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Mn-tourmaline from island of Elba (Italy): Crystal chemistry

¹ Dipartimento di Scienze della Terra, Università degli Studi di Roma "La Sapienza", P.le A. Moro 5, 00185 Roma, Italy
² Dipartimento Geomineralogico, Università di Bari - Campus, via E. Orabona 4, 70125 Bari, Italy

Correspondence: ^* E-mail: ferdinando.bosi{at}uniroma1.it

The crystal chemistry of seven crystal fragments taken from differing regions of the same colorless to yellow-greenish tourmaline macro-crystal from pegmatite pockets in aplite veins (island of Elba, Italy) was studied with a multi-disciplinary (SREF, XRDT) and multi-analytical approach (EMPA, SIMS). EMPA and XRDT studies showed relationships between color and chemical zoning and crystal-growth evolution, indicating which fragments could be considered representative of the chemical evolution of the genetic micro-environment in which the crystal developed.

Results showed that the colorless fragment is an elbaite while the yellow-greenish crystal fragments are Mn²⁺-rich (up to 1.34 apfu) and belong to the alkali group and fluor subgroup. They are characterized by dehydroxylation and alkali-defect type substitutions that cooperate in reducing Li and increasing Mn contents. The Y site is populated by Al, Li, and Mn²⁺, and the Z site by Al and Mn²⁺ (up to 0.10 apfu). In contrast with data in the literature, Mn²⁺ populates both octahedral sites according to the order-disorder reaction: ^YMn + ^ZAl ^YAl + ^ZMn. As Mn²⁺ content increases, progressive disorder takes place. This disorder is quantitatively lower than that of the ^ZMg in dravite, due to the low structural tolerance of the small Z cavity in the incorporation of larger cations by the ^ZR^{2+ Z}Al substitution. Relationships of direct proportionality between lattice parameters and both <Y-O> and <Z-O> are observed. The expansion of both octahedra, as well as of lattice parameters, increases linearly as a function of ^YMn²⁺ and ^ZMn²⁺. The latter has greater weight in dictating unit-cell variations, due to the degree of size mismatch between ^ZMn^{2+ Z}Al and ^YMn^{2+ Y}Li substitutions, and the way in which the Z octahedra are articulated in the structure.

This article has been cited by other articles:

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