Gem News International Gems & Gemology, Spring 2018, Vol. 54, No. 1

Color Origin of Phenakites from the Ural Emerald Mines

Anatoliy G. Nikolaev, Nazym M. Nizamutdinov, and Michael P. Popov

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Phenakite with beryl and yellow phenakite.
Figure 1. Phenakite with beryl in mica (left) and yellow phenakite (right). Photos by Mikhail Popov; fields of view 5 cm.

Phenakite (Be2SiO4) was first discovered at Russia’s Ural emerald mines, and was chemically identified in 1833. The mineral is widely distributed in beryllium deposits, but crystals of gem quality or large size are rare. At the Ural mines, however, there are large transparent crystals that can be used in jewelry. Jewelry-grade phenakite is generally colorless or yellowish brown (figure 1; see M.P. Popov et al., “Features of phenakite mineralization from the Ural emerald mines,” Bulletin of the Ural Branch of the RMS, No. 13, 2016, pp. 105–111). The main mineral inclusions are chlorite, talc, phlogopite, actinolite, and ilmenite; fluid inclusions with a gas-liquid composition also occur.

The color origin of phenakite using optical absorption spectroscopy has not been studied in detail. Many authors agree that the coloration is associated with electron-hole centers (see A.S. Marfunin, Spectroscopy, Luminescence and Radiation Centers in Minerals, 1975, Nedra, Moscow). This is indicated by the disappearance of color when phenakite is exposed to either ultraviolet radiation or high temperatures. According to one theory of absorption, this is related to the oxygen vacancies that have captured electrons (again, see Marfunin, 1975). Other researchers believe that the absorption features are associated with bridging electronic centers such as Al-O-Al, which are formed by isomorphous replacement of silicon by aluminum ions in the crystal lattice, in amounts up to 0.5% (see A.N. Platonov, Nature of Coloring of Minerals, 1976, Naukova Dumka, Kiev).

Absorption spectra of colorless and yellow-brown phenakites.
Figure 2. Absorption spectra of colorless phenakites (blue trace) and yellow-brown phenakites (black trace).

To determine the color origin of yellow-brown phenakites, we obtained optical absorption spectra of colorless and yellow-brown phenakite from the Mariinsky deposit. These spectra were collected in the 185–700 nm wavelength range, at room temperature, on a specialized Shimadzu UV-3600 spectrophotometer. A broad absorption band was noted in the 225–325 nm range of the yellow-brown phenakite spectrum, with a maximum at 268 nm. Absorption in the ultraviolet range of the optical spectrum was not detected in transparent colorless phenakites (figure 2).

Phosphorus ions replace silicon ions in a tetrahedral structure to form a [PO4]4– complex in yellow-brown phenakites.
Figure 3. Phosphorus ions replace silicon ions in a tetrahedral structure to form a [PO4]4– complex in yellow-brown phenakites. This complex is paramagnetic and gives several signals in the range of 320–340 mT. Splitting of the hyperfine structure of the signal occurs; lines 1, 2, and 3 appear and indicate the existence of the [PO4]4– radical. When the crystal rotates along its c-axis, it changes the intensity of these lines.

To study electron-hole centers and to reveal color centers in phenakite, crystals were examined using electron paramagnetic resonance (EPR). EPR is an informative method for studying dissymmetry of crystals, the role of the symmetry elements of the spatial group, and the distribution of impurity ions and point defects in the bulk of the crystal. Dissymmetry of crystals, as a result of the uneven distribution of point defects in the process of crystal growth, is a widespread phenomenon (see G.R. Bulka et al., “Dissymmetrization of crystals: Theory and experiment,” Physics and Chemistry of Minerals, Vol. 6, 1980, pp. 283–293; R.A. Khasanov et al., “Derivation of the conditions for equivalent positions in crystals: The dissymmetrization of barite by electron spin resonance,” Crystallography Reports, Vol. 57, No. 5, 2012, pp. 751–757; J.M. Hughes et al., “Dissymmetrization in tourmaline: the atomic arrangement of sectorally zoned triclinic Ni-bearing dravite,” The Canadian Mineralogist, Vol. 49, 2011, pp. 29–40). We used an Adani CMS8400 spectrometer (frequency ν=9, 4 GHz) at room temperature to perform EPR on our samples. During an experiment on the EPR spectra of the impurity radical [PO4]4– phenakite of deposits in Volynskii (see A.I. Novozhilov et al., “Electron paramagnetic resonance in irradiated phenakite Be2SiO4,” Journal of Structural Chemistry, Vol. 11, 1970, pp. 393–396), we found a difference in intensity of the lines of three magnetically nonequivalent centers for the impurity radical. The paramagnetic center [PO4]4– was only detected in the yellow-brown crystals (figure 3), and active centers were absent in colorless specimens. From this we concluded that the origin of the yellow-brown color of phenakite is associated with a paramagnetic complex [PO4]4– that produces absorption bands in the ultraviolet region, which in turn leads to the formation of this color in the mineral.

These results can be used in gemological identification of phenakites from the Ural emerald mines.

This study was performed in the context of the Russian Government Program of Competitive Growth of Kazan Federal University, “Paleogeodynamics and evolution of structural-material complexes in the formation of the continental-type crust ...,” theme no. 0393-2016-0019.

Anatoliy G. Nikolaev and Nazym M. Nizamutdinov are affiliated with Kazan (Volga Region) Federal University in Kazan, Russia. Michael P. Popov is with Ural State Mining University, Institute of Geology and Geochemistry, in Yekaterinburg, Russia.