Single-crystal spectra of garnets from diamondiferous high-pressure metamorphic rocks from Kazakhstan: indications for OH-, H2O, and FeTi charge transfer
Langer, Klaus; Robarick, Eckhard; Sobolev, Nikolai V.; Shatsky, Vladislav S.; Wang, Wuyi
European Journal of Mineralogy Volume 5 Number 6 (1993), p. 1091 - 1100
published: Dec 1, 1993
manuscript accepted: Jun 11, 1993
manuscript received: Oct 14, 1992
ArtNo. ESP147050506005, Price: 29.00 €
Abstract Garnet crystals from diamondiferous high-pressure metamorphic rocks of the Kokchetav Massiv, Kazakhstan, were studied by electron microprobe and single crystal microscope-spectrometry in the UV/VIS and IR spectral ranges. The crystals are quaternary solid solutions Gross48Pyr33Alm16Spess2, (I.) from garnetpyroxene- carbonate rock K7, and Gross21Pyr24Alm49Spess5, (II.) from garnet-biotite-gneiss, both with small TiO2-contents (0.3 wt%). Their yellow (I.) or deep-red colour (II.) is caused by the position of the UV absorption edge and a broad Fe2+Ti4+ charge-transfer band centered at 21500 cm-1. For comparison, a high-pressure garnet Py26, Py70Alm17Uv13 (III.), from Liaoning-50 kimberlite, NE China, was included in the IR and microprobe study. All crystals studied contained defect OH--groups, corresponding to those in synthetic OH --bearing pyropes, giving rise to a single, relatively sharp absorption band (Δṽ ½ ≃ 100 cm-1) in the range 3560 to 3610 cm-1. The exact energy position depends on the type and site fractions of ions in the OH--coordinating metal sites, A and B. This hydroxyl 'water' seems to be not uniformly distributed in the garnet crystals and ranges from 0.02 to 0.25 wt% (wt% calculated from integral intensities using εi Gross).In addition to vOH, the IR spectra show a broad band at 3400 cm-1, characteristic of (H2O)n-clusters. There seems to be a constant ratio αi,OH- /(α i0H- + α1,H2O) for the various spots on the different crystals I, II, and III.