Metamict state

Article By:

Ewing, Rodney C. Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico.

Last reviewed:November 2019

DOI:https://doi.org/10.1036/1097-8542.419600

The state of a special class of amorphous materials that were initially crystalline. W. C. Broegger first used the term metamikte in 1893 to describe minerals that were optically isotropic with a “glasslike” fracture but still retained well-formed crystal faces. In 1914 A. Hamburg correctly attributed the transition from the periodic, crystalline state to the aperiodic, metamict state as induced by alpha-decay damage. In minerals, this damage is the result of the decay of naturally occurring radionuclides and their daughter products in the uranium and thorium (²³⁸U, ²³⁵U, and ²³²Th) decay series. A wide variety of complex oxides (for example, pyrochlore structure types), silicates (such as zircon, thorite, and yttrialite), and phosphates (for example, xenotime) are reported as occurring in the metamict state. All of these structures can accommodate uranium and thorium. A renewed interest in the metamict state has been stimulated by concern for the long-term stability of crystalline materials (nuclear waste forms) that will serve as hosts for actinides (for example, plutonium, americium, curium, and neptunium). Various crystalline materials (phases) may appear in a single waste form; each phase may or may not suffer radiation damage. For some nuclear waste-form phases, the radiation-induced transformation to the metamict state has been stimulated by doping phases with highly radioactive plutonium-238 or curium-244. See also: Actinide elements; Alpha particles; Radioactivity

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