Kornerupine in Slyudites From the Usmun River Basin in the Aldan Shield. Part Ii. Chemistry of the Minerals, Mineral Reactions

Kornerupine is characterized by a relatively high content of B2O3 – 3.07 – 4.02 wt.%, moderate content of F – 0.26–0.69 wt.% and has an XMg (= atomic Mg/(Mg + Fe)) ranging from 0.73 to 0.98. Sapphirine contains 0.1–0.85 wt.% B2O3. Enstatite is highly magnesian (XMg = 0.97), contains 3.75–4.08 wt.% Al2O3. Phlogopite occurs in two generations; F content and XMg of both are similar, but phlogopite of the second generation is poor in Ti. Kornerupine formed in the slyudites either by reactions involving melt such as: tourmaline + + sapphirine + phlogopite + cordierite = kornerupine + melt, or as a result of tourmaline breakdown in the absence of melt, that is, sapphirine + enstatite + tourmaline = kornerupine ± cordierite. These reactions occurred during the progressive stage of metamorphism at T = 800–900°C, P = 6 kbar; the first in the iron-bearing slyudites in the absence of fluid, and the second in the iron-poor slyudites.