Metamictization
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Metamictization (sometimes called metamiction) is a natural process resulting in the gradual and ultimately complete destruction of a mineral's crystal structure, leaving the mineral amorphous. Affected material is therefore described as metamict.
Certain minerals occasionally contain interstitial impurities of radioactive compounds and it is the alpha radiation emitted from these compounds that is responsible for degrading a mineral's crystal structure through internal bombardment. Effects of metamictization are extensive: other than negating any birefringence previously present, the process also lowers a mineral's refractive index, hardness, and specific gravity. The mineral's colour is also affected: metamict specimens are usually green or brown. Further, metamictization diffuses the bands of a mineral's absorption spectrum. Curiously and inexplicably, the one attribute which metamictization does not alter is dispersion. All metamict materials are themselves radioactive, some dangerously so.
An example of a metamict mineral is zircon. The presence of uranium and thorium atoms substituting for zirconium in the crystal structure is responsible for the radiation damage in this case. Unaffected specimens are termed high zircon while metamict specimens are termed low zircon. Specimens falling between the two extremes are termed intermediate. Other minerals known to undergo metamictization include allanite, ekanite and titanite. Ekanite is almost invariably found completely metamict as thorium and uranium are part of its essential chemical composition.
Metamict minerals can have their crystallinity and properties restored through prolonged annealing.
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.
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