Minerals, Metamict

a group of minerals whose matter, while preserving the external crystal habit, either partly or completely transforms from a structurally ordered crystalline state into a special state of aggregation resembling solid colloids. This transition is accompanied by a disordering or disintegration of the structure and by absorption of energy and is connected with the effects of radioactive decay of U and Th, which are present in metamict minerals. On heating in the range of 400°-800°C (sometimes to 1000°C), the matter in metamict minerals reverts back to the state of an ordered, crystalline aggregate with the properties of the initial crystalline mineral. It is believed that during a metamict transition the atoms of the crystal lattice are displaced by the energy of the radioactive radiation from the ideal position to a configuration in which the lattice no longer exists but the “memory” of the lattice is preserved. Heating returns the atoms to their normal position in the crystal lattice.

Metamict disintegration is observed in minerals in which the crystallochemical structure consists of weakly bonded cationic and anionic groups (Zr, Th, U, the rare-earth elements, and others with Si, Nb, Ta, Ti, and other elements). The metamict state has been observed in various minerals including zircon, thorite, orthite, and gadolinite, and in pyrochlore, samarskite, euxenite, and other niobotantalates. The metamict disintegration is usually accompanied by the sorption of water and various other materials from the surrounding medium.