Although closely related, the feldspars fall into two sub-groups: (1) the alkali feldspars which are generally monoclinic and (2) the sodium and calcium feldspars which form a solid solution series, collectively known as the plagioclase feldspars, which are triclinic. Members of the plagioclase series thus range in composition from 100% albite (NaAlSi3O8), often abbreviated Ab, to 100% anorthite (CaAl2Si2O8), abbreviated An. Within the plagioclase series, a number of feldspar types are distinguished based on their percentage composition of albite/anorthite (Table 6).


Oscillatory Zoning Alkali Feldspar Plagioclase Feldspar Plagioclase Composition Lamellar Twinning 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


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Copyright information

© Elsevier Science Publishers Ltd 1989

Authors and Affiliations

  • R. G. Rothwell
    • 1
  1. 1.Institute of Oceanographic SciencesGodalmingUK

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