Genetic model for the color anomalies at the termination of pegmatitic gem tourmaline crystals from the island of Elba

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Marco E. Ciriotti
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Genetic model for the color anomalies at the termination of pegmatitic gem tourmaline crystals from the island of Elba

Messaggio da Marco E. Ciriotti » ven 15 set, 2023 15:21

Referenza:
▪ Altieri, A., Pezzotta, F., Andreozzi, G.B., Skogby, H., Bosi, F. (2023): Genetic model for the color anomalies at the termination of pegmatitic gem tourmaline crystals from the island of Elba, Italy. European Journal of Mineralogy, 35, 755–771.

Abstract:
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−.

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Marco E. Ciriotti

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