Single-crystal IR- and UV/VIS-spectroscopic measurements on transition-metal-bearing pyrope: the incorporation of hydroxide in garnet

Abstract

Pyrope single crystals doped with transition-metal ions (Co, Cr, Ni, Ti and V) were synthesised in a piston-cylinder device at 950–1050°C and 25 kbar. Stoichiometric oxide mixtures were used as starting materials and distilled water was used as a fluid flux. Crystals up to 2 mm in size were grown. Microprobe analysis and optical absorption spectroscopy were used to determine on which positions and in which oxidation states the transition-metal ions are incorporated in the pyrope structure. Cr^(3+)-ions occupy the octahedral site and Co^(2+) and Ni^(2+) the dodecahedral site. Although extra metallic Ti was included in the synthesis of Ti-bearing pyropes, only Ti^(4+) and no measurable Ti^(3+) could be stabilised on the octahedral site. The optical absorption spectra of V-bearing pyropes show, in addition to the spin-allowed dd-transitions ^3T_(1g)(F) ^3T_(2g)(F) at ~ 17000 cm_(-1) and ^3T_(2g)(F) ^3T_(1g)(P) at ~ 20000 cm^(-1) corresponding to V^(3+) on the octahedral site, absorption bands which are thought to be caused by dd-transitions of V^(3+) in the tetrahedral site and V^(4+) on octahedral and tetrahedral sites. V^(4+) was not observed in silicate garnets before. IR spectra in the OH- stretching region between 4000 and 3000 cm^(-1), obtained on pyrope single-crystals which only contain divalent and trivalent transition-metal ions like Ni^(2+), Co^(2+), and Cr^(3+), are similar to that normally shown by end-member pyrope (Geiger et al., 1991). At room temperature the spectra show a single band at ≈ 3630 cm^(-1), which splits at ~ 79 K into two bands of smaller FWHM's at ∼ 3618 cm^(-1) and 3636 cm^(-1). These bands are assigned to OH–stretching modes resulting from the hydrogarnet substitution. The spectra of Ti^(4+)-bearing pyrope measured at 298 K show four OH–stretching bands at approximately 3686, 3630, 3567 and 3527 cm^(-1). At ∼ 79 K the band at 3630 cm^(-1) splits into two narrow bands at 3636 cm^(-1) and 3614 cm^(-1). This suggests that additional OH- substitutional mechanisms occur in Ti-containing garnets. In the IR spectrum of a V^(4+)-bearing pyrope the same number of OH-stretching bands is observed, suggesting that higher charged cations cause additional OH- substitutions and increased OH- concentrations in garnet. The IR spectra of most natural pyrope-rich garnets appear to be different from those of the synthetics, which suggests that they are not characterised by the hydrogarnet substitution. However, the OH–substitution mechanism and concentrations in garnets from grospydite or similar parageneses are similar to those of the synthetics, which may reflect their formation in water-rich environments.

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