86.
High pressure/temperature annealing experiments are used to determine diffusivities of H
+ and D
+ in non-stoichiometric spinel, a low-pressure analogue for nominally anhydrous minerals in Earth’s mantle. Data are fitted to the following Arrhenius law: Diffusivity (m
2/s)?=?4?±?1?×?10
?12 exp(?54?±?2 kJ?mol
?1/RT). At low temperatures, H
+ and D
+ diffusion in non-stoichiometric spinel is charge balanced by flux of O vacancies, with infrared data consistent with protonation of both octahedral and tetrahedral O–O edges in non-stoichiometric spinel, and additional fine structure due to Mg–Al mixing and/or coupling of structurally incorporated H
+ with cation vacancies. Absence of changes in the fine structure of O–H absorption bands indicates that H
+ can become locally coupled and uncoupled to other defects during bulk diffusion. As such, proton conductivity in spinel group minerals, arising from faster flux of uncoupled H
+, can only be calculated from H
+ mobility data if the extent of defect coupling is constrained.
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