Large zircon crystals from a pegmatite body in the Wichita Mountains, Oklahoma, are zoned with fresh and metamict zircon. The metamict zircon is chiefly isotropic n = 1.82; some is uniaxial positive, ω = 1.85, ε= 1.86. The change from fresh to metamict zircon seems to be an abrupt and not a gradual one.

The age of the zircon is determined as 635 million years from α determinations and 641 million years from chemical determinations of uranium and thorium.

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