Formation of nanoscale Th-coffinite
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Artur P. Deditius
,
Véronique Pointeau
Abstract
U-thorite, (Th,U)SiO4, from Ambohijatrova Masindray, Madagascar, was investigated to understand the behavior of Th and U during recrystallization of amorphous radiation-damaged, (Th,U)- orthosilicates. Optical microscopy and electron microprobe analyses reveal two types of U-thorite: (i) large (about 1 cm), orange, amorphous grains with composition: (Th0.88±0.02U0.09-0.01Pb0.029±0.002 REE0.01±0.001)1.00±0.01Si1.00±0.01; and (ii) green, microcrystalline U-thorite with composition: (Th0.76±0.05U0.08±0.01Ca0.07±0.01Pb0.014±0.005REE0.009±0.001)0.92±0.07Si1.12±0.06. Ca-free U-thorite-(i) is enriched in Th, U, and Pb (7.1, 1.2, and 1 wt%, respectively), and depleted in Si (3.0 wt%) compared to U-thorite-(ii). Recrystallization of U-thorite-(i) resulted in fracturing that facilitated migration of mobilized Th and U over a distance of about 300 μm, as evidenced by precipitation of U-thorite-(ii) in the fractures in associated apatite and garnet. Transmission electron microscopy observations and selected-area electron diffraction (SAED) patterns confirm that U-thorite-(i) is amorphous. U-thorite-(ii) forms: (1) single crystals (>1 μm in size) with variable amounts of amorphous material; or (2) randomly oriented, nanocrystalline aggregates (5-10 nm in size). TEM-EDX analyses show that the Th/U ratio in U-thorite-(i) and U-thorite-(ii) is ~6. High-angle annular dark-field scanning TEM (HAADF-STEM) and high-resolution TEM reveal that nanocrystalline Th-coffinite (20-40 nm in size) with Th/U ratio = 0.6, formed during recrystallization of U-thorite-(i). The calculated chemical Th-U-Pb ages of Uthorite-( i) range from 2.1-1.9 Ga and from 1.8-1.6 Ga, whereas U-thorite-(ii) ages range from 1.6-0.5 Ga. The calculated cumulative radiation dose for U-thorite-(i) varies from 1.6-1.8 × 1018 α-decay events/mg, which is equivalent to 136-152 displacements per atom (dpa), and for U-thorite-(ii) from 3-4.4 × 1017 (α-decay events/mg) (=27-37 dpa). The cumulative dose for Th-coffinite is 9.8 × 1017 α-decay events/mg (84 dpa).
© 2015 by Walter de Gruyter Berlin/Boston
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Artikel in diesem Heft
- Veatchite: Structural relationships of the three polytypes
- Falsterite, Ca2MgMn2+2 (Fe2+0.5Fe3+0.5)4Zn4(PO4)8(OH)4(H2O)14, a new secondary phosphate mineral from the Palermo No. 1 pegmatite, North Groton, New Hampshire
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- Zaccagnaite-3R, a new Zn-Al hydrotalcite polytype from El Soplao cave (Cantabria, Spain)
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- TOF-SIMS and electron microprobe investigations of zoned magmatic orthopyroxenes: First results of trace and minor element analysis with implications for diffusion modeling
- Titanium in muscovite, biotite, and hornblende: Modeling, thermometry, and rutile activities of metapelites and amphibolites
- Polysaccharide-catalyzed nucleation and growth of disordered dolomite: A potential precursor of sedimentary dolomite
- High-pressure and high-temperature phase transitions in FeTiO3 and a new dense FeTi3O7 structure
- Compressibility and thermal expansion of hydrous ringwoodite with 2.5(3) wt% H2O
- Vibrational and elastic properties of ferromagnesite across the electronic spin-pairing transition of iron
- Electronic spin states of ferric and ferrous iron in the lower-mantle silicate perovskite
- Experimental VNIR reflectance spectroscopy of gypsum dehydration: Investigating the gypsum to bassanite transition
- Nature of rehydroxylation in dioctahedral 2:1 layer clay minerals
- Thermal behavior of afghanite, an ABABACAC member of the cancrinite group
- Experimental incorporation of Th into xenotime at middle to lower crustal P-T utilizing alkali-bearing fluids
- Sol-gel synthesis of nanocrystalline fayalite (Fe2SiO4)
- The heat capacity of fayalite at high temperatures
- Structural trends for celestite (SrSO4), anglesite (PbSO4), and barite (BaSO4): Confirmation of expected variations within the SO4 groups
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