KANONAITE-RICH VIRIDINES - FROM THE VENN-STAVELOT MASSIF, BELGIAN ARDENNES

In highly oxidized Mn- and Al-rich layers of the low-grade metamorphic metasediments north of Salmchâteau, Belgium, three types of viridine porphyroblasts can be distinguished: (a) single-phase porphyroblasts, (b) composite porphyroblasts with a core consisting of very fine-grained kaolinite, which is interpreted as a pseudomorph after andalusite, and a rim of viridine and, (c) composite porphyroblasts with a viridine core, a first mantle of kaolinite (as a pseudomorph after andalusite) and a second rim of viridine. These grains form an assemblage with muscovite, paragonite, Mg-chlorite, piemontite, braunite, hematite, quartz, rutile, and apatite. In the single-phase porphyroblasts the chemical composition of the core, (Mn0.513+Al0.44Fe0.043+)Al(O/SiO4), grades continuously into (Mn0.793+Al0.15Fe0.063+)Al(O/SiO4) of the rim. The outer zones of the analyzed grains are thus richer in kanonaite-component, MnAl(O/SiO4), than the original kanonaites described recently by Vrána et al. (1978) from the type locality in Zambia. Taking into consideration also other chemical data of viridines of the Salmchâteau locality there exists a complete solid solution series in the system Al2SiO5 (And)-MnAlSiO5 (Kan)-FeAlSiO5 at least from And61Kan37(FeAlSiO5)2 to And15Kan79(FeAl-SiO5)6 for the P-T conditions verified here (≦ 400 ° C/1-2 kb). The oxygen fugacity of the rocks is estimated to correspond to fO2 of the MnO2/ Mn2O3-buffer or to exceed it. Single-phase and composite porphyroblasts of the andalusite-type minerals contain S-shaped internal structures, which, in the marginal parts of the grains, are always oriented perpendicular to the schistosity planes. Together with the zoning of the composite grains these structures give clear evidence, that the andalusite formation is preceded by viridine growth in suitable rocks high in manganese. Thus, the viridines of the cores are formed under P-T conditions of less than 360-380 ° C/1-2 kb. The recurrent availability of Mn to form the kanonaite-rich rims in all three types of porphyroblasts may be explained by Mn release out of the recrystallizing hematites along the schistosity planes during decreasing temperatures. © 1979 Springer-Verlag.