A feldspar staining technique, based upon the method described by Gabriel and Cox, was tested for accuracy and reliability by using both artificial mixtures of quartz and feldspar, and natural sands. The sand grains are mounted on a glass slide, exposed to the fumes of hydrofluoric acid for 15 min., treated with a concentrated solution of sodium cobaltinitrite for 2 min., rinsed, and dried. This reaction forms a yellow coating on the potash feldspars, a white coating on the soda-lime feldspars, and the quartz is not affected. For greater accuracy, the sample may then be treated for a few minutes with a 0.5% solution of eosine "B." This will stain the soda-lime feldspar pink and the potash feldspars orange-yellow. Tests on 70 slides of artificial mixtures of quartz and feldspar show that the method is accurate to plus or minus 5%. Percentages determined by multiple tests on natural sands show good agreement, usually to within 5%.

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