Genetic reinterpretation of crystallographic intergrowths of jacobsite and hausmannite from natural assemblages

Abstract

Crystallographic intergrowths of jacobsite and hausmannite ("vredenburgite") occur in association with braunite in the Precambrian Sausar Group of rocks, India, that were metamorphosed under 600-700°C and P ~ 6 kb. Quartz, hematite, rhodochrosite and a later hausmannite may occasionally occur as minor associates. Detailed characterization of the intergrown phases reveals that hausmannite lamellae, oriented in 4 or 5 crystallographic directions in the jacobsite host, show a wide variation in thickness and tapered intersections at low angles. The lamellae may be locally deformed. Analytical data reveal that the composition of natural hausmannite and jacobsite in the intergrowths cannot be approximated within the system Fe₃O₄-Mn₃O₄, as has been conventionally done. These really belong to the Fe₂O₃-Mn₃O₄ subsystem. In the two phase intergrowths, hausmannite is depleted and the jacobsite is enriched in Fe in higher grade rocks. Mineral associations and petrographic considerations suggest that the jacobsite-hausmannite intergrowth originated through prograde decarbonation-oxidation reactions of a carbonatic precursor in an unbuffered X CO₂ situation, but f O₂ was held between hematite-magnetite and bixbyite-hausmannite buffers at the ambient physical conditions of metamorphism. Subsequent oxidation yielded a strong oxygenbuffering assemblage jacobsite, hausmannite, braunite, hematite and quartz. This study negates the commonly held idea that hausmannite jacobsite crystallographic intergrowth ("vredenburgite") originates through unmixing of a high gh temperature spinelss temperature spinel_ss during cooling.