Effects of dry aging and heating on the structural characteristics and transformation of hexagonal turbostratic birnessite

Abstract

AbstractNatural birnessite‐like minerals are commonly enriched in various transition metals, such as iron (Fe). Though the fates of metals associated with birnessites during mineral transformation in aqueous conditions are thoroughly studied, we determined the Fe behaviors in Fe‐doped hexagonal turbostratic birnessites during mineral evolution in dry state at room temperature for 8 years and upon thermal treatments at temperatures ranging from 323 to 773 K, covering the temperatures in extreme environments such as wildfires. These Fe‐containing birnessites were very stable upon aging in the dry state. Upon thermal treatment, the birnessite sample with a small amount of Fe (≤2.8 wt.%) was transformed to cryptomelane at 573–673 K, while for the sample with ∼5.6 wt.% Fe, the transformation temperature increased to 673–773 K. This indicated that Fe adsorption enhanced the birnessite's thermal stability. Further, there was a linear relationship between the fraction of edge‐sharing Fe–Fe(Mn) pairs and temperature over 298–473 K. This implied the migration of Fe adsorbed on vacancies into birnessite layers and/or increased edge‐sharing of Fe around vacancies from adjacent layers during heating. The average manganese (Mn) valences in the Mn dioxides were almost constant with the increase in temperature when the layer structure was kept, but greatly increased when the tectomanganate was formed. These results provided deep insights into the mechanisms of Mn dioxide mineral transformation and the fates of associated metals under extreme conditions in terrestrial environments.