Nuclear Instruments and Methods in Physics Research B 91 (1994)22-29
North-Holland
NOMBBeam Interactions
with Mattarials & Atoms
The metamict state: 1993 - the centennial
Rodney C. Ewing
~e~art~e~t of Earth and Planetary Sciences, U~~ers~~ of New Mexico, Albuquerque, New Mexico 87131, UsA
In 1893, a Norwegian mineralogist, W.C. Bragger, defined "metamikte" as a third class of naturally occurring, amorphous
materials. Metamict minerals were determined to be amorphous based on conchoidal fracture and isotropic optical properties;
however,well developed crystal forms evidenced the prior crystalline state.
In 1914, A. Hamberg, based on the observation of gleochroic haloes, first suggested that metamictization is a radiation-induced,
periodic-to-aperiodic transition caused by a-particles which originate from decay of constituent U and Th. Within a few years, F.
Rinne and L. Vegard confirmed by X-ray diffraction that metamict minerals were either amorphous or finely crystalline. By 1940,
M. v. Stackelberg and E. Rottenback had established that the decrease in density, refractive indices and birefringence correlated
with the breakdown of the structure with increasing a-decay event dose. They tested this hypothesis by bombarding a thin slab of
zircon, tetragonal ZrSiO,, with ol-particles. The result was inconclusive, but this must have been one of the first experiments in
which an "ion-beam" was used to "modify" a ceramic material. Meta~ct minerals remained a curiosity and a daunting challenge,
as compositions were exceedingly complex (lanthanides were abundant) and no structure remained, except that which was restored
on heating. In 1952, Adolf Pabst, in his presidential address to the Mineralogical Society of America, carefully tabulated the
changes in properties (e.g., release of stored energy and decreased resistance to leaching) which resulted from the radiation
damage. Pabst specifically noted that some structures are "resistant" to damage accumulation (e.g., monoclinic ThSiOs) while
other polymorphs are often found in the metamict state (e.g., tetragonal ThSiOaI.
The nse of zircon (isostructural with thorlte) in U/Pb dating, however, focused substantive studies on the process of radiation
damage. Holland and Gottfried (1955) i~estigated damage a~umulat~on as an age-dating technique in a classic study of natural
zircons (570 million years old; doses up to lOi or-decay events/mg = 0.7 dpa); and developed a modern model of damage
accumulation. Recent studies are reviewed which include Pu-doped and ion-beam irradiated zircons. With the exception of
radiation effects on alumina and silica, zircon is now probably the most studied complex ceramic.
1. In~duction
During the past ten years, the words "metamict" or
"metamictization" have been increasingly used synonymously
for "amorphous" or "amorphization", particularly
in referring to the effects of heavy-particle irradiations,
such as o-decay event damage in ceramics and
minerals [l]. Occasionally, the term has also been applied
to the amorphization of phases such as a-quartz
under electron beam irradiations [Z]. The term
"metamikte" was first defined in a Danish encyclopedia
in 1893 by W.C. Bragger [3] as part of a discussion
of "amorf" solids. Metam~ct minerals were recognized
as a third class of amorphous substances which had
originally been crystalline. The amorphous state of
such substandes was inferred from their conchoidal
fracture, optical isotropy, and "glass-like" appearance.
The previous crystallinity was evidenced by well formed
crystal faces.
On the occasion of the hundredth anniversary of
the definition of the metamict state, this paper briefly
summarizes the history of work on metamict minerals.
This summa~ draws extensively on the work of Professor
Adolf Pabst [4]. Additional reviews of the literature
of metamict minerals can be found in refs. [5-71.
