Semi-coke is a solid waste material left after oil shale retorting in oil shale chemical industry. During more than 80 years of operation more than 110 million tonnes of semicoke have been deposited at retorting plants in Estonia but so far very little is known about the mineral composition of this waste. This paper presents new data on mineral composition of semi-coke and its spatial variation in waste heaps. The composition of semi-coke reflects the changes of mineral matter during the retorting process. The changes with the terrigenous and carbonate matter is almost negligible during the main phase of retorting, only dehydration and partial transformation of clay minerals and decomposition of sulfur compounds occur. At the final step of retorting (900-1000 degrees C) a slag-like material forms, which consists of amorphous and Ca-silicate phases. The deposition and subsequent hydration causes the change in the composition of semi-coke. The most notable change is the formation of ettringite. The composition of mineral matter in semi-coke waste heap is relatively uniform. The variations are probably due to both physical and chemical separation during deposition of the sediment and to different diagenetic/hydration processes.