We have determined the room-temperature compressibility of pure, synthetic zircon (ZrSiO4). 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 room-temperature bulk modulus for the zircon structure, KT0 = 199 ± 1 GPa, and ambient pressure unit-cell volume V0 = 260.89 ± 0.03 Å3, when (∂KT0/∂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 ZrSiO4) 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.