Crystal structure and compressibility of zircon at high pressure

Unit-cell and crystal-structure parameters of zircon (ZrSi0₄, tetragonal I4₁/amd, Z = 4) have been determined at eight pressures from 1 bar to 48.1 kbar. Volume compressibility is 4.41 ± 0.04 X 10⁻⁴ kbar⁻¹ (bulk modulus 2.27 ± 0.02 Mbar), and linear compressibilities are β_∥ = 0.95 × 10⁻⁴ and β_⊥ = 1.6 × 10⁻⁴ kbar⁻¹. Zircon is less compressible than any other measured compound with tetrahedrally-coordinated silicon.

Projections of tetrahedral Si-O bond distances to high pressure suggest that ^IVSi to ^VISi transformations occur when the tetrahedral distance compresses to approximately 1.59A, on the basis of known transformation pressures, Si-0 bond compressibilities, and room-pressure structures of several silicates. For silicates with room-pressure Si-0 distances ranging from 1.61 to 1.655A, predicted transformations to 6-coordinated silicon structures will occur between approximately 100 and 300 kbar at 23°C. These transformation pressures are relatively insensitive to temperature (dP/dT ≤ 0.01 kbar/°C), and ^lvSi to ^VISi transitions are therefore predicted to occur primarily in the transition zone of high-density gradient of the earth s mantle. ZrSi0₄ is predicted to transform to a dense phase with octahedral silicon at approximately 170 kbar.