DEVELOPMENT OF TOURMALINE AT DIAGENETIC AND LOW-GRADE METAMORPHIC CONDITIONS: PETROLOGIC APPLICABILITY

J.B. Thompson Jr. advocated the investigation of complex minerals through the prism of exchange vectors (operators). This approach has been particularly effective in the investigation of compositional complexities inherent in tourmaline (tur). Tourmaline developed under low-temperature conditions (< 300°C) typically has distinctive morphological and chemical characteristics that reveal significant information about their environments of formation. Under diagenetic conditions, tourmaline can form as monopolar crystal overgrowths on detrital tourmaline grains or as distinct authigenic crystals. Clastic sedimentary rocks with a diagenetic overprint exhibit overgrowths nucleating exclusively at the positive end of the c axis (+c pole) on the detrital tur grain, a feature related its polar nature. These monopolar crystals tend to have compositions dominated by the X-site vacant tourmaline species (foitites) and can be linked to growth in aqueous environments low in Na, with other chemical characteristics related to the host rocks. As temperature increases, tur overgrowths develop at both the +c pole and -c pole, but continue to exhibit a significant degree of morphological and chemical asymmetry. These differences at either pole diminish as temperature increases. Authigenic tourmalines without detrital tur nuclei have been found in many sedimentary or weakly metamorphosed metasedimentary rocks as well as fluid-dominated geothermal systems. Foititic tourmalines characterize these occurrences, but with their Mg-Fe contents influenced by the host rock composition. Low-temperature tourmalines developed in hypersaline environments (e.g. salt domes) have distinct chemical characteristics – high Na, Fe³⁺ and ^WO and follow a trend between the “oxy-dravite” and povondraite species. Alternatively, this trend can be used as an indication for tur developed within low-temperature hypersaline and oxidizing environments. Tourmaline in some meta-evaporite deposits and in some economically important metal deposits exhibits this trend such that it is an additional indication of the importance of salinity. These complex chemical variations can be easily displayed, analyzed and visualized using exchange vectors.