Blue growth zones caused by Fe²⁺ in tourmaline crystals from the San Piero in Campo gem-bearing pegmatites, Elba Island, Italy

Abstract

Two tourmaline crystals with a blue-growth zone at the analogous pole, respectively from the San Silvestro and the Fucili pegmatites, located in the San Piero in Campo village, Elba Island (Tyrrhenian Sea, Italy), have been described for the first time through chemical and spectroscopic analyses to define their crystal-chemical aspects and the causes of the colour. Chemical data obtained by electron microprobe analysis indicate that both tourmalines belong to the elbaite-fluorelbaite series. The upper part of each crystal is characterized by an increased amount of Fe (FeO up to ⁓1 wt%) and a Ti content below the detection limit. Optical absorption spectra recorded on the blue zone of both samples show absorption bands caused by spin-allowed d-d transitions in [6]coordinated Fe 2+ , and no IVCT Fe 2+ -Ti interactions, indicating that Fe 2+ is the only colour-causing agent. Mössbauer analysis performed on the blue zone of the Fucili sample confirmed the Fe 2+

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

“…Microstructural and paragenetic observations of the cavities in which tourmaline crystals with dark-coloured overgrowths were formed, provide evidence that such pockets are characterised by: (1) thin fractures penetrating from the cavity into the enclosing pegmatite; (2) leaching and corrosion of early-formed cavity-lining and cavity-coating Fe- and Mn-rich minerals crosscut by the fractures; (3) partial collapse of quartz and feldspar crystal aggregates; and (4) the breakage of some tourmaline crystals as a consequence of a partial pocket collapse (Bosi et al , 2022; Altieri et al , 2022). These features are particularly evident in the Rosina pegmatite from which the tourmaline crystals originated (Fig.…”

“…Additionally, Felch et al (2016) inferred that the observed corrosion and leaching of cavity-lining spessartine garnet crystals, was the result of reactions with late-stage hydrothermal fluids that originated and migrated from adjacent highly evolved miarolitic cavities. More recently, Bosi et al (2022) and Altieri et al (2022) proposed that the pocket rupture event, possibly related to thermal contraction during the cooling of the rock, led to mechanical brittle deformation of the enclosing pegmatite by the formation of late-stage fractures. This allowed the highly-reactive late-stage cavity fluids to permeate the fractures surrounding the cavity where the early-crystallised cavity-lining and cavity-coating Fe- and Mn-rich minerals were hosted.…”

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

“…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.…”

“…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.…”