2. Historg
2.1. Prior to 1950
The earliest description of unique properties associated
with radiation-damaged materials was by Jijns
Jacob Berzelius [8]. Berzelius was a Swedish physician
and mineral chemist and certainly one of the most
eminent scientists of his century. Berzelius discovered
the elements cerium, selenium and thorium. His treatise
on blowpipe analysis was translated into German,
French, English, Russian, and Italian, and his systematic
application of this technique laid the analytical
foundation for the discovery of new elements during
the 19th century. The abundance of complex pegmatites
(small rock units containing higher than normal
concentrations of exotic elements, such as the rare
0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved
SsI31 OI68-583X(94)00042-T
R.C. Ewing/Nucl. Instr. and Meth. in Phys. Res. B 91 (1994) 22-29 23
earths, uranium and thorium) led to the inevitable
study of U- and Th-bearing minerals and the observation
that some glowed on qoderate heating, releasing,
sometimes violently, large amounts of energy. This
"pyrognomic" behavior was first observed in gadolinite,
(REE),FeBe,Si,O,,, a monoclinic silicate.
Bragger's definition of metamict minerals in 1893
was based in part on the knowledge that there was a
large class of materials that displayed this pyrgonomic
behavior. Other workers during the second half of the
19th century [9,10] established that these phases were
initially isotropic but became birefringent and increased
in specific gravity on heating. As early as 1864,
zircon, ZrSiO,, was the focus of study [lo].
As this work predated the discovery of radioactivity
in 1896 by Becquerel, metamictization was not recognized
as a radiation-induced transformation. Bragger
speculated that metamictization was due to "outside
influences" and that complicated structures might be
more susceptible to this effect. Spencer [ll] considered
hydration as a possible cause, as the molecular water
content of these phases could be exceedingly high (10
to 15 wt.%).
Goldschmidt [12], the first to establish principles,
"Goldschmidt's Rules", which governed the distribution
of elements in the Earth's crust, provided the first
clear statement of criteria for metamictization:
1. The structure should be weakly ionic and possibly
susceptible to hydrolysis.
2. The structure should contain ions which may change
their "state of ionization" under ambient condi-
tions.
3. The structure must necessarily be subjected to strong
radiation, either from internal sources or from outside
sources.
Hamberg 1131was the first to suggest that metamictization
is a radiation-induced, periodic-to-aperiodic
phase transition caused by a-particles which originate
from the constituent radionuclides in the uranium and
thorium decay series. Vegard [14] in studies of thorite,
ThSiO,, confirmed the absence of X-ray diffraction
maxima in metamict minerals, indicating that they were
either amorphous or finely crystalline. Later work
[15,16] supported the idea of a radiation-induced transformation.
Stackelberg and Rottenback [17,18] established
that the decrease in density, refractive indices
and birefringence correlated with the breakdown of
the structure with increasing a-decay dose. Stackelberg
and Rottenback 117,181 tried to test this hypothesis
directly by bombarding a thin slab of zircon with or-particles.
The results were inconclusive because the slab
fractured, but this must have been one of the first
experiments in which an "ion beam" was used to
"modify" a ceramic material. Machatschki [19] suggested
that the instability of zircon might be due to
structural considerations. In zircon, zirconium is in
eight-fold coordination; however, zirconium's ionic radius
is such that it is close to the limit of the range in
which Zr is in six-fold coordination. Hutton [20] suggested
that changes in ionic radius associated with
transmutation or "auto-oxidation" of phases due to the
a-particle capture of electrons may lead to an increased
susceptibility to metamictization.
2.2. 1950 to 1975
Careful, systematic work on the metamict state was
not undertaken until the 1950s. In 1952, Pabst's presidential
address to the Mineralogical Society of America
[4], "The Metamict State", resurrected the use of
the term and raised important fundamental questions
concerning the stability of different structure types in a
radiation field. In 1955, Holland and Gottfried [21]
completed the first careful and systematic study of
damage in-growth in zircon, ZrSiO,.
Pabst summarized the early studies on metamictization
and listed the known properties of metamict minerals.
More importantly, the second half of his paper
focused on the behavior of the polymorphs of ThSiO,
monoclinic huttonite and tetragonal thorite. Huttonite,
isostructural with monazite (CePO,), is always found in
the crystalline state in nature; while, thorite, isostructural
with zircon, is often partially or completely
metamict. The principal differences between the structures
(see Fig. 1) are: 1) the ThO, coordination polyhedra
in huttonite vs. the ThO, coordination polyhedra
in thorite; 2) the close-packed structure in huttonite
(p = 7.35 g/cm3) vs. a more open structure in thorite
(p = 6.69 g/cm3) in which large voids (14 A3) are
connected and form channels parallel to the c-axis.
