Neoproterozoic amorphous “ekanite” (Ca2Th0.9U0.1Si8O20) from Okkampitiya, Sri Lanka: A metamict gemstone with excellent lead-retention performance

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

doi:10.1130/g39334.1

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 x 10(20) 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 Pb-207/Pb-206 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.

Highlights

Minerals that incorporate the unstable actinides U and Th in their crystal lattices are subject to corpuscular self-irradiation that, over geologic periods of time, may cause a crystalline to amorphous transition that is referred to as metamictization (Ewing, 1994)
The Okkampitiya “ekanite” yielded a Neoproterozoic 207Pb/206Pb age of 562.1 ± 0.8 Ma that is concordant within the uncertainties of decay constants
It is well known that structural radiation damage enhances the susceptibility of minerals to secondary loss of radiogenic Pb (Krogh and Davis, 1975; Nasdala et al, 1998; Horie et al, 2006; Smye et al, 2014)

Summary

Introduction

Minerals that incorporate the unstable actinides U and Th in their crystal lattices are subject to corpuscular self-irradiation that, over geologic periods of time, may cause a crystalline to amorphous transition that is referred to as metamictization (Ewing, 1994). 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.

Results

Conclusion