In one of the December 1998 issues of Nature the seismologists Ho Kwang Mao, Russell Hemley and colleagues from the Geophysical Laboratory of The Carnegie Institution of Washington, D.C. published an account of researches into the properties of the Earth's core.

They were able to "reconnoitre" the planet's depths by using a new technique based on X-ray diffraction and ultrasonics. In the course of the experiments, the scientists discovered that the Earth's core "responded" to signals in the same way as strongly compressed iron.

The centre of the Earth is located at depths between 890 and 6,370 kilometres and subject to pressures of between 1.3 and 3.6 million atmospheres. The immense iron sphere is surrounded by a liquid mantle and has some curious properties. For example, sound waves crossing the core from East to West travel far slower than those passing from North to South. This characteristic is called seismic anisotropy

The iron making up the core is close to its melting point and densely "packed" in crystal hexagons. Hence the resonance with the hexagonal structure of quartz.

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