Note 105 — Chapter IV · The Microscopic and Optical Characters of Minerals.
Reading edition
The plagioclases have rather a complex composition; but may be regarded as forming a series from the composition NaAlSi3O8(Ab) to the composition CaAl2Si2O8(An), consisting for the most part of isomorphous mixtures of these types, with some replacement by KAlSi3O8. The compositions of only a few of the common plagioclases are given above.