The cause of spontaneous combustion of pyritic black shale has been studied by calorimetry and other techniques. The conditions required for ignition to occur are: 1. A critical mass of pyritic black shale._x000D_
2. Acidity. 3. Fine shale particle size._x000D_
4. A high ambient temperature. 5. The exclusion of air during the preparation of a calorimeter charge. The reaction is characterised by an initial incubation period during which an unknown phase is produced. The build up and oxidation of this phase results in a chain reaction culminating in ignition. The mechanism is believed to involve the interaction of pyrite, sulphuric acid, ferrous sulphate, or meta-stable oxysulphates, with ammonium nitrate. The recommended strategy for prevention of premature detonation is to neutralise the acids in the shale with quick and slow release alkaline materials.