Is ``metamictization`` of zircon a phase transition?

Journal Article · Thu Jul 01 04:00:00 EDT 1999 · American Mineralogist

DOI:https://doi.org/10.2138/am-1999-7-813· OSTI ID:684593

Salje, E K.H.; Chrosch, J ^[1]; Ewing, R C ^[2]

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1. Univ. of Cambridge (United Kingdom). Dept. of Earth Sciences
2. Univ. of Michigan, Ann Arbor, MI (United States)

Metamictization is the transition from the crystalline to an aperiodic or amorphous state due to alpha-decay event damage from constituent radionuclides ({sup 238}U, {sup 235}U, and {sup 232}Th) and their daughters. However, this transformation in minerals is part of a larger class of radiation-induced transformations to the amorphous state that has received considerable recent attention as a result of ion- and electron-beam experiments on metals, intermetallics, simple oxides, and complex ceramics and minerals. Diffuse X-ray scattering from single crystals of metamict zircon reveals residual crystallinity even at high fluences (up to 7.2 {times} 10{sup 18} {alpha}-decay events/g). The experimental evidence does not suggest that radiation-induced amorphization is a phase transition. The observations are in good agreement with a nonconvergent, heterogeneous model of amorphization in which damage production is a random process of cascade formation and overlap at increasing fluence. Instead of an amorphization transition, the existence of a percolation transition is postulated. At the level of radiation damage near the percolation point, the heterogeneous strain broadening of X-ray diffraction profiles is reduced whereas the particle-size broadening increases. Simultaneously, the macroscopic swelling of the zircon becomes larger than the maximum expansion of the unit-cell parameters. A suitable empirical parameter that characterizes this transition is the flux, D{sub s}, at which the macroscopic expansion is identical to the maximum expansion of the crystallographic unit cell. In zircon, D{sub s} = 3.5{center_dot}10{sup 18} {alpha}-decay events/g.

• Sponsoring Organization: USDOE, Washington, DC (United States)
• DOE Contract Number: FG02-97ER45656
• OSTI ID: 684593
• Journal Information: American Mineralogist, Journal Name: American Mineralogist Journal Issue: 7-8 Vol. 84; ISSN AMMIAY; ISSN 0003-004X
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ALPHA DECAY
AMORPHOUS STATE
CRYSTAL STRUCTURE
PHASE TRANSFORMATIONS
RADIATION EFFECTS
THORIUM 232
URANIUM 235
URANIUM 238
ZIRCON