Tourmaline as a petrogenetic indicator highlighted in a multicoloured crystal from the gem deposit of Mavuco (Alto Ligoña pegmatite district, NE Mozambique)

A rounded fragment of a multicoloured tourmaline crystal of sizes up to 2.5 cm, collected from the secondary gem deposit of Mavuco (Alto Ligoña pegmatite district, Mozambique), has been investigated using a multi-analytical approach, with the goal of reconstructing its growth history. The sample represents a core-to-rim section, perpendicular to the c-axis, of a crystal characterised by a variety of colours. The colours change from a black core to an intermediate zone with a series of colours such as yellow, blue-green, and purple, to a final dark-green prismatic overgrowth. This change is the result of a wide variation in Fe, Mn, Ti, and Cu concentrations and their redox state. The black core is characterised by an enrichment in Fe and Mn, with iron present in its divalent state. The yellow zone shows a progressive depletion in Fe and its coloration is caused by Mn2+ and Mn2+-Ti4+ IVCT interactions. The progressive decrease in Mn coupled with the absence of Ti, and the lack of Fe, allow Cu2+ to act as the only chromophore in the pale blue-green zone. The dominant colour-causing agent of the purplish zone is Mn3+, denoting a change in redox environment; however, despite the amount of Cu remains significant, its chromophore effect is obscured by Mn3+. The dark-green prismatic overgrowth, characterized by a sharp increase in Fe, Mn and also Ca, is interpreted as a late-stage partial re-opening of the geochemical system. This occurrence could potentially be related to mechanical instability of the cavity in which the crystal grew.