Plagioclase

Also referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (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 mineral names are antiquated and instead use the An, Ab and/or Or wt% notation.

Name Anorthite (CaAl2Si2O8) wt% Albite (NaAlSi3O8) wt% Orthoclase (KAlSi3O8) wt%
Anorthite
An90-100
Ab10-0
Or10-0
Bytownite
An70-90
Ab30-10
Or10-0
Labradorite
An50-70
Ab50-30
Or10-0
Andesine
An30-50
Ab70-50
Or~13-0 (see note 1)
Oligoclase
An10-30
Ab90-70
Or~13-0 (see note 2)
Albite
An90-100
Ab10-0
Or10-0

Notes:

  1. 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%.
  2. 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%.