Characterization of the blue-growth sectors in elbaite crystals from the San Piero in Campo pegmatites, Elba Island (Tyrrhenian Sea, Italy)

With their variety of pastel colors, tourmalines from Elba Island are easily recognizable and the main feature, which made them famous, is the dark-colored termination frequently occurring at the analogous pole. Such unusual termination includes multiple thin growth sectors of blackish, brownish, greenish, purplish, or even bluish colors. Tourmalines with blue hues are the rarest ones, making them always been sought after by mineral collectors from all over the world. However, this exceptional and unusual color variety has hindered studies concerning their crystal-chemical characteristics as well as the origin and chemical composition of fluids involved in their crystallization. Two tourmaline crystals with a blue-growth sector at the analogous pole, respectively from the San Silvestro and the Fucili pegmatites, located in the area of the San Piero in Campo village, have been described for the first time through electron microprobe (EMP) and optical absorption spectroscopy (OAS) analyses to define their crystal-chemical aspects and the causes of the color. Chemical data suggest that both tourmalines can be classified as elbaite, ideally Na(Li1.5Al1.5)Al6Si6O18(BO3)3(OH)3OH, and that the upper part of each crystal is characterized by an increased amount in Fe (FeO up to ∼1 wt.%). Spectra recorded on the blue-growth sector of both samples show two major absorption bands centered at ∼13800 cm-1 and ∼9200 cm-1 related to pure d-d transitions in [6]-coordinated Fe2+, and this suggests that Fe2+ is the main color-causing agent. The blue-growth sectors of tourmaline crystals recorded an increase in Fe2+ availability in the crystallization environment that can be explained in terms of partial opening of the geochemical system at the scale of the cavity. Microstructural observations of the cavities in which tourmalines were collected reveal that these pockets are associated with a series of micro-fractures, which crosscut also some early formed biotite crystals hosted in the surrounding solid pegmatitic rock and partially altered in white mica along the fractures and at the rim. In our genetic model, late-stage fracturing phenomenon allowed cavity fluids to infiltrate the surrounding solid pegmatite and locally react with biotite, with a subsequent release of Fe to the fluids, and the formation of a new generation of white mica. Thus, tourmaline, being an excellent petrogenetic indicator, registered such event with a bluish slightly Fe2+-rich growth sector.