Séminaire - Granitic pegmatite evolution studied via tourmaline single-crystal chemical zoning - Giovanni Andreozzi

IMPMC - Sorbonne Université - 4 place Jussieu, 75005 Paris, tour 23, 4^e étage, couloir 22-23, salle 401

Jeudi 3 mai 2018 à 10 h 30

Giovanni Andreozzi - Sapienza, University of Rome

Abstract

The present study is focused on the comparison of crystal chemistry of a large color-zoned tourmaline crystal and tourmaline chemical evolution in the internally-zoned Cruzeiro pegmatite, Brazil. The tourmaline sample investigated here is a prismatic, multi-colour-zoned single crystal, ca. 8 cm large and 10 cm long, characterized by a large, homogeneous black core in contact with a narrow blue inner rim and a thicker outer green rim. Four crystal fragments were extracted and investigated by X-ray diffraction and structure refinement, electron microprobe analysis, Mössbauer spectroscopy and optical absorption spectroscopy. Tourmaline chemical variation from core to rim is defined in terms of site population at the Y site (Al, Fe, Li etc.) and at the O1 site (O, OH, F): the zoned crystal evolved from black schorl to blue and green fluor-elbaite, that is with concomitant reduction of Fe2+ and enrichment of (Al+Li) as well as of F due to increasing melt fractionation. Along this path, a very good negative correlation between F and Fe2+ was observed, nicely approximated by the linear trend F = 1 - 033Fe2+ (apfu), which drives tourmaline composition from schorl to fluor-elbaite.