Crystal field spectra and geochemistry of transition metal ions in silicate melts and glasses

The crystal field spectra of V³⁺, Cr²⁺, Cr³⁺, Mn²⁺, Mn³⁺, Fe²⁺, Co²⁺, Ni²⁺, and Cu²⁺ doped in glasses of albite and 50 wt% albite–50 wt% diopside composition were measured. The glasses were prepared at 1 atm pressure and under controlled f_O2 by quenching the corresponding silicate melts from 1200–1500°C to room temperature. V³⁺ and Cr³⁺ occupy octahedral sites in the glasses. The sites of Cr²⁺, Fe²⁺, Ni²⁺, and Cu²⁺ appear to be strongly distorted from ideal octahedral geometry. Co²⁺ is in distorted tetrahedral coordination. The fact that only Co²⁺ occupies a tetrahedral site can be understood on the basis of the small ionic radius and low octahedral site preference energy of Co²⁺. A change of the melt composition from albite to 50 wt% albite–5O wt% diopside causes only a small increase in the crystal field splitting of the divalent ions, whereas Racah parameters and coordination numbers remain unchanged. However, the speciation of Cr in the glasses strongly depends on bulk composition. Spectroscopic evidence suggests the stabilization of Cr²⁺ in albite melt at 1500°C even under very oxidizing conditions in equilibrium with air. In a series of Co²⁺-doped glasses rangrng in composition from pure albite to pure diopside, the strongest changes in the spectra of Co²⁺ occur between the composition of pure albite and 90 wt% albite–10 wt% diopside, while only minor changes are observed in the diopside-rich portion of the system. This observation is consistent with complexing of Co²⁺ by nonbridging O atoms in the melt. Published data of crystal-melt partition coefficients of Cr³⁺ and Ni²⁺ show that their partitioning behavior is essentially determined by the difference between the crystal field stabilization energy in the melt and in the crystal. Earlier theories that assumed the partitioning behavior of transition metal ions to be controlled by octahedral site preference eneryies disagree with spectroscopic evidence. In general, the more polymerized the melt, the more are transition metal ions expected to partition preferentially into crystalline phases.