Vibrational spectroscopic study of the system α-Co2SiO4-α-Ni2SiO4

alpha -Co2SiO4 (Co-olivine) and alpha -Ni2SiO4 (Ni-olivine) can form a continuous series of substitutional solid solutions, in which the b-axis shows the largest expansion after substitution of Co2+ for Ni2+. The linear composition dependences for lattice parameters and vibrational frequencies suggest that the mixing of alpha -Co2SiO4 and alpha -Ni2SiO4 are nearly ideal, and both Co2+ and Ni2+ in the (Co, Ni)(2)SiO4 solid solutions distribute randomly over the M1 and M2 sites. Addition of Co2+ into alpha -Ni2SiO4 has significantly weakened the Raman intensity of the system. Together with other transition-metal silicate olivines, the frequencies of the lattice Raman modes appear to have a systematic variation with radius and crystal held stabilization energy of cation in the olivine structure, while the same trend has not been observed in the internal modes of SiO4. (C) 2001 Academic Press.