T. Adachi’s research while affiliated with Kyushu University and other places

We investigated the stability of Fe3+- and Fe2+-rich armalcolite at pressures 6–10 kbar and temperatures 900–1080 °C under the high oxygen fugacity. Armalcolite was synthesised in the bulk Mg/(Mg+Fetotal) of 0.2 from the mineral mixture of ilmenite, rutile and titanomagnetite (or ulvöspinel) within a platinum capsule at all experimental P–T conditions, though we could never find it in the bulk Mg/(Mg+Fetotal) of 0.1. The stability temperature of armalcolite significantly decreases with increasing oxygen fugacity and Mg/(Mg+Fetotal), indicating that armalcolite can be a new index mineral and an oxygen fugacity sensor of the ultrahigh-temperature metamorphism.