These structures were refined by Taylor and Ewing
[22], and two observations were made concerning the
susceptibility of thorite to a-decay event damage:
1) Stability criteria based on radius ratio and charge
balance are inconclusive; the Th/O radius ratio (0.76)
suggests that the nine-fold coordinated huttonite structure
should be preferred, while a calculation of Pauling
charge balance indicates that the O(1) oxygen of huttonite
is overbonded (l = 2.5). All oxygen atoms in
thorite are exactly charge balanced (l= 2.0).
2) The voids in the thorite structure provide channels
for the diffusion of molecular water into the
structure (some metamict thorites contain up to 70
mol% water); thus, highly polarizing impurities such as
water may inhibit or prevent recrystallizaion of radiation-damaged
thorite under ambient conditions. Pabst
ends his paper by noting that . . . powerful sources of
radiation are now available . . * and it is surprising that
so few attempts have been made to produce it [the
metamict state] artificially."
At the same time as Pabst's presidential address,
there was renewed interest in the use of radiation
I. FUNDAMENTAL PROCESSES
24 R.C. Ewing /Nucl. fnstr. and M&h. in Phys. Res. B 91 (1994) 22-29
a
b
a
C
Th mite
Fig. 1. Perspective polyhedral representations of the huttonite
and thorite structures. Smaller tetrahedra are SiO, groups
and the larger polyhedra are ThO, (hu~onite) and ThO,
(thorite). After Taylor and Ewing [22J.
damage as a tool for geochronology [23] either by
measuring stored energy or the degree of damage (via
changes in physical and structural properties) as a
function of a-decay event dose which depends on the
concentrations of U and Th. The watershed paper by
Holland and Gottfried [21] provided one of the most
detailed studies of damage development in a complex
ceramic, zircon, as a function of increasing o-decay
event dose. A suite of zircons (570 million years old)
from Sri Lanka with a range of U and Th concentrations
was analyzed to determine changes in density,
refractive indices and unit cell volume as a function of
increasing o-decay dose up to saturation values of lOI6
o-decay events/mg. At this dose, the zircon became
X-ray diffraction amorphous. This study was different
from previous efforts in that it: 1) clearly established
the range of dose over which the pe~odic-to-aperiodic
transition occurred, 1015-1016 a-decay events/mg; 2)
precisely determined the change in density (- 17%),
the decrease in birefringence until the material was
isotropic, and the anisotropic expansion of the unit cell
(5%). Their inte~retation: 1) clearly ~st~g~shed between
the role of o-particles, which dissipate most of
their energy by electronic excitation and ionization
events, from that of the recoil nucleus which dissipates
most of its energy by elastic collisions and causes
upwards to 2000 atomic displacements; 2) modelled
damage in-growth and proposed the formation of new
crystallites, "phase 2", of an undamaged zircon structure
type at intermediate doses (i.e., radiation enhanced
crystallization); 3) discussed the role of annealing
in determining the final state of a material.
2.3. 1975 to 1990
During this period, renewed interest in the effects
of a-decay event damage on radioactive waste forms
led to increased funding and research. Weber 124,251
studied defect accumulation in UO, as a function of
o-particle, o-recoil and fission fragment damage combined
with detailed studies of the annealing kinetics
1261.This work demonstrated the different effects of
various types of radiation and their temperature de-
pendence.
Eyal and co-workers [27,28] developed an ingenious
chemical probe of o-recoil tracks that has proved particularly
valuable in calculating the mean life, 7, of
a-recoil tracks in crystalline and metamict minerals.
Powdered samples were leached in a bicarbonate solution
at room temperature for periods up to hundreds
of days. The radioactivities of 238U, 234U, =*Th, 230Th
and 228Th in each solution were determined by ol-spectrometry.
