Also referred to as the plagioclase An alumino-silicate mineral containing a solid solution of calcium, sodium and potassium. Over half the Earth’s crust is composed of feldspars and due to their abundance, feldspars are used in the classification of igneous rocks. A more complete explanation can be found on the feldspar group page. Click on Term to Read More series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous Compositional variation resulting from the substitution of one ion or ionic compound for another ion or ionic compound in an isostructural material. This results in a mineral structure with specific atomic sites occupied by two or more ions or ionic groups in variable proportions. Solid solutions can be complete (with of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called Rare compositional variety of plagioclase and the calcium end-member of the plagioclase feldspar mineral series with the formula CaAl2Si2O8. Anorthite is found in mafic igneous rocks such as anorthosite. Anorthite is abundant on the Moon and in lunar meteorites. However, anorthite is very rare on Earth since it weathers rapidly Click on Term to Read More (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite (pure albite has the formula: NaAlSi3O8). Meteoriticists denote the composition of plagioclase feldspars in meteorites based on their overall fraction of pure anorthite (An) or pure albite (Ab). As some of the minerals below may contain K-rich feldspars (orthoclase denoted as Or), their composition requires further description based on An, Ab and Or. A more complete explanation can be found on the feldspar group page that also includes the alkali feldspars as part of the feldspar ternary diagram.
All the named minerals in the plagioclase feldspar series below can be found in meteorites. However, some meteoriticists feel the Inorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More names are antiquated and instead use the An, Ab and/or Or wt% notation.
|Name||Anorthite (CaAl2Si2O8) wt%||Albite (NaAlSi3O8) wt%||Orthoclase (KAlSi3O8) wt%|
|Andesine||Or~13-0 (see note 1)|
|Oligoclase||Or~13-0 (see note 2)|
- K-rich andesine is quite rare in meteorites. Its orthoclase content, when listed in the chemical writeups, is typically 1.0% to 2.2% with the only exception to date being NWA 7034 and its pairings (Black Beauty) whose orthoclase content spans 1.8% to 10%.
- K-rich oligoclase is more common in meteorites than K-rich andesine. Its orthoclase content, when listed in the chemical writeups, is typically 1.3% to a maximum 10%.