Distinguished for his work on the constitution of the earth's mantle and on the early chemical evolution of the earth, meteorites and the inner planets. He showed experimentally that olivines and pyroxenes transform to spinel and ilmenite structures at high pressures, and produced a model of the mantle soundly based on experiment and which accounts for the seismic observations. He has investigated the chemical processes involved in the formation of planetary bodies by accretion from a gas-dust cloud and emphasised the importance of oxidation­reduction equilibria in them. This led to his prediction of the occurrence of considerable metallic silicon in the Earth's core and to the suggestion that a body formed in this way would pass through a molten stage.