Temperature-Induced Phase Transitions of Vivianite: In Situ Analysis of a Redox-Driven Crystallization

We document a solid-state, temperature-dependent (25-700 °C), multistage redox transformation of crystalline ferrous iron phosphate, vivianite (Fe₃(PO₄)₂·8H₂O). Under anoxic conditions, vivianite breaks down at <i>T</i> > 250 °C into an anhydrous, amorphous intermediate Fe₃(PO₄)₂ phase, yet the bulk shape and morphology of the starting vivianite crystals were retained. This amorphous intermediate phase remained stable until <i>T</i> > 500 °C, after which a redox-dependent crystallization into two different minerals was observed. Under anoxic conditions, the amorphous ferrous intermediate (Fe₃(PO₄)₂) transformed into the crystalline ferrous phosphate (graftonite, (Fe²⁺)₃(PO₄)₂), while under oxic conditions it crystallized into a ferric phosphate (rodolicoite, Fe³⁺PO₄). Graftonite formation occurs via an exothermic molar enthalpy (Δ<i>H</i>_cryst) of -16.7 ± 0.2 kJ mol^-1. Rietveld refinements of the two crystalline endmembers (vivianite and graftonite) revealed a unit cell volume decrease of ∼3.1% during the transformation, which was observed by in situ electron microscopic observations as an overall shrinking of the initial vivianite crystals. Despite volume loss and bubble-like features, the original vivianite shape was preserved, indicating a solid-state pseudomorphic transformation. Ex situ XRD and TEM-EELS analyses confirmed the ferrous-to-ferric oxidation, forming rodolicoite, through changes in the Fe geometry and oxidation state.