Metamictization

Metamictization breaks down the original crystal structure of certain rare minerals to a glassy state by radioactive decay of uranium and thorium. It is accompanied by marked changes in properties and often water content. Many minerals start to lose their radioactive components upon metamictization. Because of this, scientists have questioned whether other methods, such as hydrothermal alteration, may be preferable for some crystals, such as zircon. Crystals that do not lose their radioactivity may be candidates for synthetic rocks grown from high-level nuclear wastes to isolate them until they are safe.

Metamict Mineral Properties

Metamict minerals are an anomaly. They have the form of crystals, but in all other ways they resemble glasses. They fracture like glass, are optically isotropic like glass, and to all appearances are noncrystalline. Yet minerals cannot grow without an ordered crystalline arrangement of their constituent atoms—even metamict ones. The term “metamict” is from the Greek roots meta and miktos, meaning “after mixed.” It aptly describes minerals that must originally have grown as crystals and subsequently have been rendered glassy by some process that has destroyed their original crystallinity. The discovery that all metamict minerals are at least slightly radioactive and that metamict grains have more uranium or thorium than do their nonmetamict equivalents led to the realization that radiation damage resulting from the decay of uranium and thorium causes metamictization. Although controversial at first, the concept of radiation damage is now so easy for scientists to accept that the puzzle is not why some minerals become metamict, but why others with relatively large concentrations of uranium and thorium never do.

The varieties and chemical compositions of minerals that undergo metamictization are quite diverse. Yet all metamict minerals share several common properties: They are all radioactive, with measurable contents of uranium, thorium, or both; they are glassy, brittle, and fracture like glass; they are usually optically isotropic for both visible and infrared light; they are amorphous to X-ray diffraction; and they often have nonmetamict equivalents with the same form and essentially the same composition. Finally, compared to their nonmetamict equivalents, they have lower indices of refraction, are more darkly colored, are softer, are less dense, are more soluble in acids, and contain more water. In those minerals that exhibit a range of metamictization from crystalline to metamict, partially metamict samples have intermediate properties.

For example, X-ray diffraction patterns of zircon show that radiation damage causes it to swell markedly in proportion to accumulated radiation damage up to the point of total metamictization. Beyond that point, the structure is so disordered that it can no longer diffract X-rays, even though a continued decrease in density indicates further expansion.

Changes in other properties parallel the decrease in density. Partially metamict zircons that are heated at temperatures well below the melting point recrystallize readily to grains that are aligned with their original form, while those that are completely metamict recrystallize just as readily, but not to the original alignment. Several other metamict minerals recrystallize in the same way, but many produce mixtures of different minerals on heating because either they are outside their fields of stability or their compositions have changed subsequent to metamictization.

Metamict Mineral Groups

Metamict minerals belong to only a few broad groups of chemical compounds. In each group, some have uranium or thorium as the dominant metal ion present, but most have only small amounts substituting in the crystal structure for other ions, such as zirconium, yttrium, and the lanthanide rare-earth elements (REE), which just happen to have similar sizes and charges. This is known as isomorphous substitution and is common in most mineral groups.

The largest group of metamict minerals are yttrium-, REE-, uranium-, and thorium-bearing complex multiple oxides of niobium, tantalum, and titanium. All the metal-oxygen bonds in these minerals have about equal strength and about equal susceptibility to radiation damage. As it happens, most of these minerals have little resistance to metamictization and are commonly found in the metamict state.

Samarskite, brannerite, and columbite are examples of complex multiple oxide minerals. Samarskite is always totally metamict and compositionally altered with added water. It usually recrystallizes to a mixture of different minerals on heating, making its original structure and chemical formula hard to determine. Brannerite can range from partially to totally metamict. It has been found to be the primary site for uranium in some 1,400-million-year-old granites from the southwestern United States. In those rocks, it was recrystallized as recently as 80 million years ago, but even in that geologically short period of time, it has become metamict again. Columbite is never more than partially metamict. Apparently, the presence of iron in its formula helps prevent metamictization from progressing as far as it does in other niobates. These minerals are generally found in coarse-grained pegmatites associated with granites. They may also be found as accessory minerals within the granites themselves. Even when they account for most of the rock’s uranium or thorium, however, they are so rare that it takes special concentrating techniques simply to find a few grains.

