Metamictisation

The effects of metamictisation are extensive: other than negating any birefringence previously present, the process also lowers a mineral's refractive index, hardness, and its specific gravity.

The presence of uranium and thorium atoms substituting for zirconium in the crystal structure is responsible for the radiation damage in this case.

Ekanite is almost invariably found completely metamict as thorium and uranium are part of its essential chemical composition.

A related phenomenon is the formation of pleochroic halos surrounding minute zircon inclusions within a crystal of biotite or other mineral.

The spherical halos are produced by alpha particle radiation from the included uranium- or thorium-bearing species.