Abstract
Six synthetic NaScSi2O6–CaNiSi2O6 pyroxenes were studied by optical absorption spectroscopy. Five of them of intermediate (Na1−x , Ca x )(Sc1−x , Ni x )Si2O6 compositions show spectra typical of Ni2+ in octahedral coordination, more precise Ni2+ at the M1 site of the pyroxene structure. The common feature of all spectra is three broad absorption bands with maxima around 8,000, 13,000 and 24,000 cm−1 assigned to 3 A 2g → 3 T 2g, 3 A 2g → 3 T 1g and →3 T 1g (3 P) electronic spin-allowed transitions of VINi2+. A weak narrow peak at ∼14,400 cm−1 is assigned to the spin-forbidden 3 A 2g → 1 T 2g (1 D) transition of Ni2+. Under pressure the spin-allowed bands shift to higher energies and change in intensity. The octahedral compression modulus, \( k^{{{\text{loc}}}}_{{{\text{Ni}}{\left( {{\text{M}}1} \right)}}} , \) calculated from the shift of the 3 A 2g → 3 T 2g band in the (Na0.7Ca0.3)(Sc0.7Ni0.3)Si2O6 pyroxene is evaluated as 85±20 GPa. The Racah parameter B of Ni2+(M1) is found gradually changing from ∼919 cm−1 at ambient pressure to ∼890 cm−1 at 6.18 GPa. The Ni end-member pyroxene [(Ca0.93 Ni0.07)NiSi2O6] has a spectrum different from all others. In addition to the above mentioned bands of Ni2+(M1) it displays several new relatively intense and broad extra bands, which were attributed to electronic transitions of Ni2+ at the M2 site. In difference to CaO8 polyhedron geometry of an eightfold coordination, Ni2+(M2)O8 polyhedra are assumed to be relatively large distorted octahedra. Due to different distortions and different compressibilities of the M1 and M2 sites the Ni2+(M1)- and Ni2+(M2)-bands display rather different pressure-induced behaviors, becoming more resolved in the high-pressure spectra than in that measured at atmospheric pressure. The octahedral compression modulus of Ni2+(M1) in this end-member pyroxene is evaluated as 150 ± 25 GPa, which is noticeably larger than in Ni0.3 pyroxene. This is due to a smaller size and, thus, a stiffer character of Ni2+(M1)O6 octahedron in the (Ca0.93Ni0.07)NiSi2O6 pyroxene compared to (Na0.7Ca0.3)(Sc0.7Ni0.3)Si2O6.
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Notes
Rossman et al. (1981) found this bands in a variety of complex nickel oxides appearing at ∼720–740 nm (13,500–13,900 cm−1), i.e. at somewhat lower energies than in our case. These differences may be due to different degree of covalence of Ni–O bonds in oxides and pyroxene structures. Note that Ito and Sone (1985) also attributed a band at 721 nm (13,870 cm−1) in spectrum of [Ni(H2O)6]2+ to 3 A 2g → 1 E g transition of Ni2+.
Tejedor-Tejedor et al. (1983) attributed some features in diffuse reflectance spectra of nickel-bearing phyllosilicates to Ni2+ in fourfold coordination replacing Si4+. However, latter this attribution was discriminated by Manceau and Calas (1987), who found “no spectroscopic support of tetrahedrally coordinated Ni in phyllosilicates”.
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Taran, M.N., Ohashi, H. & Koch-Müller, M. Optical spectroscopic study of synthetic NaScSi2O6–CaNiSi2O6 pyroxenes at normal and high pressures. Phys Chem Minerals 35, 117–127 (2008). https://doi.org/10.1007/s00269-007-0202-6
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DOI: https://doi.org/10.1007/s00269-007-0202-6