An experimental study on mineral layer formation at the coke-iron boundary during coke dissolution has been carried out. In previous articles by the authors the focus was on morphology of layer development and its effects on the kinetics of coke dissolution. In this article the formation of calcium sulphide at the coke-iron boundary is discussed. The compositions and temperatures used in the experimental set up were chosen to represent coke iron mass ratios of the lower zone of an iron blast furnace. SEM images and EDS analysis of the coke-iron interface revealed a mineral layer at the interface. This layer was found to be primarily calcium aluminate based that over time (0-120 minutes) became progressively enriched with calcium. The ratios of calcium and aluminium found in the mineral layer indicate that the layer is composed of alumina, CA6, CA2 and CA. A sulphide layer was also observed in these experiments after the establishment of a calcium rich calcium aluminate (CA2 and in particular CA). Thermodynamic analysis of the calcium aluminates formed during coke dissolution was carried out to establish the stability of in the experimental set up. From the results presented and the coke composition studied it can be concluded that formation of calcium rich calcium aluminates are a necessary precursor to formation.