SINGLE-CRYSTAL ABSORPTION-SPECTRA OF SYNTHETIC TI, FE-SUBSTITUTED PYROPES

Synthetic pyrope crystals up to 0.5 mm in diameter, substituted by tit anium or by titanium plus iron, were grown under defined conditions of P, T, x, fo(2) in the presence of water using a piston-cylinder devic e. The crystals were characterized by X-ray and microprobe techniques. Their single-crystal optical absorption spectra were measured by mean s of a microscope-spectrometer. Two absorption bands at 16 100 and 22 300 cm(-1) in the spectra of pale-blue Fe-free Ti-bearing pyropes, gro wn under reduced conditions, were identified as originating from spin- allowed transitions, derived from T-2(2g)-->(2)E(g) of octahedral Ti3 ions. The splitting value of the excited (2)E(g) state, 6200 cm(-1), and the crystal field parameter of Ti3+ in pyrope Delta(0) = 19 200 cm (-1) are both in agreement with literature data. In spectra of brown F e, Ti-bearing garnets, a broad band at 23 000 cm(-1) was interpreted a s a Fe-2+[8]-->Ti-4+[6] charge-transfer band. The spectral position an d width of this band agree with those observed for a FeTi charge trans fer band in natural garnets. Fe, Ti-containing garnets synthesized at relatively high oxygen fugacity (10(-11,0) atm), which permits a fract ion of Fe3+ to enter the garnet, show an additional Fe-2+[8]-->Fe-3+[6 ] charge transfer band at 19 800 cm(-1).