Metamictization

Metamictization is a process by which crystalline minerals become amorphous or glass-like due to internal radiation damage. It occurs when radioactive elements within the crystal lattice, primarily uranium and thorium (and to a lesser extent potassium-40), decay and emit energetic particles. The cumulative damage from alpha decay and recoil displaces atoms and creates defect clusters that disrupt the long-range order of the mineral structure.

The effect is most pronounced in minerals with significant records of radiogenic elements, such as zircon (ZrSiO4), monazite, titanite, uraninite, and apatite. Over geologic timescales, the accumulation of radiation damage can drive a transition from a well-ordered crystal to a metamict, amorphous state. The degree of metamictization depends on the dose of radiation, temperature history, and the mineral’s ability to anneal damage.

Metamict minerals typically show reduced crystallinity, often with broadened or absent X-ray diffraction peaks, and altered physical and chemical properties. They may exhibit increased chemical solubility and diffusion rates for certain elements, which can complicate radiometric dating and elemental transport studies. The radiometric ages of metamict minerals can be biased if damage-induced phenomena affect closed-system behavior.

Recrystallization or partial restoration can occur under high-temperature or hydrothermal conditions, leading to partial annealing and re-crystallization. The extent and reversibility of metamictization are influenced by the mineral type, radiation dose, and geological history. Metamictization remains an important consideration in mineralogy, geochronology, and studies of the thermal evolution of rocks.