Selective staining of K-feldspar and plagioclase on rock slabs and thin sections^*

This note describes a new method of selectively staining plagioclase in which a surface etched with hydrofluoric acid is dipped into a solution of barium chloride, rinsed, and treated with a solution of potassium rhodizonate. Plagioclase feldspars other than pure sodium feldspar are thus stained a brick red by barium rhodizonate. This procedure is combined with the cobaltinitrite staining of K-feldspar to stain K-feldspar yellow and plagioclase red on polished rock surfaces and uncovered thin sections.