Metamict

Metamict describes minerals whose crystal structures have been damaged by radiation from within the mineral itself. The term, derived from the Latin metamictus meaning mixed or confused, refers to a state in which internal alpha decays from radioactive elements such as uranium and thorium create displacement cascades that disrupt bonds and progressively erode long-range crystallinity. In many cases the material becomes poorly crystalline or amorphous, though some short-range order may persist and the mineral may retain a detectable, albeit degraded, crystal framework.

Radiation damage from self-irradiation leads to changes in physical and chemical properties. Metamict minerals often exhibit reduced hardness and altered elasticity, along with increased susceptibility to dissolution. Their X-ray diffraction patterns typically show broad halos rather than sharp reflections, reflecting the loss of long-range order. Spectroscopic methods like Raman or infrared analysis also reveal signs of disorder. Radiometric dating can be complicated by metamictization, especially in heavily damaged zircons where lead mobility can occur, though certain microanalytical approaches can still yield ages with appropriate caveats.

Zircon is the classic and best-known example of a metamict mineral. Other minerals commonly described as metamict include monazite and titanite, as well as some xenotime and phosphate-bearing minerals that host substantial radiogenic elements. Metamictization is a progressive process that may be at least partially reversible through high-temperature annealing, which can encourage recrystallization and restore some degree of order, though the original radiation damage may not be completely eliminated.