Metamictisation (sometimes calledmetamictization ormetamiction) is a natural process resulting in the gradual and ultimately complete destruction of amineral'scrystal structure, leaving the mineralamorphous. The affected material is therefore described asmetamict.
Certain minerals occasionally contain interstitial impurities ofradioactive elements, and it is thealpha radiation emitted from those compounds that is responsible for degrading a mineral'scrystal structure through internal bombardment. The effects of metamictisation are extensive: other than negating anybirefringence previously present, the process also lowers a mineral'srefractive index,hardness, and itsspecific gravity. The mineral's colour is also affected: metamict specimens are usually green, brown or blackish. Further, metamictisation diffuses the bands of a mineral'sabsorption spectrum. Curiously and inexplicably, the one attribute which metamictisation does not alter isdispersion. All metamict materials are themselves radioactive, some dangerously so.
An example of a metamict mineral iszircon. The presence ofuranium andthorium atoms substituting for zirconium in the crystal structure is responsible for the radiation damage in this case. Unaffected specimens are termedhigh zircon while metamict specimens are termedlow zircon. Other minerals known to undergo metamictisation includeallanite,gadolinite,ekanite,thorite andtitanite. Ekanite is almost invariably found completely metamict as thorium and uranium are part of its essentialchemical composition.
Metamict minerals can have their crystallinity and properties restored through prolongedannealing.
A related phenomenon is the formation ofpleochroic halos surrounding minute zircon inclusions within a crystal ofbiotite or other mineral. The spherical halos are produced by alpha particle radiation from the included uranium- or thorium-bearing species. Such halos can also be found surroundingmonazite and other radioactive minerals.
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