Silicates are the largest group of minerals in the earth’s crust, but they account for only the second largest number of metamict minerals. They are characterized by having very strongly bonded silicon-oxygen groups in their crystal structures. Each silicon ion is surrounded by four oxygen ions in a tetrahedral (three-sided pyramid) shape. The chemical bonding between silicon and oxygen in the tetrahedral group is considerably stronger than the bonding between oxygen and any other metal ions in the structure, and is more resistant to radiation damage. The only silicates susceptible to metamictization are those in which the tetrahedral groups are not linked to form strong chains, sheets, or networks that make the structure resistant to radiation damage. The most commonly occurring metamict mineral is zircon, a zirconium silicate with isolated tetrahedral groups. Only a small amount of uranium can substitute for zirconium in the structure—up to about 1.5 percent, but usually less than 0.5 percent. Up to about 0.1 percent of thorium can also be present. These small amounts are sufficient to cause zircon to occur in a wide range of degrees of metamictization. Zircon is only one member of a group of silicates that grow with essentially the same crystal structure and subsequently become metamict. Thorite (thorium silicate) and coffinite (uranium silicate) also belong to that group. They each require about the same amount of radiation damage as zircon to become metamict, but because their concentrations of radioactive elements are much higher, metamictization occurs much more quickly.

Phosphates are the smallest group of metamict minerals, and they are usually found only partially metamict. One of particular interest is xenotime (yttrium phosphate), which has the same crystal structure as zircon. It commonly occurs in the same rocks as zircon, with equal or even greater contents of uranium and thorium, but is usually much less radiation damaged. Similarly, monazite (a rare-earth phosphate) takes up about the same amount of uranium and considerably more thorium than zircon but is seldom more than slightly damaged. Apparently, the substitution of phosphorus for silicon in these structures makes the phosphates less susceptible to metamictization than the equivalent silicates.

Radiation Damage to Mineral Structure

In order to understand metamictization fully, one must also understand how the radioactive decay of uranium and thorium damages crystal structures. In nature, uranium and thorium are both made up of a number of isotopes. Their most common isotopes are uranium-238, uranium-235, and thorium-232. Each of these isotopes decays, through a series of emissions of alpha particles, into an isotope of lead—lead-206, lead-207, and lead-208, respectively. The alpha particle is emitted from the decaying nucleus with great energy, and the emitting nucleus recoils simultaneously in the opposite direction. The energy transmitted to the mineral structure by the alpha particle and the recoiling nucleus is the major cause of radiation damage. The alpha particle has a very short range in the mineral structure (only about 20 wavelengths of light) and imparts most of its energy to it by ionizing the atoms it passes. Near the end of its path, when it has slowed down enough, it can collide with hundreds of atoms. The much larger recoil nucleus has a path that is about one thousandth as long, but it collides with ten times as many atoms. Thus, the greatest amount of radiation damage is caused by the recoil nucleus rather than by the alpha particle itself. Both particles introduce an intense amount of heat in a very small region of the structure, disrupting it but also increasing the rate at which the damage is spontaneously repaired. The accumulation of radiation damage depends on a balance between the damage and self-repair processes. In simple terms, radioactive minerals that remain crystalline have high rates of self-repair, while those that become metamict do not.

Another contribution to radiation damage in uranium-bearing minerals is the spontaneous fission of uranium-238, in which its nucleus splits into two separate nuclei of lighter elements. That process is much more energetic than is alpha emission, but it happens at a much lower rate. It probably produces only about one-tenth to one-fifteenth the damage that alpha emission and alpha recoil cause. The radiation damage done by the decay of uranium and thorium is not always confined to the metamict mineral itself. The decay process also involves the emission of gamma rays that penetrate the surrounding rock, often causing the development of dark halos in the host minerals. Metamict grains are also often surrounded by a thin, rust-colored, iron-rich rim at their contact with other minerals. These rims are enriched in uranium and lead that have leaked out of the damaged grain as well as the iron from the surrounding rock. These phenomena draw dark outlines around radioactive grains and make them easy to spot in most granites and pegmatites.

X-Ray Studies

X-ray diffraction results from the reinforcement of X-ray reflections by repeated planes of atoms in a crystal. The diffraction pattern is characteristic of each crystal structure and depends on the spacing of the planes. The strength of diffraction peaks depends on the atomic density in the diffracting planes and on the regularity of the crystal structure. If the structure is damaged, both the intensity and the sharpness of the diffraction peaks decrease. Glasses produce no X-ray diffraction pattern because there is no long-range order or regularity in their structures, and thus no planes of atoms exist on which X-rays can diffract; they are said to be “X-ray amorphous.”

X-ray diffraction studies of metamict minerals have shown them to resemble glasses in being devoid of a regular crystal structure. In partially metamict samples, the spacing between planes increases and the regularity of the structure decreases with progressive radiation damage. For example, in some 570-million-year-old zircons, 0.25 percent uranium is enough to make them X-ray amorphous—completely metamict. Much older zircons from other localities with higher uranium contents, however, yield X-ray patterns that reveal them to be only slightly damaged. In those areas, a relatively recent geologic event caused the zircons to be completely recrystallized with partial loss of lead, produced by radioactive decay of uranium and thorium, but little or no loss of uranium or thorium themselves.

