Kornerupine-bearing gneiss from Inanakafy near Betroka, Madagascar

This paper presents mineralogical data for the constituents of a kornerupine-bearing gneiss from Inanakafy near Betroka, Madagascar. Chemical analyses are given of the phlogopite, kornerupine, orthopyroxene and cordierite occurring in the rock. The unit cell content of kornerupine is discussed. On a visit to Madagascar in 1967 the first two authors were privileged to obtain a couple of specimens of a highly metamorphic gneiss containing abundant kornerupine. The rock was said to come from Inanakafy near Betroka. The field association of the rock is not known to the authors. Because more recent data for the Madagascar kornerupine and for its mineral paragenesis are not available in literature, the Inanakafy rock was subjected to a mineralogical study. The results will be presented in this paper. The rock contains the following constituents listed in order of estimated abundance: phlogopite, kornerupine, orthopyroxene and cordierite, with tourmaline, plagioclase, spinel, sapphirine and zircon as accessories. Pure fractions or the four main constituents were prepared by hand picking, heavy liquids and magnetic separator. The results of the chemical analyses made on these constituents are summarized in Table 1. Phlogopite Black phlogopite represents the most abundant constituent of the rock. The flakes are mostly anhedral. Sometimes, however, prismatic faces and, very rarely, poorly developed pyramidal ones are to be seen. The flakes range up to 1 cm in diameter. The chemical composition is given in Table 1. Based on (O, OH, F) = 24, the unit cell content is as follows: Si 5.67, Al 2.80, Fe3+ 0.18, Fe2+ 0.32, Mg 5.02, Ti 0.11, Ca 0.18, Na 0.15, K 1.20, F 0.42, OH 3.14. The powder pattern is characteristic of a trioctahedral mica. Precession photographs were taken about the aand b-axes. The hkl reflections with k ^ 3» appear as streaks parallel to c*. These streaks contain intensity maxima consistent with the 37" polytype. Accordingly, the mica represents a somewhat disordered stacking sequence with dominating 3 7" or twinned 1M. 80 Oleg v. Knorring, T h. G. Sahama and Martti Lehtinen