The leaching data are presented in terms of
the quantity, R, which is the relative leaching rate
given by the fractions of activities leached. The ratios
are with respect to the initial radioactivities in the
mineral. The most important feature of the technique
is that the ma~itude of the fractionation varies for
different isotope pairs 228Th/232Th and 234U/238U.
Commonly the 228Th/232Th fractionation is much enhanced
relative to the 234U/238U fractionation. Taking
into account the time scales for radioactive decay, this
observation suggests the annealing of indi~dual tw-recoil
tracks. The short-lived nuclide "*Th (mean life =
2.7 yr) is formed by the decay of the short-lived a-recoil
atom `=Ra (mean life = 8.3 yr). Therefore the
enhanced leaching of "`Th displays the effect of o-recoil
damage that is always relatively fresh (mean life =
11 yr). On the other hand, the nuclide w4U is relatively
long-lived (mean life = 3.53 X 10' yr), and its enhanced
dissolution relative to =`U is governed by the older
damage. Using this method, Eyal and Fleischer [27,28]
dete~ined a mean annealing time for an fu-recoil
track in minerals (e.g., UO,, ThO, and CeP04) as
being in the range of 15000 to 18000 yr. These phases
are important because they retain their crystallinity
even at high u-decay doses (> 10 dpa). This is in
contrast to the calculated mean life of an o-recoil track
of lo8 yr in complex pyrochlore structure types which
are commonly found in the metamict state.
Another important development was the use of
highly active actinides, mainly z38Pu (87.7 yr half-life)
and 244Cm(18.1 yr half-life), to accelerate the damage
R.C. Ewing/N&. Instr. and Meth. in Phys. Rex B 91 (1994) 22-29 25
in-growth process and to allow the observation of the
periodic-to-aperiodic transition in phases which were
potential actinide hosts in nuclear waste forms 129,301.
Although these highly radioactive samples were difficult
to handle, it was possible to precisely monitor
density changes and follow damage in-growth using
X-ray diffraction analysis and electron microscopy. The
"4Cm-doped phases included: Ca,Nds(SiO,),O~a8[31-
331; Gd,Ti,O, [34-361; CaZrTi,O, [36]. The Pudoped
phases included: zircon 137,381;CaZrTizO, [39-
421.
Finally, nearly 30 years after the initial suggestion
by Pabst, Cartz and others [43] irradiated powders of
monoclinic huttonite and tetragonal thorite, ThSiO,,
with Arf ions at 3 MeV to investigate structural controls
on radiation damage. Using X-ray diffraction
analysis, they demonstrated that both thorite and huttonite
can become metamict (the damage cross-section
for thorite is nearly twice that of huttonite); however,
low temperature annealing studies showed that the
huttonite recrystallized more easily than thorite. Under
ambient conditions over geologic time, huttonite may
recrystallize; therefore, huttonite is not found in the
metamict state. Unfortunately, to date, there are still
no systematic studies of the ThSiO, polymorphs; however,
Cartz's work anticipated the use of ion beam
irradiations to study the susceptibility of different minerals
to radiation damage.
Work continued on natural zircons. Chakoumakos
et al. [44] examined in detail a single zircon crystal with
zones (5 to 400 urn thick) of variable U and Th
concentrations which lead to a range of a-decay doses
which spanned the periodic-to-aperiodic transition.
Their work 144,451confirmed the earlier work of Holland
and Gottfried [21]. Recent work [46] on this
crystal was extended by using the mechanical properties
microprobe to measure changes in mechanical
properties with increasing dose (40% decrease in hardness,
60% decrease in elastic moduli, and an increase
in fracture toughness). Also, using a suite of natural
zircons, the periodic-to-aperiodic transition was studied
for the first time using high resolution transmission
electron microscopy 1471.The same samples were used
in leaching experiments [48] and showed a factor of ten
increase in leach rate as a function of this radiation-induced
transformation.
