Micro-Raman spectroscopy of gem-quality chrysoprase from the Biga–Çanakkale region of Turkey

The commercial quantities of gem-quality dark green chrysoprase are found as the fracture fillings covered with a weathering crust in the silicified serpentinites throughout the border of a metamorphic zone in the Biga–Çanakkale region of Turkey. However, the green-stained opaque quartz materials are also present in the same deposit, but these materials are common and in low-demand according to chrysoprase in terms of gemmological importance. Thus, it is necessary to distinguish these two similar materials from each other non-destructively. In addition, all chrysoprase roughs in this deposit also have alpha-quartz and moganite inclusions. Accordingly, dispersive (visible) confocal micro-Raman spectroscopy (DCμRS) allows us to distinguish clearly the chalcedonic-quartz silica phase (fibrous quartz (chalcedony)) from the crystalline-quartz silica phase (fine-grained alpha-quartz) in the case of both quartz inclusions in the chrysoprase material and itself of the green-stained quartz material in the same deposit.This study characterizes the Biga chrysoprase (Turkey) in terms of silica building phases, chemical content, and individual Raman bands, using several destructive and non-destructive analytical techniques.The Raman spectra show that the most characteristic intensive and the widest Raman bands peaked at about 498 and 460 cm−1 can be inferred to ν2 doubly symmetric bending mode of [SiO4/M] centers. The “M” includes the some cationic substitutions of Si by Fe, Cr, Mn, As, Ni, Pb, Sb, and Zn, and K and Na as well. The second characteristic Raman band peaked at about 206 cm−1 can be inferred to single translational libration mode. The last readable Raman bands peaked at about 139 and 126 cm-1 can be inferred to doubly translational libration modes as well. In addition, the weaker Raman bands peaked at about 1577, 1430, 1303, 1160, 1082, 549, 394, 352, and 259 cm−1 are also present.As a result, the dispersive confocal micro-Raman spectrum of chrysoprase is directly related to its silica building phases and trace element implications.

► Mineralogical characterization of the Biga (Çanakkale) chrysoprases from Turkey. ► A total of 14 vibrational micro-Raman bands of Biga chrysoprase were established. ► Five of these bands, at about 498, 460, 206, 139, and 126 cm-1, are distinctive. ► Fe and Cr ions modify the chrysoprase green colour by an adding brownish hue. ► Opalline-quartz (opal-CT, opal-C) silica phases do not exist in Biga chrysoprase.