Information AboutFeldspar |
| CATEGORIES ABOUT FELDSPAR | |
| tectosilicates | |
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Ferroan Anorthosite #60025 ( Plagioclase Feldspar). Collected by Apollo 16 from the Lunar Highlands near Descartes Crater . This sample is currently on display at the National Museum Of Natural History in Washington, DC . (''Unknown scale'')]] Feldspar is the name of a group of rock-forming 2007 . Feldspars 2007 . Rock formed entirely of Plagioclase feldspar (see below) is known as Anorthosite . Feldspars are also found in many types of Sedimentary Rock . ETYMOLOGY Feldspar is derived from the German ''Feld'', field, and ''Spat'', a Rock that does not contain Ore . "Feldspathic" refers to materials that contain feldspar. The alternative spelling, ''felspar'', has now largely fallen out of use. COMPOSITIONS This group of minerals consists of framework or Tectosilicate s. Compositions of major elements in common feldspars can be expressed in terms of three Endmembers : K-feldspar endmember K Al Si 3 O 8 Albite endmember Na Al Si 3 O 8 Anorthite endmember Ca Al 2 Si 2 O 8 Solid Solution s between K-feldspar and albite are called alkali feldspar. Solid solutions between albite and anorthite are called ''' Plagioclase '''. Only limited solid solution occurs between K-feldspar and anorthite, and in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the earth. Sanidine ( Monoclinic ), ''' Orthoclase ''', and ''' Microcline ''' ( Triclinic ) refer to Polymorphs of K-feldspar. Sanidine is stable at the highest temperatures, and microcline at the lowest. 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 granites are coarse enough to be visible to the naked eye. Compositions of the plagioclase series have been labeled as follows (percent anorthite in parentheses):
Intermediate compositions of plagioclase feldspar also may exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes. The immiscibility gaps in the plagioclase Solid Solution are complex compared to the gap in the alkali feldspars. The play of colors visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae. USES
In 2005, Italy was the top producer of feldspar with almost one-fifth world share followed by Turkey, China and Thailand, reports the International Monetary Fund . REFERENCES SEE ALSO |
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