Structural and Chemical Transformations of Isocubanite CuFe₂S₃ Upon Cooling from the Melting Point

A specific regime of continuous slow cooling of melted isocubanite CuFe₂S₃ from 1020 °C to 638 °C is realized, and then the crystallized ingot is quenched. This regime ensures both equilibrium crystallization of primary phases and their subsequent non-equilibrium solid-phase transformations. A large series of samples cut along a 90 mm long ingot are studied by optical and electron microscopy, powder XRD, and the stoichiographic differential dissolution method. Phase compositions are determined by the electron microprobe analysis. It is established for the first time that isocubanite releases cubic chalcopyrite rather than tetragonal one upon cooling in the following order: first cationic ordering of the isocubanite matrix and then steady metastable coherent pre-precipitations of cubic chalcopyrite in the matrix to finally transform into polycrystals free from the matrix.