Chalcopyrite, chalcopyrite-rich ore, talnakhite concentrates, and synthetic Cu–Fe–S compositions were heated, in separate experiments, and quenched. Those heated above 550 °C produced well-formed lenticular lamellae of chalcopyrite arranged in three directions at right angles to one another and set in a darker matrix with distinctly different optical and X-ray properties and different composition. The composition and structure of the matrix depends upon the bulk composition of the starting materials.When heated, chalcopyrite exsolves 10 to 15% pyrite; the remaining phase on quenching yields a grid of near-stoichiometric chalcopyrite in a matrix of talnakhite of variable composition. The presence of pyrite in the starting material does not change the result. Heated mixtures of chalcopyrite, pyrrhotite, and pyrite yield a similar grid of chalcopyrite in a matrix of high-temperature cubanite solid solution.These exsolutions form on quenching, probably at the cubic/tetragonal inversion and are not equilibrium assemblages. Twinning was not observed in samples cooled through the inversion temperature, and, therefore, the fine twin lamellae sometimes observed in natural chalcopyrite may not necessarily have been formed at the cubic/tetragonal inversion.