ISSN:
1432-2021
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Notes:
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 A 1−m B 2O6(O,OH,F)1−n ·pH2O. As defined by the XRD intensity ratio I/I 0 , both the saturation dose (for which I/I 0 =0.1−0.0) and the dose which signifies the initial loss of crystallinity (for which I/I 0 =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, τa, 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/I 0 is observed to decrease exponentially to zero over the dose range 0.02–1.0 × 1016 alpha-events/mg. The relationship between I/I 0 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 0.04–1.7 × 1017 alpha-events/mg. Using a value of τa=108 years, this dose range is corrected down to 0.02–1.2 × 1016 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 EXAFS/XANES studies (Greegor et al. 1985a, 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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00201325
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