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X-ray spectroscopy already occupies a unique position as one of the most powerful of the nondestructive analytical tools. Great strides forward in instrumentation and technique have led to an increased range of application on a routine basis. Although greater sensitivities are constantly being reported, there is need for still further improvement, and these needs can clearly be demonstrated by reference to specific geochemical and mineralogical problems. Recent researches in the geochemistry of sulfide minerals have shown that the distribution of trace elements between coexisting phases provides important data on the pressures and temperatures of formation of the minerals and quantitative data on the approach to thermodynamic equilibrium between the minerals and the solutions depositing them. This has given rise to a need for nondestructive methods of trace analysis of elements in concentrations of a few parts per million on grains that may be as small as 5 μ in diameter. In petrological studies, problems of compositional zoning of minerals, diffusion across grain boundaries, composition of exsolution bodies, fine-grained intergrowths, and inclusions require very specialized X-ray instrumentation to supply point by point chemical analysis on a microscopic scale.
U. S. Geological Survey, Washington, D. C.,