Once a mineral is X-ray amorphous, X-ray diffraction is not as effective as is X-ray absorption spectroscopy for studying its structure. The way that a material absorbs X-rays depends on the spacing of the atoms in it. X-ray absorption spectroscopy has shown that the spacing between adjacent ions is little changed in metamict minerals, but the regularity at greater distances has been lost. It has also shown that among complex multiple oxides, their crystal structures are highly regular and easily distinguished, but their metamict structures are nearly indistinguishable. Thus, the oxides all approach the same glassy state on metamictization.

Electron Microscopy and Infrared Spectroscopy

Electron diffraction from single grains of metamict minerals yields results very similar to those of X-ray diffraction techniques. For example, it has been shown that partially metamict zircons are made up of misaligned crystalline domains that are destroyed with progressive metamictization.

High-resolution electron microscopy allows the scientist to examine a mineral’s structure directly. While actual atoms cannot be seen, the regularity of the array of atoms making up the structure can. Applied to zircons, this technique has shown that fission particles from the spontaneous fission of uranium-238 leave long tracks of disruption behind them as they pass through the crystal structure. In partially metamict zircons, highly disordered damaged patches (called domains) are interspersed with undamaged domains. As metamictization proceeds, the damaged domains begin to overlap and eventually wipe out all remaining crystalline domains.

Infrared spectroscopy is a powerful tool for studying the bonds between atoms in minerals. The absorption of infrared light by a crystal structure depends on the strength and regularity of bonds within the structure. Infrared study has shown that the silicon-oxygen bonds of the tetrahedral groups in zircon remain intact to a large extent even in metamict samples, while the regularity of virtually every zirconium-oxygen bond has been disturbed. Thus, silicon remains surrounded by four oxygen atoms even in the most metamict zircons when the regularity of the formerly crystalline structure has been destroyed. Infrared spectroscopy is also often used to study the occurrence of water in minerals. Because most metamict minerals are enriched in water to some extent, its role in producing or stabilizing the metamict state is a question of some interest. Infrared studies of zircons in a wide range of metamict states show that while there is no water in their structures, small amounts of hydroxyl are common. The existence of “dry” zircons in all states from crystalline to metamict, however, shows that neither water nor hydroxyl is necessary to the metamictization process. The tendency for metamict zircons to have more hydroxyl than less damaged samples suggests that hydroxyl enters zircons only after metamictization has opened up their structures sufficiently for it to diffuse in.

Role in Economy and Public Safety

Metamict minerals are important to people for two principal reasons. First, some gemstones, such as zircon, occur in the metamict state, and several others, such as topaz, can be enhanced in appearance by irradiation. Metamict gemstones are often considerably more valuable than the crystalline varieties because the anisotropic optical properties of the crystalline gems make them look “fuzzy” inside and less beautiful than the isotropic metamict stones. Radiation damage also often imparts color to the gemstone, increasing its value. (Artificial means of imparting color to gemstones by irradiation have been developed, but the gem often fades back to its original color with time. Consequently, it is best to know the entire history of a colored gem, or have it examined by a knowledgeable and trustworthy expert, before investing large amounts of money in it.)

Second, and more importantly, the understanding of metamictization may someday be invaluable to the safe disposal of high-level nuclear wastes. Ways must be found to keep high-level wastes from leaking into the environment and poisoning groundwater for a period of at least ten thousand years, after which they will have lost most of their radioactivity. The only way that scientists can determine what could conceivably lock up hazardous, highly radioactive atoms for that length of time is to examine results of long-term experiments on potentially leak-proof containers—solid materials that could be grown from the radioactive wastes that would be impervious to alteration, breakdown, or damage. The “experiments” of that duration are those that have already occurred in nature: radioactive minerals that have been damaged internally for millennia. Geochemists have found that some minerals retain their radioactive elements over millions of years, despite metamictization, while others do not. Synthetic analogues of the minerals that do may be grown from a mixture of the radioactive elements and added compounds to produce artificial rocks that are resistant to leaching and that have the potential to trap the hazardous substances for the required time.

Principal Terms

alpha particle: a helium nucleus emitted during the radioactive decay of uranium, thorium, or other unstable nuclei

crystal: a solid made up of a regular periodic arrangement of atoms; its form and physical properties express the repeat units of the structure

glass: a solid with no regular periodic arrangement of atoms; that is, an amorphous solid

granite: a light-colored igneous rock made up mainly of three minerals (two feldspars, and quartz), with variable amounts of darker minerals

isotopes: atoms of the same element with identical numbers of protons but different numbers of neutrons in their nuclei

isotropic: having properties that are the same in all directions—the opposite of anisotropic, having properties that vary with direction

pegmatite: a very coarse-grained granitic rock, often enriched in rare minerals

silicate: a substance whose structure includes silicon surrounded by four oxygen atoms in the shape of a tetrahedron

spontaneous fission: uninduced splitting of unstable atomic nuclei into two smaller nuclei, an energetic form of radioactive decay

X-ray: a photon with much higher energy than light and a much shorter wavelength; its wavelength is about the same as the spacing between atoms in crystal structures

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