Fibrous tourmaline from San Piero in Campo (Elba, Italy)

Abstract

An unusual variety of fibrous tourmaline, not previously described on the island of Elba, was studied using a multi-analytical approach, including powder X-ray diffraction, scanning electron microscopy, electron probe micro-analysis, Raman spectroscopy, and in situ B-isotopic analyses. The investigated sample comes from a miarolitic cavity of the Rosina granitic pegmatite outcropping in the San Piero in Campo area (Elba, Italy) and consists of three generations of tourmaline: a pale green prismatic tourmaline about 200 µm thick and 1 mm long (Tur I); a co-axial tight overgrowth of a dark fibrous cap at the analogous pole termination of the prismatic tourmaline (Tur II); and colourless acicular single crystals about 1 µm thick and 28 µm long (Tur III), which form radial aggregates on both sides of the prismatic crystal, mostly concentrated around the fibrous termination. The prismatic crystal has a large core of elbaite (Tur Ia) and a relatively thin rim, consistent with Ca-bearing fluor-elbaite (Tur Ib). The fibrous cap is made of an inner dark-coloured (Fe,Mg)-bearing darrellhenryite (Tur IIa) terminated by an outer black schorl prone to foitite (Tur IIb). The acicular single crystals Tur III have a complex composition corresponding, on average, to (Ca,F)-rich, (Fe,Mg)-bearing darrellhenryite. On top of the radial aggregates of acicular single crystals, there are globular terminations formed by rose-shaped lath-like particles, which are made of a mixture of layer silicates. The most likely scenario for the formation of the studied tourmaline assemblage involves the miarolitic cavity fracturing due to a mechanical shock, the subsequent circulation of the highly reactive cavity fluids, and the leaching of accessory biotite in the surrounding pegmatite, which account for the overgrowth of the fibrous cap of dark Tur IIa and black Tur IIb on top of the prismatic elbaite. Leaching of accessory fluorapatite provided late (Fe,Mg)-poor, (Ca,F)-rich fluids that determined the crystallization of the acicular Tur III. The late, highly reactive fluids also interacted with the prismatic tourmaline, locally modifying its rim composition and forming Tur Ib. Moreover, the leaching of local petalite provided Li, Al, and SiO2 that contributed to the crystallization of Tur Ib and Tur III, as well as the final rose-shaped lath-like particles.