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

<p>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: <span class="inline-formula">^X</span>Na<span class="inline-formula">⁺</span> <span class="inline-formula">+</span> <span class="inline-formula">^Y</span>(Li<span class="inline-formula">_1.5</span> <span class="inline-formula">+</span> Al<span class="inline-formula">_0.5</span>)<span class="inline-formula">³⁺</span> <span class="inline-formula">+</span> <span class="inline-formula">^W</span>F<span class="inline-formula">⁻</span> <span class="inline-formula">↔</span> <span class="inline-formula">^X□</span> <span class="inline-formula">+</span> 2<span class="inline-formula">^Y</span>(Fe,Mn)<span class="inline-formula">²⁺</span> <span class="inline-formula">+</span> <span class="inline-formula">^W</span>OH<span class="inline-formula">⁻</span>.</p>