Calcite readily incorporates minor and trace amounts of divalent transition metal cations such as Cu from mineralizing fluids. Reflectance spectroscopy provides a rapid, inexpensive, nondestructive laboratory tool for detecting Cu (super +2) in solid solution in calcites, and for determining relative variations in Cu concentration. In reflectance spectra of calcites, Cu (super +2) causes a strong absorption centered near 0.9 mu m and weaker bands near 1.3 and 1.5 mu m. With increasing copper content the intensity of the 0.9-mu m band increases and shifts to shorter wave-lengths. The weaker bands appear to shift to shorter wavelengths with increasing Cu concentration as well. A Cu (super +2) concentration of 0.04 percent by weight produces a clearly discernible absorption feature, and detection limits for Cu (super +2) in the absence of other absorbing species (e.g., Fe, H 2 O) should be as good as this or better.Because carbonate rocks form important host rocks for Cu deposits and because calcite is a gangue mineral frequently associated with such deposits, this technique should provide a useful compliment to existing techniques for geochemical exploration. Since all the diagnostic spectral features occur in the region of the spectrum available with natural illumination, these data can be used to interpret remotely obtained spectra as well.