In addition to detailed studies of zircon, similar
systematic studies 1491were initiated on natural, isometric
pyrochlore structure types, A,_,B,O,(O,OH,
F), _-IIpH,O, and its monoclinic derivative, zirconolite,
CaZrTi,O,. These phases are important actinide hosts
in the titanate nuclear waste form, Synroc. Although
results from these studies were similar to those for
zircon, Lumpkin [49] made a significant contribution by
noting that if one analyzed suites of pyrochlores from
different localities (of different ages), the calculated
100
n
0.1
10
-1 x
3
-0.1
0:1 i 1'0
t (109 yr)
Fig. 2. Variation of a-decay event dose as a function of age
for natural pyrochlores. The upper curve and data represent
the saturation dose estimated from slightly crystalline and
metamict samples. The lower curve and data represent the
onset of detectable a-decay damage defined by X-ray diffraction
analysis (I/I, = 0.8-1.0). After Lumpkin and Ewing [49].
dose for the crystalline-to-metamict transition increased
with the age of the geologic deposit (Fig. 2).
This is clear evidence for the annealing of a-recoil
damage over geologic time. The change in critical
amorphization dose with age was modelled assuming
"fading" of the recoil-nucleus "tracks" in a fashion
similar to that used to describe fission fragment track
fading. Critical parameters are the o-decay event dose,
age of the sample, and the mean life of an o-recoil
track in the mineral. The variation of dose as a function
of age for natural pyrochlores can then be corrected
by consideration of recoil track annealing. Fig. 3
illustrates such a correction when the mean life of a
track, r,, is assumed to be 100 million years. A similar
analysis of data for zircon and zirconolite gave mean
lifetimes of 400 million years and 700 million years,
respectively. This is in contrast to the mean lives determined
by Eyal and co-workers of tens of thousands of
years for phases which retain their crystallinity despite
high doses (> 10 dpa). Thus, the natural samples provide
clear evidence for annealing over geologic time
under ambient conditions. The final state of the material
depends on the mean life of the o-recoil track, a
material dependent property.
2.4. 1990 to present
The most recent work is not summarized in detail,
as the references cited provide the best description of
approach and results. During the 199Os, much effort
has been devoted to the synthesis of data for related
I. FUNDAMENTAL PROCESSES
26 R.C. Ewing/N&. Instr. and Meth. in Phys. Res. 3 91 (1994) 22-29
2 0
:!
0
t TRANSITION ZONE Q
t(l 09/Yf)
Fig. 3. Variation of dose as a function of age for natural
pyrochlores after correcting the annealing of a-decay damage
using a mean track life, r,, of 100 million years. The upper
line is the saturation dose and the lower line is the onset of
a-decay damage detectable by X-ray diffraction analysis. The
corrected dose in dpa, cdpa, includes the correction for annealing.
After Lumpkin and Ewing [.50].
structure types under different types of irradiation.
[50-521.
The study of natural zircons cul~nated with the
very detailed analysis [53] of damage in-growth in suites
of zircons from Sri Lanka (570 million years old) which
had experienced variable o-decay event doses due to
variable U and Th contents (0.06 X 1015 to 6.8 X 1015
o-decay events/m~. In addition to the physical and
optical property changes with dose determined by Holland
and Gottfried in 1955, later workers applied detailed
X-ray diffraction analysis, electron microprobe
analysis, high resolution transmission electron microscopy,
infrared spectroscopy, and X-ray abso~tion
spectroscopy to the study of this radiation-induced
transition [53-561. Murakami et al. [531described three
stages of `damage in-growth. Stage-I ( < 3 x 10rs o-decay
events/mg) is characterized by sharp Bragg diffraction
maxima with a minor ~nt~bution from the diffuse-scattering
component. Electron diffraction patterns
were sharp. Damage is dominated by the accumulation
of isolated point defects which cause unit-cell
expansion and distortion that account for most of the
decrease in density. These defects may partially anneal
over geologic time. Stage-II (3 X 1Or5to 8 X lox5 ar-decay
events/mg) is characterized by significant decreases
in the intensity of the Bragg dif~action maxima
which become asmetric due to increased contributions
from the diffuse-scattering component. High-resolution
transmission electron microscopy revealed that
the microstructure consisted of distorted crystalline
regions and amorphous "tracks" caused by o-recoil
nuclei. With increasing o-decay dose, damaged crystalline
regions are converted into aperiodic regions,
but with no further significant expansion of the unit
cell in the remaining crystalline regions. Stage-III (> 8
X 1015 o-decay events/mgl consists of material which
is entirely aperiodic as far as can be determined by
X-ray or electron diffraction. There was no evidence
for the formation of ZrO, or SiO, as final products
during the last stage of metamictization. Based on
modeled density changes, aperiodic regions continued
to experience a change in structure as they were
redamaged. During Stage-II of the process, the modeled
density of aperiodic regions changed from 4.5
g/cm3 to 4.1 g/cm3. Fission fragment damage does
not contribute to the process of metamictization. The
amorphization process is consistent with a model for
the multiple overlap of displacement cascades, suggesting
amorph~ation occurs as a result of defect accumulation
rather than directly within a single displacement
cascade. Comparison of results for natural zircon with
those for Pu-doped zircon [57] showed that dose-rate
variations (even as great as a factor of 10') had no
substantial effect on the damage accumulation process.
