SYNTHESIS AND PROPERTIES OF MN-BEARING YODERITE AND OF MN-BEARING KORNERUPINE AS BY-PRODUCT

Yoderite with compositions close to those of the natural purple variet y were synthesized from gels at high water pressures (15-16 kbar) and temperatures (650, 800-degrees-C) at the oxygen fugacities of the Mn2O 3/MnO2-buffer with yields up to 95%. Chemical formulae based on microp robe data and water analyses are .01Fe3+0.28Mn3+0.11)SIGMA=6.40Si3.80O 18.21(OH)1.79 and l5.77Fe3+0.36Mn3+0.04)SIGMA=6.17Si4O18.20(OH)1.80. M anganiferous, but iron-free yoderite with the formula 1.85(Al6.26Mn3+0 .10)SIGMA=6.36Si3.91O18.15(OH)1.85 was also obtained and proves that M n3+ alone may stabilize the yoderite structure, although this does not necessarily imply thermodynamic-stability. All these synthetic yoderi tes exhibit the typical purple color known from the natural mineral wi th pleochroism of dark blue parallel-to b to colorless perpendicular-t o b, which confirms the earlier spectroscopic conclusion that Mn is re sponsible for the purple color of yoderite. Compared to ferric iron, M n3+ is incorporated into yoderite in much smaller amounts, although th e maximum attained here (0.11 p.f.u.) is still below the 0.15 found in new analyses of natural yoderite from Tanzania. In some runs yoderite coexisted with kornerupine containing Mn and Fe as well and showing s pectacular pleochroism from dark green parallel-to b to light red para llel-to c. Relative to yoderite Mn is fractionated into kornerupine. T he analytical data suggest that most of the manganese is incorporated as Mn2+, although some Mn3+ may be the reason for the color. Coexistin g braunite contains high amounts of Mg and Al substituting for Mn2+ an d Mn3+, respectively. Garnet obtained from the Fe-free gel contains on ly Mn2+ and has the end member composition Pyrope79Spessartine21 despi te high oxygen fugacity.