Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Milan Novák, Radek Škoda

Mn³⁺-rich andalusite to kanonaite and their breakdown products from metamanganolite at Kojetice near Třebíč, the Moldanubian Zone, Czech Republic

Journal of Geosciences, volume 52 (2007), issue 1-2, 161 - 167

DOI: http://doi.org/10.3190/jgeosci.003

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.