What does mean metamict in mineralogy ?

Metamict : definition

This term designates the state of a mineral whose crystal lattice is totally or partially disorganized under the action of radiation emitted by the uranium or thorium atoms it contains. This disorganization leads to color modifications (smoky quartz, antozonite fluorite, etc...) and can lead to an amorphous state with the acquisition of an isotropy in certain minerals (betafite, fergusonite, zircon, etc...). Metamict minerals can sometimes return to their original state by simple heating.

This phenomenon remains poorly understood and seems independent of the content of radioactive elements. Indeed, minerals containing less than 0.5% of radioactive elements can become metamict, while highly radioactive minerals retain their crystal structure intact ; this apparent paradox would be due to the strongly ionic bonds of the very radioactive minerals which would ensure a rapid ordering of the damage caused by the irradiation.