Green to blue-green quartz from Rakowice Wielkie (Sudetes, south-western Poland) - a re-examination of prasiolite-related colour varieties of quartz
The green colour of prasiolite, defined as naturally occurring transparent macrocrystalline α-quartz with primary colouration, results from the optical absorption centred at 13,660 cm-1 and attributed to the FeVI2+→FeVI3+ intervalence charge transfer (Platonov et al. 1992). However, optical absorption spectroscopy of blue-green to green quartz from Rakowice Wielkie, Sudetes, south-western Poland, shows that its primary colouration results from the combination of this band and absorptions at 18,500 cm-1 and 16,250 cm-1. The first is assigned to a hole centre Fe4+s(Fe3+s-e-) combined with an electron centre Fe2+I6(Fe3+I6+e-), while the second from AlO44- defects. The quartz is blue-green if the 18,500 cm-1 prevails and becomes pale green if the 16,250 cm-1 band predominates. These colours seem to represent intermediate colour varieties between amethyst and prasiolite. We also suggest that spectral features of coloured quartz varieties might be useful indicators of changes in the physical- and chemical characteristics of the mineral-forming fluids.