Neoproterozoic amorphous "ekanite" (Ca2Th0.9U0.1Si8O20) from Okkampitiya, Sri Lanka: A metamict gemstone with excellent lead-retention performance
| Authors | |
|---|---|
| Year of publication | 2017 |
| Type | Article in Periodical |
| Magazine / Source | Geology |
| MU Faculty or unit | |
| Citation | |
| Doi | http://dx.doi.org/10.1130/G39334.1 |
| Keywords | PB ZIRCON AGES; RADIATION-DAMAGE; ALTERED ZIRCON; GEOCHRONOLOGY; DISSOLUTION; METAMORPHISM; MICROPROBE; ACCURACY; URANIUM; SYSTEMS |
| Description | 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. |
| Related projects: |