Neoproterozoic amorphous "ekanite" (Ca₂Th_0.9U_0.1Si₈O₂₀) from Okkampitiya, Sri Lanka

Author(s)

Lutz Nasdala, Fernando Corfu, Doris Blaimauer, Chutimun Chanmuang, Katja Ruschel, Radek Skoda, Manfred Wildner, Richard Wirth, Manuela Zeug, E. Gamini Zoysa

Abstract

A rare gemstone form of "ekanite" from Okkampitiya, eastern Sri Lanka, has a chemical composition of Ca2Th0.9U0.1Si8O20, corresponding to that of the tetragonal mineral ekanite. The Okkampitiya material, however, has undergone amorphization and is found in a fully metamict state, which is ascribed to the long-term accumulation of high levels of self-irradiation damage (1.38 × 1020 alpha decay events per gram). The "ekanite" nevertheless does not show any post-growth chemical alteration, and the radiogenic Pb has been retained. The Okkampitiya "ekanite" yielded a Neoproterozoic 207Pb/206Pb age of 562.1 ± 0.8 Ma (uncertainty quoted at the 95% confidence level) that is concordant within the uncertainties of decay constants. The remarkable Pb-retention performance of "ekanite" contradicts the common hypothesis that metamictization in minerals results in U-Pb discordance. The exceptional chemical durability of the heavily radiation-damaged geological material described here has also implications for materials-science research. High radiation resistance, a key criterion in the search for advanced nuclear waste forms, may not be a prerequisite for high aqueous durability of a solid.

Organisation(s)

Department of Mineralogy and Crystallography

External organisation(s)

Federal Office for Radiation Protection, University of Oslo, Masaryk University, Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum, Mincraft Company

Journal

Geology

Volume

45

Pages

919-922

No. of pages

4

ISSN

0091-7613

DOI

https://doi.org/10.1130/G39334.1

Publication date

08-2017

Peer reviewed

Yes

Austrian Fields of Science 2012

104026 Spectroscopy, 105116 Mineralogy

Keywords

Portal url

https://ucrisportal.univie.ac.at/en/publications/c2aed23b-b3bf-407b-83bd-2cf4ba497822