Note 114 — Chapter IV · The Microscopic and Optical Characters of Minerals. (continued 2)

These extinction angles, as well as those previously given, are those of only a few type feldspars of definite composition. As the composition varies through a long series, so the extinction angle changes, one being a function of the other.

For a complete list of compositions and related extinction angles, see Iddings’ Rock Minerals and Lévy & Lacroix’s Les Minéraux des Roches.