Elba tourmalines are renowned for the beauty and elegance of their pastel colors. Characteristic for these tourmalines are their dark-colored terminations at the analogous pole typically rich in Fe and/or Mn. Tourmaline crystals are excellent petrogenetic indicators due to the ability to register chemical-physical variations of the crystallization environment during their growth. Consequently, dark-colored terminations likely reflect processes that occurred in tourmaline-rich pegmatites during the latest stages of crystallization. However, the mechanisms that led to the availability of Fe and/or Mn in the crystallizing fluids are not yet completely clear. For this purpose, two tourmaline crystal fragments characterized by dark-colored terminations, found within a wide miarolitic cavity of the Rosina pegmatite, located in the area of the San Piero in Campo village, have been studied. Electron microprobe (EMP) and optical absorption spectroscopy (OAS) data suggest that these fragments were originally a whole crystal that underwent a breakage event followed by the simultaneous growth of Mn-rich dark terminations. We assume that a pocket rupture destabilized the crystallization environment within the pocket itself and was responsible for both the crystal to break and the chemical alteration of early-crystallized minerals, such as spessartine. This led to a release of Mn in the pegmatitic melt allowing the formation of the observed dark colored Mn-rich terminations. Thus, these tourmaline crystal fragments and their dark overgrowths represent the memory of the events that led to the mechanical pocket rupture and the subsequent compositional variation of the geochemical system. Additionally, a comparison of the two dark overgrowths formed on both of the broken surfaces, can provide further information on the different growth processes at the antilogous and the analogous poles.
Dark colored Mn-rich terminations in a tourmaline crystal from Elba Island (Tyrrhenian Sea, Italy): memory of the late-stage opening of the geochemical system / Altieri, Alessandra.; Pezzotta, Federico; Skogby, Henrik; Hålenius, Ulf; Bosi, Ferdinando.. - (2022). (Intervento presentato al convegno Congresso Congiunto SGI-SIMP 2022 "Geosciences for a sustainable future" tenutosi a Turin; Italy).
Dark colored Mn-rich terminations in a tourmaline crystal from Elba Island (Tyrrhenian Sea, Italy): memory of the late-stage opening of the geochemical system
Altieri Alessandra.
;Bosi Ferdinando.
2022
Abstract
Elba tourmalines are renowned for the beauty and elegance of their pastel colors. Characteristic for these tourmalines are their dark-colored terminations at the analogous pole typically rich in Fe and/or Mn. Tourmaline crystals are excellent petrogenetic indicators due to the ability to register chemical-physical variations of the crystallization environment during their growth. Consequently, dark-colored terminations likely reflect processes that occurred in tourmaline-rich pegmatites during the latest stages of crystallization. However, the mechanisms that led to the availability of Fe and/or Mn in the crystallizing fluids are not yet completely clear. For this purpose, two tourmaline crystal fragments characterized by dark-colored terminations, found within a wide miarolitic cavity of the Rosina pegmatite, located in the area of the San Piero in Campo village, have been studied. Electron microprobe (EMP) and optical absorption spectroscopy (OAS) data suggest that these fragments were originally a whole crystal that underwent a breakage event followed by the simultaneous growth of Mn-rich dark terminations. We assume that a pocket rupture destabilized the crystallization environment within the pocket itself and was responsible for both the crystal to break and the chemical alteration of early-crystallized minerals, such as spessartine. This led to a release of Mn in the pegmatitic melt allowing the formation of the observed dark colored Mn-rich terminations. Thus, these tourmaline crystal fragments and their dark overgrowths represent the memory of the events that led to the mechanical pocket rupture and the subsequent compositional variation of the geochemical system. Additionally, a comparison of the two dark overgrowths formed on both of the broken surfaces, can provide further information on the different growth processes at the antilogous and the analogous poles.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
