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Genetic model for the color anomalies at the termination of pegmatitic gem tourmaline crystals from the island of Elba, Italy
Ist Teil von
European journal of mineralogy (Stuttgart), 2023-09, Vol.35 (5), p.755-771
Ort / Verlag
Göttingen: Copernicus GmbH
Erscheinungsjahr
2023
Quelle
EZB Electronic Journals Library
Beschreibungen/Notizen
Tourmaline crystals from the island of Elba commonly display a
sharp transition to dark colors at the analogous termination due to the
incorporation of Fe and/or Mn during the latest stages of crystallization
in pegmatites. The formation of such color anomalies is related to a
dramatic physicochemical change in the crystallization environment as a
consequence of an opening of the geochemical system. However, mechanisms
that may lead to the availability of Fe and/or Mn in the residual cavity fluids
have been unclear. On the basis of chemical and spectroscopic
investigations, combined with structural and paragenetic observations of the
cavities, we propose a general genetic model in which, as a consequence of a
pocket rupture event, chemical alteration of Fe- and Mn-rich
minerals that formed early in the pegmatitic rock surrounding the cavities occurred through
leaching processes, produced by the action of the highly reactive late-stage
cavity fluids. Such processes were responsible for the release of Fe and Mn
in the geochemical system, allowing the formation of the late-stage dark-colored terminations in the tourmaline crystals. In some cavities, a high
availability of Mn and/or Fe determined the evolution of the crystals from
an initial elbaite/fluor-elbaite composition to celleriite, foitite or
schorl. This compositional evolution trend can be described by the following general
chemical substitution: XNa+ + Y(Li1.5 + Al0.5)3+ + WF− ↔ X□ + 2Y(Fe,Mn)2+ + WOH−.