Zircon (ZrSiO) is commonly found in felsic igneous rock. Because both Ce and Ce can substitute for zirconium, Zircon often has a positive Ce anomaly. Ce substitutes with Zr much more easily than Ce because Ce (ionic radius 0.97Å) has the same charge and a similar ionic radius as Zr (ionic radius 0.84Å). Therefore, the oxidation state of the magma is what determines the Ce anomaly in Zircon. If the oxygen fugacity is high, more Ce will oxidize to Ce and create a larger positive Ce anomaly in the zircon structure. At lower levels of oxygen fugacity, the level of Ce anomaly will also be lower.