Role of natural radiation in tourmaline coloration

Ansrn-lcr The optical spectra of elbaite tourmalines subjected to large, controlled doses of gamma radiation have been compared to those of natural specimens. Both naturally pink and laboratory-irradiated elbaites show the same spectroscopic features. Optical absorption features of Mn2* in nearly colorless elbaites are lost during laboratory irradiation, indicatifig a Mn2+ r Mn3+ transformation during the radiation process. Measurements of the radiation levels in tourmaline pockets in southern California pegmatites have been used to compute the doses that natural samples should have experienced over geologic time. These doses generally correspond to the doses required to restore the color to elbaites that have been decolorized by laboratory heat treatment, indicating that color in naturally pink tourmaline is a product ofnatural radiation. This radiation could have been effective only after the pegmatite cooled below the decolorizing temperature of tourmaline, suggesting that most pink elbaites originally grew nearly colorless in the pegmatites and only later attained their pink color through oxidation of Mn via ionizing radiation. INrnorucrroNr High-energy ionizing radiation is capable of changing the color of several minerals and inducing a variety of radiation-damage centers that include trapped electrons and oxidized or reduced cations and anions. The detailed atomic processes accompanying these changes are often poorly understood. It has long been recognized that many minerals change color when they are irradiated in the Iaboratory. The new colors that develop frequently resemble the colors of naturally occurring varieties of the mineral. Furthermore, the natural color of many minerals can be cyclically removed by heating and restored by irradiation. Therefore, it is often stated that the colors induced by laboratory irradiation are identical to natural colors with the tacit assumption that the underlying atomic-level color origins are also identical. There are few quantitative studies that have examined these concepts in detail. Two questions need to be addressed: what are the detailed transformations that occur when a mineral is irradiated, and do the changes induced in the laboratory correspond to those that occur in nature? These questions can be answered in part from a comparison of the spectroscopic data from natural and laboratory-irradiated minerals. It is possible that the answer will be different for different minerals. It is also necessary to estimate the radiation doses experienced by minerals in nature in order to determine whether the radiation doses applied to laboratory samples are similar to those that a mineral experiences in its natural setting. In this paper we consider the coloration ofpink elbaite. Bershov et al. (1969), Nassau (1975), and others have