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Dark-coloured Mn-ri...
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
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Altieri, Alessandra (author)
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Pezzotta, Federico (author)
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- Skogby, Henrik, 1956- (author)
- Naturhistoriska riksmuseet,Enheten för geovetenskap
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- Hålenius, Ulf (author)
- Naturhistoriska riksmuseet,Enheten för geovetenskap
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Bosi, Ferdinando (author)
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(creator_code:org_t)
- 2022-11-28
- 2023
- English.
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In: Mineralogical magazine. - : Mineralogical Society. - 0026-461X .- 1471-8022. ; 87:1, s. 130-142
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https://nrm.diva-por... (primary) (Raw object)
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https://urn.kb.se/re...
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https://doi.org/10.1...
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Abstract
Subject headings
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- Multicoloured tourmalines from Elba Island, commonly display dark-coloured terminations due to incorporation of Fe, and also occasionally Mn. The mechanisms which led to the availability of these elements in the late-stage residual fluids are not yet completely understood. For this purpose, we investigated a representative tourmaline crystal found naturally 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, together with compositional and spectroscopic data (electron microprobe and optical absorption spectroscopy), provide evidence which shows that the formation of the dark-coloured Mn-rich overgrowths are the result of a pocket rupture. This event caused alteration of the cavity-coating spessartine garnet by highly-reactive late-stage cavity fluids by 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 natural breakage followed by the simultaneous growth of Mn-rich dark terminations at both breakage surfaces. This conclusion supports the evidence for a pocket rupture event, responsible for both the shattering of the tourmaline crystal and the compositional variation of the cavity-fluids related to the availability of Mn, which was incorporated by the tourmaline crystals. Additionally, a comparison of the dark overgrowths formed at the analogous and the antilogous poles, provides information on tourmaline crystallisation 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 a considerable amount of Mn3+ in the system. This uneven uptake of Mn ions resulted in the yellow–orange colouration of the antilogous overgrowth (Mn2+ dependent) rather than the purple-reddish colour of the analogous overgrowths (Mn3+ dependent).
Subject headings
- NATURVETENSKAP -- Geovetenskap och miljövetenskap -- Geologi (hsv//swe)
- NATURAL SCIENCES -- Earth and Related Environmental Sciences -- Geology (hsv//eng)
- NATURVETENSKAP -- Geovetenskap och miljövetenskap -- Geokemi (hsv//swe)
- NATURAL SCIENCES -- Earth and Related Environmental Sciences -- Geochemistry (hsv//eng)
Keyword
- The changing Earth
- Den föränderliga jorden
Publication and Content Type
- ref (subject category)
- art (subject category)
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