Single-crystal polarised-light FTIR study of an historical synthetic water-poor emerald

This paper describes a single-crystal polarised-light FTIR study of the original emerald synthesised by Hautefeuille & Perrey (1888) using the flux method. The microchemical data show the analysed sample to be very close to the stoichiometric composition and homogeneous except for a significant variation in the Cr2O3 content (from 1.45 to 2.59 wt%). Trace amounts of Ti, Mg, Fe, Zn, Na, K and F were also detected. The cell parameters are (in Å) a = 9.2264(25) and c = 9.1904(2), c/a = 0.9961. Despite the synthesis conditions, FTIR spectra in the OH-stretching region show the presence of weak but significant H2O vibrations in the crystals. The polarised FTIR spectra collected with E?c consist of a unique sharp and intense band at 3643 cm-1, whereas the E//c spectra consist of two minor bands at 3643 and 3587 cm-1. These bands are assigned to the ?3 antisymmetric stretching, and ?1 symmetric stretching modes of type II water in the structural channels, respectively. These water molecules are probably associated with Li impurities in the mineral, due to the flux employed for the syntheses. Using the molar absorption coefficient ei = 27126 l * mol-1 * cm-2 from the calibration curve of Libowitzky & Rossman (1997), a water content of ~ 30 ppm is derived for the studied emerald.