Sodium cobaltinitrite can be used to stain muscovite, to distinguish it from paragonite. The stain is inhibited in the neighbourhood of feldspar crystals.
Laduron, D. (1971). A staining method for distinguishing paragonite from muscovite in thin section. The American Mineralogist (Print) : an international journal of earth and planetary materials, 56(5-6), 1117-1119. https://hdl.handle.net/2078.5/108397 (Original work published 1971)