Definition

1. Noun:
   • A mineral of the plagioclase feldspar group: A calcium-rich, triclinic feldspar mineral, typically white, gray, or reddish in color, with the chemical formula CaAl₂Si₂O₈.
   • A rock-forming mineral: A primary constituent in many igneous rocks, especially those of basaltic composition, and in some metamorphic rocks.

Usage Examples

• Noun:
  • The sample contained abundant anorthite, indicating a calcium-rich magma source.
  • Geologists identified anorthite as the dominant feldspar in the anorthosite rock.

Advanced Usage

• In petrology: The term is used to specify the calcium endmember of the plagioclase solid solution series, which ranges from sodium-rich albite (NaAlSi₃O₈) to calcium-rich anorthite (CaAl₂Si₂O₈).
  • The plagioclase composition was determined to be An₈₀, meaning it is 80% anorthite and 20% albite.

Variants and Related Words

• Anorthitic (adj): Relating to or containing anorthite.
  • The anorthitic composition of the lava was unusual.
• Anorthosite (n): A coarse-grained intrusive igneous rock composed almost entirely of plagioclase feldspar, predominantly anorthite.
  • The lunar highlands are largely composed of anorthosite.

Synonyms

• Calcic plagioclase: A technical synonym emphasizing the high calcium content.
• Calcium feldspar: A descriptive synonym.

Related Terms and Contexts

• Plagioclase (n): The series of feldspar minerals of which anorthite is the calcium-rich endmember.
• Igneous petrology: The branch of geology where this term is most commonly applied to describe mineral composition and rock genesis.

Noun

1. rare plagioclastic feldspar occurring in many igneous rocks