Unit cell parameters increased and density decreased
more for the Pu-doped zircon than for natural zircon
in the early stages of damage accumulation (< 3 X 1015
a-decay events/mg), suggesting that annealing of point
defects in the early stages of the damage accumulation
process occurs in natural zircon under ambient conditions.
This accounts for the distinct sigmoidal shape of
the damage curves for natural zircon and the apparent
incubation period before the onset of amorphization.
Most recently, particle irradiations (2 MeV He+
[58]; 0.8 MeV Ne+, 1.5 MeV Art, 1.5 MeV Kr+`, 0.7
MeV Kr+, 1.5 MeV Xe+ 1591) have been used to
induce amorphization in zircon, and the results are
compared to the results of a-decay event damage [52].
The time scales in this wmp~ison range from 0.5
hours to 570 million years! The defects are produced in
varying concentrations along each particle track, depending
on mass and energy. The amo~hization process
is nearly independent of the damage source (o-decay
events vs. heavy-ion beam irradiations) and is consistent
with models based on the multiple overlap of
particle tracks, indicating amorphization occurs as a
result of a critical defect concentration [52]. These
results from natural zircons, Pu-doped zircons, and
ion-beam irradiated zircons provide a basis for an
understanding of not only the long-term behavior of
materials in natural and man-made radiation environments
(e.g., minerals used in geochronoIo~ and nuclear
waste forms), but also the relatively short-term
processes associated with ion-beam modification of
27
ceramic properties for specific technologic applications
(e.g., optical waveguides).
The work on mineral phases has continued and has
been extended beyond those phases which become
metamict due to their natural abundances of U and
Th. Minerals now provide a wide variety of structure
types by which irradiation-induced amorphization can
be investigated as a function of structural topology,
structural complexity and bond type. Initial studies by
Wang et al. 1601and Wang and Ewing [61-631 have
shown the relationship between structural topology
(e.g., degree of ~rnple~~), melting point (e.g., bond
strength), and bond type (e.g., degree of ionic&> to the
critical amorphization dose. A much more systematic
study of twenty-five silicate structure types [64] has
shown that melting point, structure (e.g., degree of
polymerization and atomic packing), and bonding (e.g.,
proportion of Si-0 bonds) correlate with the critical
amorphization dose (Fig. 4). As an example, Fig. 5
illustrates the variation of the mean amorphization
dose,with the degree of silicate polymerization (DOSP).
The required dose for amorphization decreases as the
connectivity of the SiO, tetrahedral monomers increases
(DOSP = 0 for structures in which the siliconoxygen
tetrahedra are not connected, that is there are
no bridging oxygens; DOSP = 3 for framework silicates
in which every corner of every silicon-oxygen tetrahedron
is connected to another silicon-oxygen tetrahedron,
that is there are 100% bridging oxygens in the
structure).
