A speedy and accurate staining procedure for the quantitative determination of the feldspars in rocks and sands of soils is proposed. The minerals are treated, by direct contact, with 48 per cent hydrofluoric acid, and then immersed in a concentrated sodium cobaltinitrite solution which stains the potash feldspars yellow. This treatment is followed by immersion in a buffered hematein solution which stains the plagioclase feldspars purple. The depth of colour on the plagioclase depends upon the position in the isomorphous series. Quartz, gypsum and calcite are not affected by the treatment.The quantity of the potash and plagioclase feldspar present is then easily determined by grain counts under the petrographic microscope.
