Chemical-structural-spectroscopic variability of a zoned tourmaline, Cruzeiro pegmatite, Brazil

Corso di Laurea Geologia di Esplorazione Università degli Studi di Roma “La Sapienza”

Anno Accademico 2012-2013.

Laureanda: Claudia Gori (attualmente iscritta al primo anno del 29°ciclo del Dottorato in Scienze della Terra presso l’Università degli Studi di Parma).
Relatore:Ferdinando Bosi, Dipartimento di Scienze della Terra-Università degli Studi di Roma “La Sapienza”.
Correlatore:Giovanni Battista Andreozzi, Dipartimento di Scienze della Terra-Università degli Studi di Roma “La Sapienza”.

Tourmaline is a mineral widespread in several geological (igneous, metamorphic and sedimentary) environments and this makes it a good petrogenetic indicator. In fact, it shows compositional and structural changes related to the conditions of P-T-X-fO2 in the environment of crystallization and interaction with the wall rock.
This thesis focuses on the study of the crystal chemistry of a zoned macrocrystal of tourmaline coming from the Cruzeiro pegmatite (Minas Gerais, Brazil). The purpose of this work was the identification of the chemical-structural relationshipswithin the macrocrystal. Results clarified the relationships between the crystal chemistry of the various zones of the macrocrystaland chemical-physical conditions that occurred at the time of theirformation.
The study focused on four representative samples coming from the same macrocrystal, which were studied using the techniques of electron microprobe (EMP), single-crystal X-ray diffraction (SREF), Mössbauer spectroscopy (MS) and optical absorption spectroscopy (OAS). Results obtained consistedin atomic frequencies, Fe2+ and Fe3+ quantification, bond lenghts, bond valence and UV-VIS-NIR absorption spectra. Through the convergence of all the information acquired (using a mathematical model based on the minimization of the differences between the observed and calculated parameters), were obtained intracrystalline cation distributions of the four samples and crystal chemical formulas were defined. In addition, the color variation of the samples (from green outer rim and blue inner rim to black core), was explained in terms of variable combinations of Fe2+ absorption, Fe2+-Ti4+ intervalence charge-transfer (IVCT) and Fe2+-Fe3+ IVCT.

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