Related Experiment Video
Updated: Sep 9, 2025

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets
Published on: May 15, 2017
Temperature-Induced Phase Transitions of Vivianite: In Situ Analysis of a Redox-Driven Crystallization
Alice Paskin1,2, Thaïs Couasnon1, Roberts Blukis1,3
1GFZ Helmholtz Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany.
Abstract:
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 T > 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 T > 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 (ΔHcryst) 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.
Related Concept Videos
Phase Transitions: Melting and Freezing
Phase Transitions: Sublimation and Deposition
Phase Transitions: Vaporization and Condensation
Recrystallization: Solid–Solution Equilibria
Crystal Growth: Principles of Crystallization
Initiating crystallization involves manipulating the concentration of the solute and the temperature of the solution. Since crystal growth occurs when the ratio of concentration and solubility of the solute in the solvent...
Phase Transitions

