Morphological and compositional changes of chromite as it descends in the black top of a platinum group metals smelter

•Partial reduction of Fe3+ to Fe2+ at chromite surface between 800 and 1100 °C.•Chromite dissolution in molten matte from 1000 °C and in liquid silicate from 1200 °C.•MgCr2O4-rich spinel precipitation in liquid silicate.•FeCr2O4-based spinel precipitation in matte droplets.•1.1 mass% Cr in PGM matte and 0.14 mass% CrO in liquid silicate at 1480 °C.