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Webster 1913 Edition

Noun.

[Gr.

πλάγιοσ

oblique +

κλᾶν

to break.]

(Min.)

A general term used of any triclinic feldspar. See the Note under

plagioclase ‎(usually uncountable, plural plagioclases)

(mineralogy) Any of a group of aluminum silicate feldspathic minerals ranging in their ratio of calcium to sodium.
- 2004, Richard Fortey, The Earth, Folio Society 2011, p. 51:
  These crystals are of one of the most abundant minerals in nature – feldspar; furthermore, they are of a particular kind of feldspar rich in the element calcium, termed plagioclase.

group of feldspar minerals