Different crystallization events of Li-bearing pegmatitic tourmaline from San Piero in Campo, Elba Island, Italy


Ertl A1,2, Pezzotta F3, Hughes JM4, Meyer HP5, Ludwig T6, Dyar MD7, Rossman GR8



 
1 Naturhistorisches Museum, Mineralogisch-Petrographische Abt., Burgring 7, 1010 Wien, Austria
2 Universität Wien, Institut für Mineralogie und Kristallographie, Josef‑Holaubek‑Platz 2, 1090 Wien, Austria
3 MUM – Mineralogical Museum “Luigi Celleri”, Via Cavour 73, San Piero in Campo, Campo nell’Elba, 57034 Livorno, Italy
4 Department of Geology, University of Vermont, Burlington, Vermont 05405, U.S.A.
5 Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234-236, 69120 Heidelberg, Germany
6 Planetary Science Institute, 1700 East Fort Lowell, Tucson, Arizona 85719, U.S.A.
7 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125-2500, USA
 

Abstract

To learn more about the growth zones of distinctly zoned tourmalines from an aplite-pegmatite dyke near San Piero in Campo, Elba Island, four tourmaline crystals from three different small pockets and from the pegmatite were chemically characterized, including the light elements, and their lattice parameters were determined. Black Fe2+-rich tourmaline (grown directly within the pegmatite) with lattice parameters a = 15.97–15.98 Å, c = 7.15–7.16 Å, can be assigned to schorl and fluor-schorl with only 4% Fe3+. During crystallization the Fe content decreases, while Mn and F contents increase (up to 8 wt% MnO and 1.4 wt% F). These olive-green tourmaline zones exhibit lattice parameters of a = 15.87–15.94 Å, c = 7.12–7.14 Å and can be assigned to fluor-tsilaisite and Mn2+-rich fluor-elbaite. Later, the Mn content decreases while the Al and Li contents increase (up to 44 wt% Al2O3 and 1.7 wt% Li2O). These zones can have a Ga content of up to ~1100 ppm and a Pb content of up to ~500 ppm. The color of such tourmaline can be pale olive-green, pale pink or the samples are colourless. These tourmaline zones have lattice parameters of a = 15.78–15.82 Å, c = 7.08–7.10 Å and can be assigned to fluor-elbaite, elbaite, rossmanite, and darrellhenryite. In the final stage of tourmaline crystallization, the F content in the investigated crystal terminations can drop down to 0.1 wt% F, while the (Fe + Mn) content increases slightly. Usually these terminations can be assigned to elbaite (a = 15.79–15.83 Å, c = 7.08–7.10 Å). When a correlation is plotted between F content and X-site charge (Na, Ca), all zones grown in the pegmatite pocket, except for the termination, show a strong positive correlation. This is interpreted as an evidence that the terminations of the tourmaline crystals in the pegmatitic pockets grew during a different event than the previously grown crystals.