|1993||pages=532-536}} Sanidine is stable at the highest temperatures, and microcline at the lowest.<ref name="orthoclase" /><ref name="sanidine" /> [[Perthite]] is a typical texture in alkali feldspar, due to [[exsolution]] of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many [[granite]]s can be seen with the naked eye.<ref>Ralph, Jolyon and Chou, Ida. "Perthite". [http://www.mindat.org/min-6683.html Perthite Profile on mindat.org]. Retrieved on February 8, 2008.</ref> Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.