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INTRODUCTIONZircon (ZrSiO 4 ) is an ubiquitous accessory mineral. Its presence in a wide variety of sedimentary, metamorphic, and igneous settings, combined with its ability to retain trace-element and isotopic information, makes it an important phase for geochemical and geochronological studies (e.g., Heaman and Parrish 1991;Dickin 1995;Hanchar and Hoskin 2003). The refractory nature of zircon has also prompted suggestions for its use as a storage material for radioactive waste (e.g., Ewing 1999).At ambient conditions, zircon is tetragonal (space group = I4 1 /amd), and is among the few silicates containing only nominally tetravalent cations (Zr 4+ and Si 4+ ) that ensure a strong linkage between the structureʼs polyhedra. At high pressure, zircon transforms to reidite (scheelite structure, space group = I4 1 /a, see Glass et al. 2002). Reidite is approximately 10% more dense than zircon and can be quenched to ambient pressure. Knittle and Williams (1993) observed this transition at room temperature in natural zircon at a pressure of 23 ± 1 GPa using Raman spectroscopy. Scott et al. (2000) subsequently presented infrared (IR) spectroscopic measurements that were consistent with this transition pressure. High-pressure crystal-to-crystal phase transitions at room temperature are usually hampered by sluggish kinetics, so that the observation of this transition in zircon sug-* ABSTRACTWe have determined the room-temperature compressibility of pure, synthetic zircon (ZrSiO 4 ). Unit-cell volumes of a powdered sample were determined in situ as a function of pressure up to 27 GPa in a diamond anvil cell (DAC), by using angle-dispersive synchrotron X-ray diffraction (XRD) techniques. Unit-cell volumes were fitted to a Birch-Murnaghan equation of state, resulting in a roomtemperature bulk modulus for the zircon structure, K T0 = 199 ± 1 GPa, and ambient pressure unit-cell volume V 0 = 260.89 ± 0.03 Å 3 , when (∂K T0 /∂P) T = K' T0 is fixed at 4. This bulk modulus is over 12% lower than that suggested by earlier measurements using impure, natural zircon sample. In addition, we observed the start of the transformation of zircon to reidite (scheelite-structured ZrSiO 4 ) at a pressure of 19.7 GPa, over 3 GPa lower than previously determined for natural (impure) zircon. Together with compressibility measurements of a trace-element-doped zircon, these observations suggest that impurities affect the phase transition kinetics and compressibility of zircon, and by analogy, perhaps of other silicate minerals.