TOURMALINE: AN IDEAL INDICATOR OF ITS HOST ENVIRONMENT

Tourmaline-supergroup minerals are ubiquitous accessory minerals in rocks of the Earth's crust. They can adjust their composition to suit a wide variety of environments, and therefore display a remarkable range in stability in terms of pressure, temperature, fluid composition, and host-rock composition. Because of this compositional sensitivity, tourmaline is an excellent indicator of the environmental conditions in its host. This is further enhanced by negligible diffusion up to high temperatures and a strongly refractory character during subsequent host-rock alteration and weathering, as well as mechanical transport of grains. Whereas most prior research on tourmaline has focused on chemical and crystallographic characterizations and systematics of the tourmaline-supergroup minerals, recent studies are shifting the focus to a quantitative reconstruction of environmental conditions in the host using a combination of structural, compositional and crystallographic characteristics of the tourmaline. This thematic issue, which follows a special session at the 2009 GAC-MAC-AGU meeting in Toronto, highlights these exciting advances; here we discuss some of the obstacles that will need to be overcome to insure the practical applicability of tourmaline. The papers presented in this thematic issue of The Canadian Mineralogist show that we are standing on the brink of a major breakthrough in the use of tourmaline as a quantitative indicator of the chemical and physical properties of its host environment these properties may well make tourmaline the prime mineral for this purpose.