Preliminary report of high-pressure and high-temperature synthesis of an Fe-rich armalcolite as a key mineral of the ultrahigh-temperature metamorphism

Abstract

Armalcolite, found on the Moon in Mare Tranquillitatis at Tranquility Base, on July 20, 1969 by the Apollo 11 mission and named after the Apollo 11 crew (ARMstrong, ALdrin, and COLlins), is stable at low pressure and high temperature in the low oxygen fugacity conditions and breaks down to ilmenite and rutile below 1000 C. Fe³⁺-rich armalcolite has been found from an ultrahigh-temperature metamorphosed quartzofeldspathic gneiss from Mt. Riiser-Larsen, Napier Complex, East Antarctica ​(​M​i​y​a​k​e​ ​H​o​k​a​d​a​ ​2​0​1​3​). We (2013) reported a possible armalcolite pseudomorph-bearing garnet-sillimanite gneiss from Skallevikshalsen, Lützow-Holm Complex, East Antarctica. These indicate armalcolite can be a new index mineral of the ultrahigh-temperature metamorphism. We present a preliminary report on the stablility of the Fe³⁺-rich armalcolite at the high-oxygen fugacity.