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. Cr3+-ions occupy the octahedral site and Co2+ and Ni2+ the dodecahedral site. Although extra metallic Ti was included in the synthesis of Ti-bearing pyropes, only Ti4+ and no measurable Ti3+ 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(1g)(F) → 3T(1g)(P) at ~ 2000 cm-1 corresponding to V3+ on the octahedral site, absorption bands which are thought to be caused by dd-transitions of V3+ in the tetrahedral site and V4+ on octahedral and tetrahedral sites. V4+ 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 Ni2+, Co2+, and Cr3+, 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 Ti4+-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 V4+-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.