Dark-coloured Mn-rich overgrowths in an elbaitic tourmaline crystal from the Rosina pegmatite, San Piero in Campo, Elba Island, Italy: witness of late-stage opening of the geochemical system

Abstract

Multicoloured tourmalines from Elba Island quite frequently display dark-coloured terminations due to the incorporation of Fe, but also occasionally Mn, formed during the latest-stages of crystallisation in cavities. However, the mechanisms that led to the availability of these elements in the late-stage residual fluids are not yet completely clear. For this purpose, we studied a representative tourmaline crystal found naturally disrupted in two fragments within a wide miarolitic cavity in the Rosina pegmatite (San Piero in Campo, Elba Island, Italy), and characterised by late-stage dark-coloured overgrowths. Microstructural and paragenetic observations, as well as chemical and spectroscopic analyses (electron microprobe and optical absorption spectroscopy), provide evidence that the formation of the observed dark-coloured Mn-rich overgrowths is the https://doi.org/10.1180/mgm.2022.125 Published online by Cambridge University Press result of a pocket rupture. This event allowed chemical alteration of the cavity-coating spessartine garnet by highly-reactive late-stage cavity fluids through leaching processes, with the subsequent release of Mn to the residual fluids. We argue that the two fragments were originally a single crystal, which underwent a natural breakage followed by the simultaneous growth of Mn-rich dark terminations at both breakage surfaces. This supports the pocket rupture event, responsible for both the shattering of the tourmaline crystal and the chemical variation of the cavity-fluids through the availability of Mn, which was incorporated by tourmaline crystals. Additionally, a comparison of the dark overgrowths formed at the analogous and the antilogous poles, can provide additional information on tourmaline crystallisation process at the two different poles. The antilogous pole is characterised by a higher affinity for Ca, F and Ti, and a selective uptake of Mn 2+ even in the presence of considerable amount of Mn 3+ in the system. This uneven uptake of Mn ions caused the yellow-orange colouration of the antilogous overgrowth (Mn 2+ dependent) instead of the purplereddish colour displayed by the analogous overgrowths (Mn 3+ dependent).

Genetic model for the color anomalies at the termination of pegmatitic gem tourmaline crystals from the island of Elba, Italy

“…1). Although the samples "Rosina 2" and "Violet Cap" were characterized in previously studies (Bosi et al, 2022;Altieri et al, 2023), they are included in this work because they are representative of two typical color and texture types.…”

“…Tourmalines are also distinguished by the dominant W anion into hydroxy, fluor and oxy species (Henry et al, 2011). In particular, the occupancy of the X and Y site is useful to reconstruct the chemical history of the fluids involved in tourmaline crystallization (van Hinsberg et al, 2011a, b;Dutrow and Henry, 2018;Bosi et al, 2022;Altieri et al, 2022Altieri et al, , 2023.…”

“…Furthermore, all intermediate morphological cases may occur. However, in some pegmatitic veins the tourmaline crystal terminations are not simply black, but they are made up of multiple thin growth zones of bluish, greenish, brownish or even purplish-to-reddish colors (Pezzotta, 2021;Bosi et al, 2022;Altieri et al, 2022Altieri et al, , 2023. These dark-colored terminations can be generally described as color anomalies.…”

“…Additionally, Felch et al (2016) observed corrosion and leaching of cavity-lining spessartine garnet crystals as the result of reactions with late-stage hydrothermal fluids originating from adjacent, highly evolved miarolitic cavities. More recently, Altieri et al (2023) provided clear evidence that in the pegmatites of the island of Elba the chemical changes responsible for the late-stage tourmaline generation are the result of an opening of the geochemical system related to a pocket rupture event, and they proposed a genetic model.…”

“…Each crystal was selected to represent a dark overgrowth with a different color and crystal texture. Chemical and spectroscopic investigations on such tourmaline crystals, combined with structural and paragenetic information of the cavities, are used to refine the general genetic model for the color anomalies of island of Elba tourmaline crystals previously proposed by Bosi et al (2022) and Altieri et al (2022Altieri et al ( , 2023.…”