Tourmaline petrogenetic indicator highlighted in a multicolored crystal from the Mavuco area (Alto Ligoña pegmatite district, NE Mozambique)

Tourmaline is well known to be an efficient geological tool for investigating P–T–X conditions in all crustal settings within the Earth given its ability to register and preserve the chemical composition and the redox conditions of the environment in which it crystallized (Dutrow & Henry, 2011). These features are well highlighted in a tourmaline grain collected from the secondary deposit of Mavuco in the Alto Ligoña pegmatite district (NE Mozambique). The marked polychromism from core-to-rim that characterize this sample, suggested studying it to reconstruct its growth history. Electron microprobe analyses of the sample revealed a wide variation in FeO- and MnO-concentrations, resulting in marked changes in color from black, yellow, blue-green, pink-red in the core zone, to dark-green at the prismatic overgrowth. The black inner core is characterized by a peculiar enrichment in Fe and Mn (FeO ∼5 wt.% and MnO ∼4 wt.%), as result of the availability of such elements in the pegmatitic melt. Iron is exclusively present in the divalent state, as confirmed by Mössbauer analysis. The yellow core zone shows an increase in MnO, reaching values up to 7 wt.%, and a simultaneous decrease in Fe (FeO < 1 wt.%). The increased incorporation of Mn could be promoted by the depletion of Fe in the pegmatitic melt, reflecting the competition of both elements for the Y-site in the tourmaline structure. The yellowish coloration is caused by an intervalence charge transfer Mn2+- Ti4+ interaction and to a minor extent by Mn2+ spin-forbidden transition, as revealed by OAS analysis. A drop in the concentration of Mn2+ occurs in the blue-green core zone, reaching values too low to contribute to the color (MnO < 2 wt.%), as Mn2+ is a weak chromophore. In the absence of FeO in this zone, the bluish coloration is mainly due to a slight increase in CuO, which contributes through Cu2+ spin allowed transitions. In the pinkish core zone, the lacking of FeO and the presence of a strong broad band at 19000 cm-1 indicates that Mn3+ is the main color-causing agent as the result of a change in redox environment. However, the oxidation of Mn2+ to Mn3+ is not related to an opening of the geochemical system, as supported by the lack of sudden change in the chemical composition in the tourmaline crystal. The oxidation can be instead explained by the presence of a natural radiation source in the proximity of the pocket, which effects became evident when Mn content reached a low level (MnO < 1 wt.%). The dark-green prismatic overgrowth is characterized by a sharp increase in Fe and Mn, as a consequence of partial destabilization of the pocket environment (Altieri et al., 2022; 2023). A pocket rupture event, followed by leaching and corrosion processes of the early crystallized Fe- and Mn-rich minerals, allowed the release of Fe and Mn in the environment and their incorporation in the overgrowth. Moreover, Mössbauer analysis revealed a Fe3+/Fetot-ratio of ∼ 4%, confirming mild oxidizing conditions.