Ilvaite: A study of temperature dependent electron delocalization by the mössbauer effect

The mixed valence iron silicate ilvaite, CaFe ₂ ²⁺ Fe³⁺Si₂O₈(OH), displays electron delocalization associated with Fe²⁺→Fe³⁺ charge transfer as observed by Mössbauer spectroscopy. Previous studies report the observation of an ‘electron hopping phenomenon’ with resolution of discrete valence states below 320 K. Mössbauer spectra of a suite of naturally occurring ilvaites were recorded over a temperature range, 80 K to 575 K. Five quadrupole doublets were resolved by computer fitting and assigned to Fe²⁺(A), Fe²⁺(B), Fe³⁺(A), and Fe²⁺(A)→Fe³⁺(A)‖c and ⊥c. Contrary to prior work, doublets associated with electron delocalization are resolved at 80 K and preclude the use of a Verwey-type order-disorder model. We propose a thermal activation model and discuss its criteria from molecular orbital and mineralogical viewpoints.