Mn3+-rich andalusite to kanonaite and their breakdown products from metamanganolite at Kojetice near Třebíč, the Moldanubian Zone, Czech Republic
A zoned Mn3+-rich andalusite to kanonaite occurs as shortly prismatic, black-green crystals, up to 2 mm in size, randomly distributed in quartz matrix of quartzitic rock with metamanganolite portions from Kojetice near Třebíč. Minor spessartine, braunite, K-feldspar, albite, hyalophane, rare paragonite and accessory magnetite-rutile intergrowths, tephroite, chernovite-(Y), monazite-(Ce), pyrophanite, Sb-rich rutile, cobaltite, pseudobrookite(?) and pale pink flakes of late muscovite were identified in this rock. Zoned subhedral grains of Mn3+-rich andalusite to kanonaite have strong pleochroism: X = yellow, Y = green, Z = deep green, when narrow Mn3+-enriched rims feature slightly more intensive colors. Electron microprobe study shows that the core has always lower contents of Mn2O3 relative to commonly narrow rim of kanonaite. Chemical composition varies from Mn3+-rich andalusite with 0.21 apfu Mn3+ (9.72 wt. % Mn2O3) to kanonaite with up to 0.60 apfu Mn3+ (26.09 wt. % Mn2O3). Minor amount of Fe2O3 from 0.07 to 0.14 apfu Fe3+ apfu (6.12 wt. % Fe2O3) is typical. In thin sections, subhedral grains of Mn3+-rich andalusite to kanonaite are enclosed in massive quartz and locally overgrown or replaced by spessartine plus albite/paragonite and later replaced by muscovite. No difference in degree of muscovitization was observed between Mn3+-rich andalusite and kanonaite. In particular, the abundance of feldspars as well as the total absence of Al2SiO5 minerals and the presence of some rare accessory minerals in Kojetice are different from the other localities of Mn3+-rich andalusite and kanonaite. The presence of K-feldspar may suggest higher temperature relative to the other mostly low- to medium-grade kanonaite-bearing metamorphic rocks. The following mineral reactions are indicated from textural relations: 3 Mn3+-rich andalusite (kanonaite) + Na+ + 3 quartz = spessartine + albite/paragonite, and 3 Mn3+-rich andalusite (kanonaite) + K+ + H2O = muscovite + 3 Mn. The newly formed spessartine requires a decrease in fO2, which is also indicated by the presence of accessory tephroite. This is in contrast to the other localities of Mn3+-rich andalusite to kanonaite described to date, where increasing fO2 during replacement of these minerals is typical.
IF (ISI, 2020): 1.525
5 YEAR IF (ISI, 2020): 1.446
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