Dark-coloured Mn-rich overgrowths in an elbaitic tourmaline crystal from the Rosina pegmatite, San Piero in Campo, Elba

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Marco E. Ciriotti
Messaggi: 31675
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
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Dark-coloured Mn-rich overgrowths in an elbaitic tourmaline crystal from the Rosina pegmatite, San Piero in Campo, Elba

Messaggio da Marco E. Ciriotti » lun 28 nov, 2022 12:14

Prossima pubblicazione.

Referenza:
▪ Altieri, A., Pezzotta, F., Skogby, H., Hålenius, U., Bosi, F. (2023): Dark-coloured Mn-rich overgrowths in an elbaitic tourmaline crystal from the Rosina pegmatite, San Piero in Campo, Elba Island, Italy: witness of late-stage opening of the geochemical system. Mineralogical Magazine, 87, (in press).

Abstract:
Multicoloured tourmalines from Elba Island quite frequently display dark-coloured terminations due to the incorporation of Fe, but also occasionally Mn, formed during the latest-stages of crystallisation in cavities. However, the mechanisms that led to the availability of these elements in the late-stage residual fluids are not yet completely clear. For this purpose, we studied a representative tourmaline crystal found naturally disrupted in two fragments within a wide miarolitic cavity in the Rosina pegmatite (San Piero in Campo, Elba Island, Italy), and characterised by late-stage dark-coloured overgrowths. Microstructural and paragenetic observations, as well as chemical and spectroscopic analyses (electron microprobe and optical absorption spectroscopy), provide evidence that the formation of the observed dark-coloured Mn-rich overgrowths is the result of a pocket rupture. This event allowed chemical alteration of the cavity-coating spessartine garnet by highly-reactive late-stage cavity fluids through leaching processes, with the subsequent release of Mn to the residual fluids. We argue that the two fragments were originally a single crystal, which underwent a natural breakage followed by the simultaneous growth of Mn-rich dark terminations at both breakage surfaces. This supports the pocket rupture event, responsible for both the shattering of the tourmaline crystal and the chemical variation of the cavity-fluids through the availability of Mn, which was incorporated by tourmaline crystals. Additionally, a comparison of the dark overgrowths formed at the analogous and the antilogous poles, can provide additional information on tourmaline crystallisation process at the two different poles. The antilogous pole is characterised by a higher affinity for Ca, F and Ti, and a selective uptake of Mn2+ even in the presence of considerable amount of Mn3+ in the system. This uneven uptake of Mn ions caused the yellow-orange colouration of the antilogous overgrowth (Mn2+ dependent) instead of the purplereddish colour displayed by the analogous overgrowths (Mn3+ dependent)
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 31675
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
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Re: Dark-coloured Mn-rich overgrowths in an elbaitic tourmaline crystal from the Rosina pegmatite, San Piero in Campo, E

Messaggio da Marco E. Ciriotti » sab 18 feb, 2023 11:00

Pubblicazione effettuata.

Referenza:
▪ Altieri, A., Pezzotta, F., Skogby, H., Hålenius, U., Bosi, F. (2023): Dark-coloured Mn-rich overgrowths in an elbaitic tourmaline crystal from the Rosina pegmatite, San Piero in Campo, Elba Island, Italy: witness of late-stage opening of the geochemical system. Mineralogical Magazine, 87, 130-142.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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