Alpha-decay damage in minerals of the pyrochlore group

Abstract

Abstract. X-ray diffraction analysis and transmission electron microscopy have been used to study the effects of alpha-decay damage in pyrochlore group minerals, characterized by the general formula A1 ,~BzO6(O,OH,F)I -n'pH20.As defined by the XRD intensity ratio I/Io, both the saturation dose (for which I/Io =0.1-0.0) and the dose which signifies the initial loss of crystallinity (for which I/Io = 1.0-0.8) increase as a function of geologic age. The increase is attributed to annealing of isolated alpha-recoil tracks back to the original crystalline structure. The tracks have calculated mean lives, za, on the order of 108 years. In contrast, minerals which remain crystalline (e.g., uraninite, UO2) despite doses of up to 1018 alpha-events/mg have mean alpha-recoil track lives ~ 104 years (Eyal and Fleischer 1985).After correcting the calculated dose for annealing of alpha-recoil damage, I/Io is observed to decrease exponentially to zero over the dose range 0.02-1.0 x 1016 alphaevents/rag. The relationship between I/Io and "corrected" dose was used to calculate an average alpha-recoil track diameter of 4.6 nm, in which < 2600 atoms are displaced. XRD line broadening due to strain dominates the first half of the crystalline-to-metamict transition, reaching a maximum of 0.003, then decreasing to < 0.001. Line broadening due to decreasing crystallite size dominates the latter half of the transition. Estimated crystallite dimensions decrease from 450 nm to < 15 nm prior to reaching the fully metamict state. With increasing dose HRTEM images of microlites from the Harding pegmatite sequentially exhibit: 1) mottled diffraction contrast, 2) isolated 1-5 nm aperiodic areas, 3) coexisting aperiodic and crystalline areas, 4) relict" islands" of crystalline material in an aperiodic matrix, and 5) complete loss of lattice fringe periodicity. With no consideration given to alpha-recoil track fading, the transition covers a dose range of 0.04-1.7x 1017 alpha-events/mg. Using a value of za=108 years, this dose range is corrected down to 0.02-1.2 x 10 ~6 alpha-events/mg.The metamict state is characterized by a range of M-M and M-O distances which give rise to bands of diffuse scattering centered at 0.30 nm and 0.18 nm, respectively, in x-ray and electron diffraction patterns. Random image contrast shown by HRTEM is consistent with a random network type structure, an interpretation supported by EX-AFS/XANES studies (Greegor et al. 1985a(Greegor et al. , b, 1987. The structure of metamict pyrochlore consists of an aperiodic framework of corner-sharing B--O polyhedra. Compared to the crystalline precursor, the metamict state displays a reduced M-O coordination number and mean bond length, increased distortion of the B-site, and a slight increase in the average M-M distance.

Growth and Alteration of Uranium-Rich Microlite

“…In the U-rich rim, the maximum dose was approximately 2*1017 alphas/mg, suggesting that ,most of the U-rich rim should be completely metarnict in the absence of annealing. This conclusion is based on previous studies which have shown that pyrochlore-group minerals become amorphous at a dose of > 10IG alphasdmg [6,12], and is in agreement with preliminary TEM and XRD results. Our TEM investigations revealed the presence of diffuse rings in electron difiaction images, and these rings could be due to radiation damage microstructure resulting from alpha recoil collisions.…”

“…5% crystallinity) in the studied rim sample (Fig. 4) is likely the result of partial annealing [12].…”

“…5) which are observed in the U-rich rim only. Microfractting commonly takes place as a result of the crystalline-to-amorphous transition which is associated with an isotropic volume increase of -5% in pyrochlore-group minerals [6,12]. The fine cracks observed in the rim are arcuate and unevenly distributed, being particularly abundant around small inclusions of quartz and potassium feldspar.…”

Thermal annealing of natural, radiation-damaged pyrochlore

“…Mainly Na, Ca, U, Th, Y, and REEs occupy the A-site, but also minor elements like Fe 2+ , Mn, Sn 2+ , Sb, Bi, Sr, Ba, Pb, K, and Cs have been found [16]. The B-site can be occupied by Nb, Ta, Ti, Fe 3+ , Zr, and Sn 4+ [16]. O can occupy the X-as well as the Y-site, whereas OH and F only occupy the latter [16].…”

Expected radiation effects in plutonium immobilization ceramic

“…Natural pyrochlore mineral samples have received extensive study, particularly by Lumpkin, Greegor, Ewing, and co-workers (GREEGOR, 1985a;1985b;LUMPKIN, 1980;1986d;1988a;1988b;1994;. Lumpkin and Ewing analyzed alpha-decay damage in pyrochlores having a wide range of chemical compositions, from 15 locations, with ages ranging up to 1.4 billion years.…”