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

Abstract

A zoned Mn 3+-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 Mn 3+-rich andalusite to kanonaite have strong pleochroism: X = yellow, Y = green, Z = deep green, when narrow Mn 3+-enriched rims feature slightly more intensive colors. Electron microprobe study shows that the core has always lower contents of Mn 2 O 3 relative to commonly narrow rim of kanonaite. Chemical composition varies from Mn 3+-rich andalusite with 0.21 apfu Mn 3+ (9.72 wt. % Mn 2 O 3) to kanonaite with up to 0.60 apfu Mn 3+ (26.09 wt. % Mn 2 O 3). Minor amount of Fe 2 O 3 from 0.07 to 0.14 apfu Fe 3+ apfu (6.12 wt. % Fe 2 O 3) is typical. In thin sections, subhedral grains of Mn 3+-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 Mn 3+-rich andalusite and kanonaite. In particular, the abundance of feldspars as well as the total absence of Al 2 SiO 5 minerals and the presence of some rare accessory minerals in Kojetice are different from the other localities of Mn 3+-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 Mn 3+-rich andalusite (kanonaite) + Na + + 3 quartz = spessartine + albite/paragonite, and 3 Mn 3+-rich andalusite (kanonaite) + K + + H 2 O = muscovite + 3 Mn. The newly formed spessartine requires a decrease in fO 2 , which is also indicated by the presence of accessory tephroite. This is in contrast to the other localities of Mn 3+-rich andalusite to kanonaite described to date, where increasing fO 2 during replacement of these minerals is typical.

Mn-Andalusite, Spessartine, Mn-Grossular, Piemontite and Mn-Zoisite/Clinozoisite From Trikorfo, Thassos Island, Greece

“…% (0.351 apfu). The Mn 3+ substitutes for Al 3+ in the formula of the mineral which corresponds to (Al,Mn)2SiO5 as also reported by Novák and Škoda (2007). The Mn-poor clinozoisite/epidote and Mn-poor zoisite from the Trikorfo area contain up to 0.9 wt.…”

Metamorphic and Metasomatic Kyanite-Bearing Mineral Assemblages of Thassos Island (Rhodope, Greece)

“…Orange kyanite is associated with green andalusite in quartz lenses (Figure 3f). Mn 3+ might substitute for Al 3+ in andalusite mineral formula, resulting in deep green, Mn-rich andalusite (Al, Mn) 2 SiO 5 [10,78]. This andalusite variety is named viridine in reason of its brilliant green color [12].…”

“…This andalusite variety is named viridine in reason of its brilliant green color [12]. Viridine occurrences (Germany, Belgium, Japan, Australia, Brazil and Tanzania) usually provide small or altered crystals [16,78]. Large viridine crystals of the Trikorfo area reach up to ten centimeters large, are vivid-to-deep green and unaltered.…”