Unstable, green, irradiation-induced color in grossularite

Kurt Nassau
Lebanon, New Jersey

George R. Rossman
California Institute of Technology, Pasadena, California

Darwin L. Wood
Murray Hill, New Jersey


Although conventional wisdon has been that irradiatoin will not alter the color of garnets, exposure of some colorless to pale-hued Africian grossulars to either 60Co or 137Cs gamma rays turns them light yellow-green. The resulting color is unstable and fades within hours to days in sunlight, and within weeks in the dark. The spectra of the radiation-enhanced garnets differ from those of similiar-appearing naturally colored grossulars. In the dark, the irradiation-related absorption features undergo two stages of fading: an initial period of relatively rapid fading, followed by a longer period of slow fading. In daylight, a single rate of even faster fading is observed.