Speed and accuracy may be increased in the quantitative determination of the more abundant mineral constituents in thin sections or polished surfaces of igneous rocks, or in the sands or grains derived from them, by decomposing the surface of the mineral constituents with hydrofluoric acid, and then very carefully immersing this in a concentrated sodium cobaltinitrite solution. This reagent produces a precipitate of yellow potassium cobaltinitrite on the potash feldspars which clings tenaciously to them. No precipitate is formed on the plagioclases. After this simple treatment the potash feldspars are intensely yellow, quartz is transparent, the plagioclases are white and opaque to transmitted light, the micas remain white or black, depending on the variety, but they can be differentiated by their flat surfaces. The percentages of the minerals can be determined by grain count.