Emerald pebble 960nm water absorption

The emerald (beryl) crystal structure contains channels, parallel to the optical c-axis, enclosed by rings of Si tetrahedra stacked above each other in a staggered arrangement (Wood & Nassau, 1958, J. Min. Soc. Am., 53, 777). Volatile impurity molecules can be trapped within these channes for geological timescales. These include water, carbon dioxide, nitrogen and inert gases (Damon & Kulp, 1958, J. Min. Soc. Am,, 1958, 433). The water can be seen in the near and mid-irfrared spectra of most natural and hydrothermally-grown synthetic emeralds but not those grown by the flux method.

The green and red spectra are the high resolution and smoothed telluric water absorptions from the HITRAN database which have to be shifted redwards by 27nm to match the emerald. This shift results from the influence on the water of the ions in the channel walls and is different for the water molecules aligned with their symmetry axes parallel or perpendicular to the c-axis.

These nir spectral measurements were made using white light filtered with an RG715 filter and used a minimum length of spectrometer fibre to avoid the effects of the 940nm OH glass absorption. The 960nm water band is clearly shown and there is even, perhaps, some evidence for a rather weak feature at 840nm that is visible in all five of the independent spectra used to build the purple line spectrum in the plot.

Damon & Kulp propose that the noble gases found in these channels, He_3 and A_40, are the products of radioactive decay in the magmatic rocks at the time of crystal formation and so represent the level of radioactivity in the magma at the time of crystallisation. The dependence of the abundance of He in beryl was found to increase with the age of the rocks by Lord Rayleigh (1933, Proc. Roy. Soc. A142, 370). In contrast to the later conclusions of Damon & Kulp, Rayleigh had concluded that the beryl had gradually accumulated helium over geological time after the crystal had formed.

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