Insights into the goethite to magnetite transformation pathway

Abstract:

Goethite is a widespread Fe-oxyhydroxide phase found in soils, dust, and sediment that is known to undergo reductive alteration during heating in the presence of organic matter. Dehyroxylation of goethite to hematite at temperatures as low as 150°C produces poorly crystalline, psuedomorphic hematite that may become rapidly recrystallized and converted to magnetite or maghemite in a reducing environment. New experiments were conducted to understand the influence of goethite grain size and crystallinity and the concentration of organic matter on the production of remanence-bearing phases during alteration. Pyrogenic alteration and deep burial metamorphic processes in organic-rich soils and sediments were simulated in a series of heating experiments to observe the evolution of secondary (chemical) remanence with increasing alteration temperatures. The implications for long-term goethite stability at ambient temperatures in soils and sediments will also be discussed in the context of the solid-state reaction kinetics and the influence of non-solid-state processes.