Argon diffusivities and solubilities in single gem-quality crystals of corundum

were determined from experiments conducted under 0.5 to ~6000bar Ar pressures

at 425 to 1200C. Surface-sensitive analyses were conducted in order to confirm

that the near-surface regions of the crystals were crystalline, unreacted and free

from structural damage. Polished single-crystal slabs and specimens with natural

facets were placed in open containers and exposed to an Ar atmosphere in a

pressure vessel or in a gas-flow tube furnace at near-atmospheric pressure. The Ar

atoms from the pressure medium diffused into the crystals to produce near-surface

concentration gradients which were directly profiled using Rutherford backscattering

spectrometry. The Arrhenius relationship was:

D (m2/s) = 1.2 x 10-20exp[-33(kJ/mol)/RT]

The activation energy was probably an apparent one which was a combination of

the effects of the lattice diffusion of Ar atoms, with trapping in point-defect

vacancies. There were no discernible differences in Ar diffusion in different

crystallographic directions, and the diffusivity did not vary as a function of the

intrinsic oxygen fugacity of the experimental vessel. The uptake gradients also

yielded lattice solubilities of argon, as represented by the concentrations at the

mineral surfaces. The solubility was of the order of 2000ppm[wt]. The Ar

solubilities were independent of the Ar pressure above 1bar, but were slightly

lower at 0.5 to 1bar. This suggested that all of the sites capable of accommodating

Ar atoms were filled at Ar pressures of ~1bar. The Ar appeared to be compatible;

although the degree of compatibility was expected to depend upon assumptions

concerning the minimum Ar fugacity required to fully populate the available

vacancies.

Lattice Diffusion and Solubility of Argon in Forsterite, Enstatite, Quartz and

Corundum. J.B.Thomas, D.J.Cherniak, E.B.Watson: Chemical Geology, 2008,

253[1-2], 1-22