Beryllium incorporation in corundum

The structure of corundum is not amenable to accommodating beryllium, yet Be is commonly added to corundum for purposes of colour modification. Where the main sites in corundum are octahedrally coordinated, Be is tetrahedrally coordinated in all known Be minerals. To attempt to explain this discrepancy, the configurations of Be in the Be-Al-O +/- H system were determined using density functional theory (DFT). In the Be-Al-O system, the most likely Be configuration is in triangular coordination, replacing Al and bonded to three oxygen on the basal plane (large triangle) of the M-vacant octahedral site. When H is added, OH groups form at the opposite face of the same octahedron, pointing either into or out of the site. Be-H (beryllium hydride) defects are also stable at the DIT level, with Be bonded to three O and one H. As some defects contain O-H or Be-H bonds, the DFT calculations can be corroborated by FTIR spectroscopy. Spectra recorded from 1000 corundum crystals with elevated Be confirm that bands centred at 3062 and 2490 cm(-1) correspond to Be-associated defects. Polarised spectra show that the band at 2490 cm(-1) resolves into two, one centred at 2491 cm(-1), with a shoulder at 2523 cm(-1), one of which is probably associated with a Be-H bond. The 3062 cm(-1) band resolves into at least three, including a major band at 3058 cm(-1) , a shoulder at 3140 cm(-1) and a small, sharp band at 3082 cm(-1). Whilst there are caveats relating to curve fitting and baseline subtraction, the 3058 cm(-1) band can probably be attributed to the defect where H points into the Al-vacant octahedral site, and the 3140 cm(-1 )band to the defect where the H points out. To our knowledge, this is the first occurrence of Be in triangular coordination proposed for a naturally occurring mineral. This substitution mechanism may also explain the high diffusivity of Be, as Be is effectively associated with its own Al vacancy.