ORIGINAL PAPER V. P. Solntsev Æ E. G. Tsvetkov Æ A. I. Alimpiev R. I. Mashkovtsev Valent state and coordination of cobalt Ions in beryl and chrysoberyl crystals Received: 27 September 2002 / Accepted: 28 August 2003 Abstract We have studied the polarized optical absorption and EPR spectra of Co-doped beryls grown by hydrothermal, flux, and gas-transport methods, and chrysoberyl grown by the Czochralski method. In beryls three groups of bands, belonging to three vari- ous Co centers, were distinguished by analysis of the absorption band intensities. The first group, bands with maxima at 22 220 (E ? c), 17 730 (E k c), and 9090 (E k c), 7520 (E ? c) cm )1 are due to Co 2+ in octahedral site of Al 3+ . The second group is bands at 18 940, 18 250, 17 700 (E ? c), 18 300, 17 700, 17 000 (E k c) and 8830 (E ? c), 7350 (E k c) cm )1 and 5320 (E ? c), 3880 (E k c) cm )1 , which are caused by Co 2+ in tetrahedral site of Be 2+ . A weak wide band in flux and gas-transport beryl in the region of 12 500– 8300 cm )1 (E k, ? c) is related to Co 3+ in octahedral Al 3+ site. In hydrothermal beryl, bands 13 200 (E ? c), 10 900 (E k c), and 8500 (E ? c) cm )1 are caused by an uncontrolled impurity of Cu 2+ ions. For Co-doped chrysoberyl one type of center of Co has been established: Co 2+ in the octahedral site of Al 3+ . In the approximation of the trigonal field with regard to Trees’ correction, the energy levels of Co 2+ have been calculated in octahedral and tetrahedral coordi- nation. There is good agreement between the obtained experimental and calculated data. The polarization dependence of the optical absorption bands is explained well in terms of the spin-orbit interaction. Keywords Beryl  Chrysoberyl  Optical absorption  EPR  Cobalt Introduction It is known that ions Ni and Co can occupy both octahedral and tetrahedral sites in beryl and chrysoberyl structures (Solntsev 1981). In this work the valency and coordination of Co ions in these structures are studied using EPR and optical absorption spectroscopy. Beryl crystallizes in a hexagonal space group P6/ mcc ¼ D 2 6h (Morosin 1972). The structure is sixfold rings [Si 6 O 18 ] consisting of SiO 4 tetrahedra connected with each other by BeO 4 tetrahedra and AlO 6 octahedra. The point symmetry of Si, Be, and Al sites is C s , D 2 , and D 3 , respectively. In chrysoberyl structure (space group Pnma ¼ D 2h 16 ) there are two types of the oxygen octahedron in which Al 3+ ions occupy sites with symmetry C i and C s , whereas Be 2+ ions (point symmetry C s ) occupy tetra- hedral sites (Farrell et al. 1963). Valence state and coordination of Co ions in hydrothermal synthetic beryl have been discussed in some works (Solntsev 1981; Evdokimova et al. 1989; Taran and Rossman 2001). The available interpretations of the optical spectra, EPR spectra, and data of X-ray diffraction experiment sug- gest several sites for Co 2+ and Co 3+ in the structure and different explanations for the optical absorption bands. Sontsev (1981) proposed that intense bands at 22 200 (E ? c), 17 730 (E k c) cm )1 , and at 9090 (E k c), and 7520 (E ? c) cm )1 were due to Co 2+ substituting for the Al 3+ ion in the octahedral site: transitions 4 T 1g ( 4 E) fi 4 T 1g ( 4 A 2 , 4 E) and 4 T 1g ( 4 E) fi 4 T 2g ( 4 A 1 , 4 E), respectively. Weak bands at 18 940, 18 250, 17 700 (E ? c), 18 300, 17 700, 17 000 (E k c) cm )1 , and 8830 (E ? c), 7350 (E k c) cm )1 and 5320 (E ? c), 3880 (E k c) cm )1 were related to Co 2+ , which occupy the Be tetrahedral site: transitions 4 A 2 ( 4 A) fi 4 T 1 ( 4 B 2 , 4 B 3 , 4 B 1 ) and 4 A 2 ( 4 A) fi 4 T 1 ( 4 B 2 , 4 B 3 , 4 B 1 ) and 4 A 2 ( 4 A) fi 4 T 2 ( 4 B 2 , 4 B 3 , 4 B 1 ), respectively. The Co 3+ ions in the octahedral site were responsible for the weak broad absorption band in the region 12 500–8300 cm )1 (E k; ? c). Taran and Rossman (2001) also assigned Phys Chem Minerals (2004) 31: 1–11 Ó Springer-Verlag 2004 DOI 10.1007/s00269-003-0363-x V. P. Solntsev Æ E. G. Tsvetkov Æ A. I. Alimpiev R. I. Mashkovtsev (&) Institute of Mineralogy and Petrography, Novosibirsk 630090, Russia e-mails: rim@uiggm.nsc.ru; tsvetkov@online.sinor.ru