Gemmological investigation of a synthetic blue beryl: a multi-methodological study

摘要

A multi-methodological investigation of a synthetic Cu/Fe-bearing blue beryl [IV(Be_2.86Cu_0.14)_=3.00 VI(Al_1.83Fe_0.143+Mn_0.032+Mg_0.03)_=2.03 IV(Si_5.97Al_0.03)_=6.00O₁₈·(Li_0.12Na_0.04·0.40H₂O)] has been performed by means of gemmological standard testing, electron microprobe chemical analyses, laser ablation inductively coupled plasma mass spectroscopy, thermo-gravimetric analyses, infrared spectroscopy and single-crystal X-ray diffraction in order to determine the gemmological properties, crystal structure and crystal-chemistry of this material. The increasing production of marketable hydrothermal synthetic beryls with ‘exotic’ colours and the small number of studies on the accurate location of chromophores in the crystal structure inspired this multi-methodological investigation. The X-ray structural refinements confirm that the space group of the Cu/Fe-bearing blue beryl is P6/mcc, with unit-cell parameters: 9.2483 a 9.2502 Å and 9.2184 c 9.2211 Å. The analysis of the difference Fourier maps of the electron density suggests that Cu is located at the tetrahedral site (Wyckoff 6f position) along with Be, whereas Fe shares the octahedral site with Al (4c position). No evidence of extra-framework Cu/Fe-sites (i.e. channel sites) has been found. The Li is probably located at the extra-framework 2b site. Infrared spectra show that the H₂O molecules are present with two configurations: one with the H···H vector oriented ||[0001] and the other with H···H vector oriented [0001].