Zircon (ZrSiO4) is a common accessory mineral in nature, occurring in a wide variety of sedimentary, igneous, and metamorphic rocks. Known to incorporate an assortment of minor and trace elements, zircon has the ability to retain substantial chemical and isotopic information, leading to its use in a wide range of geochemical investigations, including studies on the evolution of Earth’s crust and mantle (e.g., Hanchar et al. 1994, Bowring 1995, Vervoort et al. 1996, Hoskin and Schaltegger, this volume; Valley, this volume) as well as age dating (e.g., Gibson and Ireland 1995, Bowring et al. 1998, Solar et al. 1998, Bowring and Schmitz, this volume; Ireland and Williams, this volume; Parrish et al., this volume). The physical and chemical durability of zircon is a major factor in it being the mineral by which many of Earth’s oldest known rocks have been dated (Bowring et al. 1989, Maas et al. 1992, Buick et al. 1995, Bowring and Williams 1999, Wilde et al. 2001) and is also an important factor in zircon being proposed as a candidate waste form for the geologic disposal of excess plutonium from dismantled nuclear weapons (Ewing and Lutze 1997, Ewing 1999, Burakov et al. 2002, Burakov et al. 2003, Ewing et al., this volume). The chemical and physical properties of zircon and its ability to incorporate and retain trace elements are largely determined by its crystal structure. The zircon structure is adopted by numerous minerals and synthetic compounds with the general formula AT O4, in which high field-strength T -site cations occupy isolated tetrahedra, and A -site cations occupy larger eight-coordinated structural sites. Zircon-type compounds share many physical properties, as well as displaying variable degrees of solid solution among end members. …