Definition of a genetic model for the dark-colored overgrowths in pegmatitic gem tourmaline crystals

Elba gem tourmalines are renowned for the delicate pastel colors and the perfection of their crystal shapes in small size. Characteristic for these tourmalines is the presence of color anomalies that occur as dark-colored terminations, frequently at the analogous pole that typically are rich in Fe and/or Mn. The formation of such dark-colored overgrowths is related to sudden physicochemical changes in the crystallization environment during the latest-stages of tourmaline crystal growth. However, the detailed events that led to the availability of Fe and/or Mn in the pocket environment, resulting in the growth of late-stage tourmalines, remained unclear. The goal of this Ph.D project is the definition of a genetic model for the dark-colored overgrowths in pegmatitic gem tourmaline crystals. Since tourmaline is an excellent petrogenetic indicator, the origin and chemical composition of fluids involved in tourmaline crystallization, responsible for such color anomalies, have also been investigated. To achieve this goal, selected gem tourmaline crystals with dark overgrowths of different colors and textural features from several miarolitic Li-bearing aplite-pegmatite veins, located in the eastern border of Monte Capanne monzogranite pluton (Elba Island, Italy), were studied. Chemical and spectroscopic investigations were applied as experimental approach in order to describe the crystal-chemical characteristics of the zones characterized by such color anomalies. This information, along with microstructural and paragenetic observations of the cavities in which the studied tourmaline samples were collected, were used to determine and propose a general genetic model for the color anomalies observed at the termination of Elba gem tourmaline crystals. This study was further extended to tourmaline samples collected from the Alto Ligonha pegmatite district (Nampula Province, NE Mozambique). Gem-quality tourmalines from the Alto Ligonha pegmatite district are among the most renowned for the combination of color and rarity. These features make them unique from a gemological viewpoint and therefore with a high commercial value, limiting the information regarding their crystal-chemical characteristics. To fill this gap, several tourmaline samples from the secondary deposit of Mavuco, in the eastern portion of the Alto Ligonha pegmatite district, were studied. These tourmaline samples were provided by ongoing mining projects, and were subjected to an in-depth analysis. Specifically, a detailed chemical characterization as well as a correlation study between compositional data and color displayed by the tourmaline crystals, were conducted. In addition, on selected samples with particular chromatic features, optical absorption spectroscopy analyses were performed. For all the tourmaline samples analyzed, the gemological variety, according to the color displayed, was identified and the relative color mechanisms were investigated. Tourmaline grains from the secondary deposit of Mavuco are also characterized by a dark-colored prismatic overgrowth. The genetic mechanisms responsible for these prismatic overgrowths have not been defined so far, as the pegmatitic source from which these tourmalines originated was still uncertain. This uncertainty was essentially due to the strong NYF (Niobium-Yttrium-Fluorine) geochemical affinity (Li- and B-poor composition) of the Alto Ligonha pegmatites located quite close the secondary deposit of Mavuco, as well as the rudimentary excavation techniques, which did not allow the exploration of new pegmatitic deposits in the Alto Ligonha district. To gain information about the processes that led to the formation of the prismatic dark-colored overgrowths, selected tourmaline crystals from a recently discovered pegmatitic field, the so-called “Marina” pegmatite (Mavuco area), were analyzed. The “Marina” pegmatite was hypothesized to be a source of the detrital tourmaline grains of the Mavuco area, given its proximity to the secondary deposit, and its LCT geochemical signature. Studies on tourmaline crystals from the “Marina” pegmatite were carried out to confirm the above hypothesis, through the comparison of the compositional data obtained from such tourmaline crystals with those obtained from the detrital tourmalines of the secondary deposit of Mavuco. The same studies allowed obtain detailed information on the genesis of the dark-colored prismatic overgrowths, which also characterize some of the detrital tourmalines. During the chemical characterization activity, two new mineral species of the tourmaline supergroup were discovered. Both of them were approved by the IMA-CNMNC (International Mineralogical Association-Commission on New Minerals, Nomenclature and Classification) and named celleriite, ideally □(Mn2+2Al)Al6(Si6O18)(BO3)3(OH)3(OH), and ferro-bosiite, ideally NaFe3+3(Al4Fe2+2)(Si6O18)(BO3)3(OH)3O. The results obtained in this PhD research improved the knowledge on the opening of geochemical systems in which tourmaline crystallized, and allowed the definition of a genetic model for the formation of the dark-colored overgrowths in tourmaline crystals. This model can be exported to explain late-stage color and composition anomalies of gem tourmaline crystals of many other gem-pegmatite deposits in the world.