Blue-growth sectors in gem-tourmaline crystals: insights into their genesis

With their variety of pastel colors, tourmalines from Elba Island (Italy) 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, purplish, or even bluish colors. Tourmalines with blue hues are the rarest ones, making them always been sought after by mineral collectors. 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. A study, conducted on two tourmalines with a blue-growth sector at the analogous pole, revealed that the upper part of each crystal is characterized by an increased amount in Fe2+. The formation of such blue-growth sectors 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 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. Thus, tourmaline registered such event with a bluish slightly Fe2+-rich growth sector.