Metamictisation (sometimes called metamictization or metamiction) is a natural process resulting in the gradual and ultimately complete destruction of a

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

crystal structure In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that repeat ...

, leaving the mineral

amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid) is a solid that lacks the long-range order that is a characteristic of a crystal. The terms "glass" and "glassy solid" are sometimes used synonymousl ...

. The affected material is therefore described as metamict. Certain minerals occasionally contain interstitial impurities of

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

elements, and it is the

alpha radiation Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus). The parent nucleus transforms or "decays" into a daughter product, with a mass number that is reduced by four and an atom ...

emitted from those compounds that is responsible for degrading a mineral's

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

structure through internal bombardment. The effects of metamictisation are extensive: other than negating any

birefringence Birefringence, also called double refraction, is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are described as birefrin ...

previously present, the process also lowers a mineral's

refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

hardness In materials science, hardness (antonym: softness) is a measure of the resistance to plastic deformation, such as an indentation (over an area) or a scratch (linear), induced mechanically either by Pressing (metalworking), pressing or abrasion ...

, and its

specific gravity Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...

. The mineral's colour is also affected: metamict specimens are usually green, brown or blackish. Further, metamictisation diffuses the bands of a mineral's

absorption spectrum Absorption spectroscopy is spectroscopy that involves techniques that measure the absorption of electromagnetic radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, ...

. Curiously and inexplicably, the one attribute which metamictisation does not alter is dispersion. All metamict materials are themselves radioactive, some dangerously so. An example of a metamict mineral is

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of th ...

. The presence of

uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...

and

thorium Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and ha ...

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''. Other minerals known to undergo metamictisation include allanite,

gadolinite Gadolinite, sometimes known as ytterbite, is a silicate mineral consisting principally of the silicates of cerium, lanthanum, neodymium, yttrium, beryllium, and iron with the formula . It is called gadolinite-(Ce) or gadolinite-(Y), depending o ...

, ekanite, thorite and titanite. Ekanite is almost invariably found completely metamict as thorium and uranium are part of its essential

chemical composition A chemical composition specifies the identity, arrangement, and ratio of the chemical elements making up a compound by way of chemical and atomic bonds. Chemical formulas can be used to describe the relative amounts of elements present in a com ...

. 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 Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more al ...

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 surrounding monazite and other radioactive minerals. Crockers-Well-metamict-Uranitite-Th

References

Woodhead, James A., et al., ''The metamictization of zircon: Radiation dose-dependent structural characteristics,'' American Mineralogist, Volume 76, pages 74-82, 1991

External links

Metamictisation of natural zircon: accumulation versus thermal annealing of radioactivity-induced damage
Mineralogy Radioactivity Geological processes {{mineralogy-stub

See also

References

External links