The plagioclase feldspars are those within the plagioclase solid solution series.
Albite (Na-rich end member)
Anorthite (Ca-rich end member)
Plagioclase feldspars are easy to identify in thin section: they display first order grays and stripey twins. You’ll often see plag grains covered in something that looks like dirt (PPL) or tiny confetti (XPL); that is plag caught in the act of altering to sericite, which is a catch-all name for fine-grained white mica which can’t be positively identified without detailed chemical studies. Unless it’s heavily altered, you’ll still be able to identify the underlying plag by rotating the stage and watching the extinction patterns.
Sericitized Plagioclase, Plane Polars
Plagioclase feldspar altering to white mica in plane polarized light. Magnification: 40x Click here to view this mineral in crossed polars: https://youtu.be/…
Sericitized Plagioclase, Crossed Polars
Plagioclase feldspar altering to white mica in cross polarized light. Magnification: 40x Click here to view this mineral in plane polars: https://youtu.be/1z…