In the course of the potash investigations of the U. S. Geological Survey, several thousand samples of drill cuttings from saline deposits have been examined by the immersion method. The first mount of the powder representing a sample is ordinarily made with an oil whose refractive index is near that of halite (1.544), because in such a medium the halite, which is the chief constituent of most samples, is “flattened out,” so that the other constituents are clearly visible. The appearance of the common minerals under these conditions becomes familiar with practice, and they are usually recognized at sight in the initial mount. When a mineral that is not thus easily recognized occurs, it can, as a rule, be identified with certainty and with economy of both labor and material by means of the procedure that forms the subject of the present article. This procedure consists essentially of transferring individual grains from the initial immersion medium to one or more other liquids—usually to other immersion media with whose refractive indices the indices of the grain are compared. The grains are washed in xylol prior to accurate measurement of indices or to microchemical tests. It is hoped that this technique may occasionally be useful to other persons who study mixtures of minerals or artificial compounds by the immersion method.

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