The precipitation behaviour of nickel in silicon after rapid cooling (at about 103 Ks−1) from high temperatures (850-850°C) has been studied using high-resolution transmission electron microscopy. It is shown that (1) coherent platelets on Si{111} planes are formed, which consist of two {111} layers of NiSi2 and (2) the precipitate-matrix interface is built up by Si-Si bonds. It is proposed that these precipitates, in which all nickel atoms have a deficient coordination, are formed as a compromise to maximize the energy degradation rate. It is estimated that a major fraction of the total content is contained in these precipitates. Further annealing of samples quenched from 1050°C leads to a considerable increase in the particle thickness, leaving the diameter of the platelets almost unaffected. This behaviour is attributed to the reduction in interfacial energy. It is found that the precipitate orientation depends on the annealing temperature, that is, we obtain (1) the same orientation as the silicon matrix at 900°C, (2) a twin orientation with respect to the silicon matrix at 800°C and (3) both types of orientation at 500-500°C.