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Fe-rich antigorite; a rock-forming mineral from low-temperature/high-pressure meta-ophicarbonates

Simonpietro Di Pierro, Chiara Groppo, Roberto Compagnoni, Giancarlo Capitani and Marcello Mellini
Fe-rich antigorite; a rock-forming mineral from low-temperature/high-pressure meta-ophicarbonates
European Journal of Mineralogy (June 2019) 31 (4): 775-784

Abstract

This study provides the first characterization of Fe-rich antigorite (FeO (sub tot) up to 12 wt%), a rock-forming mineral occurring in ophicarbonate rocks from different low-temperature/high-pressure meta-ophiolitic suites: Acceglio (Traversiera Valley, external Piemonte Zone, NW Italy), Macedonia and Verias (Thessaloniki and Vurinos-Kozani ophiolitic complexes, NE Greece), Tinos (Tinos Island, Cyclades Archipelago, Greece). Fe-rich antigorite has been characterized through optical and transmission electron microscopy (TEM), and its mineral chemistry has been investigated by means of wavelength-dispersive and TEM-based energy-dispersive spectrometry. In thin section, Fe-rich antigorite is characterized by a strong, peculiar pleochroism (alpha = green-dark green; gamma = bright-yellow-orange). It occurs in both mesh and bastite microstructures, and it is locally associated with relics of lizardite and/or chrysotile. The modulated lattice parameters of disordered Fe-rich antigorites have been determined by electron diffraction in the transmission electron microscope. The values are highly variable, even within each ophicarbonate sample. Verias dominantly has a superlattice parameter a clustering around 43.5 Aa (corresponding to the m = 17 polysome); Tinos and Macedonia have around 35.4 Aa (m = 14); Acceglio may even go down to 29 Aa (m = 12). Globally, the disorder features (i.e., reduced size of crystals, polysomatic faults, wobbling, misalignment among sublattice and superlattice reflections, etc.) increase from Macedonia to Verias, Tinos and Acceglio, respectively. The analyzed Fe-rich antigorites accommodate up to 12 wt% FeO (sub tot) , with X (sub Fe) values (X (sub Fe) = Fe (sub tot) /[Mg + Fe (sub tot) ]) in the range 0.10-0.16 for Macedonia, 0.05-0.17 for Acceglio, 0.10-0.12 for Tinos and 0.05-0.10 for Verias. The intensity of the pleochroism seems to be directly correlated with the Fe content, with the Fe-richer samples showing the deeper absorption colours. Mineral relationships and TEM observations suggest that Fe-rich antigorite replaces former mesh and bastite microstructures consisting of lizardite + or - chrysotile, only locally preserved as relict phases. The thermodynamic modelling approach (i.e., P/T-X(CO (sub 2) ) pseudosection) qualitatively shows that the stability of Fe-rich antigorite is compatible with low-temperature, high-pressure conditions (i.e., blueschist-facies metamorphic conditions), and is enhanced by the occurrence of CO (sub 2) in the fluid, consistent with the systematic occurrence of this mineral in meta-ophicarbonate rocks.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 31
Serial Issue: 4
Title: Fe-rich antigorite; a rock-forming mineral from low-temperature/high-pressure meta-ophicarbonates
Affiliation: University of Turin, Department of Earth Sciences, Turin, Italy
Pages: 775-784
Published: 20190611
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 58
Accession Number: 2019-063824
Categories: Mineralogy of silicatesIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
N37°40'29" - N37°40'29", E25°00'28" - E25°00'28"
N41°31'10" - N41°31'10", E22°12'07" - E22°12'07"
N44°28'51" - N44°28'51", E06°58'51" - E06°58'51"
Secondary Affiliation: University of Milano-Bicocca, ITA, ItalyUniversity of Siena, ITA, Italy
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201933
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