Intercalation and dynamics of hydrated Fe²⁺ in the vermiculites from Santa Olalla and Ojén

Abstract

Although the intercalation of Fe³⁺ into layered phyllosicilicates—especially into smectites—attracted much attention in the past two decades, the information about Fe²⁺ loaded phyllosilicates is sparse. Here we present an investigation of the Fe²⁺ exchanged vermiculites from Santa Olalla and Ojén (Andalusia, Spain) by means of Mössbauer spectroscopy. The room temperature Mössbauer spectra are very similar to those of the starting compounds (Na forms) except for a decrease of the contribution of structural Fe³⁺ and a concomitant increase of the contribution of Fe²⁺ sites, indicating an internal redox process. The extent of this redox reaction is different for the two vermiculites. Thus, the intercalated Fe²⁺ acts as an electron mediator from the external medium to the structural Fe³⁺ ions. A new component attributable to intercalated Fe²⁺ is practically invisible in the room temperature Mössbauer spectra, but increases strongly and continuously during cooling to 4.2 K, where it is the dominant feature of the Mössbauer patterns. At 4.2 K, its quadruple splitting amounts to 3.31 mm/s, which is in excellent agreement with the quadrupole slitting of Fe²⁺ coordinated to six water molecules in a highly symmetric octahedral arrangement. The strong decrease of the Mössbauer–Lamb factor of this component with increasing temperature indicates a weak bonding of the Fe²⁺ in the interlayer space.