Zircon is a common accessory mineral found in many different rocks. It occurs as small grains in granitic rocks, syenites, schists and gneisses. It may form larger crystals in pegmatitic rocks or marbles. Since it is a dense, reasonably resistent mineral, it can be found in heavy mineral fraction of sedimentary rocks, possibly even in concentrates in placer deposits.
Some zircon contains significant uranium and thorium.
The radiaction produced by these impurities may damage or destroy zircon's crystal structure. Zircon can often be spotted when as small grains because of its tendancy to fluoresce yellow under short wave ultraviolet light. Wilcox reports on a more speciifc locality near Mellen in the SW sec. 29 T.45N. R.2W. where a pegmatite dike contained cinnamon brown zircon crystals up to 18 cm. Th zircon was associated with plagioclase, hornblende and bitotite.