Metamictization
From Wikipedia, the free
encyclopedia
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.
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.
The original wiki link can be found at: http://en.wikipedia.org/wiki/Metamictization
The term is used with rare-earth minerals. Rare-earth minerals are used in speakers and earphones, for example.
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