Understanding genesis and color origin of the very rare leek-green tourmaline variety

Tourmalines from Elba Island are easily recognizable and distinguishable from those of all other world localities for their delicate pastel colors and the perfection of their crystal shape, even in small sizes. Some colors displayed by tourmalines were recorded for the first time in crystals from Elba Island. Among these, a peculiar color is the “leek-green”. The term leek-green was firstly used by the Italian naturalist Targioni Tozzetti in 1825 for describing tourmaline crystals characterized by an unusual bright acid green-yellow coloration. In order to define the color mechanisms and the characteristics of the fluids involved in their crystallization, two tourmaline crystals, with a vivid greenish-yellow color, occurring in the Catri and the Cechi pegmatitic veins from Sant’Ilario and San Piero in Campo (Elba Island, Italy), respectively, were studied with a multianalytical approach. This approach includes electron microprobe analysis (EMP), spectroscopic investigations (OAS and FTIR) and single-crystal XDR diffraction. Experimental data reveal that both tourmalines can be classified as fluor-tsilaisite, with MnO up to 9 wt.%. Spectra recorded on both samples show absorption bands caused by Mn2+ spin-forbidden transition (∼24000 cm-1) and intervalence charge transfer Mn2+-Ti4+ interaction (∼31000 cm-1), with TiO2 up to 0.40 wt.%, indicating that the leek-green coloration is due to Mn2+ and Ti4+ without the influence of Fe2+/3+ (FeO ≤ 0.02 wt.%). The rarity of the leek-green color in the crystals is linked to specific conditions of formation: the original pegmatite-forming melt must be enriched in Mn and, at the same time, relatively depleted in Fe during tourmaline crystallization in gem pockets. Such conditions are favored by a B-rich peraluminous melt that can remove Fe as early-crystallized schorl (e.g., Laurs et al., 2007). Furthermore, Mn must be preserved when highly evolved Li-rich tourmaline starts to crystallize; this can be only achieved when spessartine garnet, the main competitor for Mn in Elba pegmatites, is absent or has crystallized during the very early stages of pegmatite crystallization (Altieri et al., 2023). In the Catri and the Cechi veins, paragenetic observations revealed that the early-stage of cavity evolution is marked by the crystallization of abundant primary schorl and a lack of spessartine. This scenario supports a high enrichment in B in the initial pegmatitic melt, which promoted schorl crystallization rather than the formation of micas or garnet (e.g., Dingwell et al., 1996). In addition, the scarce presence of biotite aids in conserving Ti until the late stages of pegmatite evolution, thus producing the leek-green color in Elba tourmalines via Mn-Ti intervalence charge transfer. The presence of Mn only in the divalent state suggests that reducing conditions are required for the formation of such colored tourmalines.