Despite the recent emphasis on ion beam irradiations,
mineralogists still continued their detailed studies
of natural phases. Hawthorne and colleagues [651
completed a monumental description of a-decay event
damage in titanite, CaTi(SiO,)O, a phase that shows
only the earliest stages of damage, as uranium concenI
Amomhkafion Dose 1
Fig. 4. Summary of the critical materials properties that
correlate with the critical amorphization dose based on ion
beam irradiations of different silicate structure types. After
Eby et al. [64].
r-square = 0.79
1600j+--bpT12004 I
~~ , / 1 , I `p-0.5
0.0 0.5 1.0 I.5 2.0 2.5 3.0 5
Tetrahedral Polymerization (DOSP)
Fig. 5. The mean values (error bars are standard deviations)
of the critical amorphization dose as a function of the degree
of SiO, group polymerization (DOSP). Polymerization increases
with increasing values of DOSP. DOSP = 0 for orthosilicates
(no polymerization); DOSP = 1.0 for single chain
silicates; DOSP 5 1.5 for ring silicates; DOSP = 2.0 for sheet
silicates; DOSP ==3 for framework silicates. After Eby and
others [64].
trations are low (up to 0.2 wt.%/,).I describe the study
as monumental because the suite of ten specimens was
examined by a full array of techniques - electron
microprobe analysis, thermal gravimetric and evolved
gas analysis, X-ray powder diffraction, single-crystal
X-ray diffraction structure refinements, Mijssbauer
spectroscopy, magic-angle spinning NMR for *`Si, high
resolution transmission electron microscopy, and X-ray
absorption spectroscopy. One of the most interesting
results was that the damage process is accompanied by
the reduction of Fe3+ to Fe*+, which resides mainly in
the aperiodic domains.
Work also has continued on minerals of interest for
nuclear waste form development. Despite its relative
rarity, zirconolite, CaZrTi,O,, has been a subject of
continuous study. Recent work using EXAFS/XANES
has systematically examined the nearest neighbor environments
of Ca, Zr, Ti, and U in fully metamict
specimens. Farges and others 1661have confirmed that
the stereochemistry of large cations such as Zr4'+ and
Th4'in fact does play an important role in controlling
the short-range environments of these cations; however,
the structural environments of these elements in
silicate and borosilicates glasses (6-coordinated environments
in glasses quenched horn a me&) are clearly
different from radiation-induced aperiodic phases. Such
studies emphasize the importance of structural descrlptions
of the aperiodic state that allow one to distinguish
between different amorphization processes.
In phosphates, high performance liquid chromatography
has been used to determine the degree of polymerization
of PO, groups in aperiodic solids, e.g., the
I. FUNDAMENTAL PROCESSES
28 R.C. Ewing/Nucl. Imtr. and Meth. in Phys. Res. B 91 (1994) 22-29
mineral griphite, as well as in quenched glasses and
radiation-induced aperiodic phases [67,68].
3. Future work
Future work will inevitably mean the examination of
more complex ceramic structures in carefully controlled
ion beam irradiation experiments. There are
three outstanding areas of research that will require
attention:
1) We must determine the displacement energies of
the different atoms in complex ceramics in order to
model basic damage parameters, such as the range of
particles, the distribution of defects along the path of
the particle, and finally, the cascade geometry.
2) We must complete systematic studies, particularly
at low temperatures, of related structure types,
over a range of compositions in order to investigate the
roles of structural topology, bond type/strength, and
defect mobility during the relaxation (1O-11 to lo-r2 s)
and annealing (seconds to millions of years) stages of
irradiated ceramics.
3) We must investigate the aperiodic structure of
radiation-induced amorphized materials and compare
them to other aperiodic structures, e.g., quenched melts
or pressure-induced amorphized solids.
Acknowledgements
This summary is written in tribute to Professor
Adolf Pabst whose work and encouragement focused
me on the subject of metamict minerals, a topic considered
esoteric even by mineralogists. The results summarized
in this paper come from nearly twenty-years of
collaborations, as indicated by the co-authors of the
papers in the references. Much of the insight into
radiation damage in ceramics and minerals comes from
these colleagues and students. Any errors and shortcomings
in this paper are the responsibility of the
author. This work has been supported by the Office of
Basic Energy Sciences of the Department of Energy,
Grant DE-FG03-93ER45498.
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I. FUNDAMENTAL PROCESSES