Abstract

Unpolarized FTIR and Raman spectra were collected in the regions 4000–4600 cm–1 (NIR) and 100–4000 cm–1 from previously synthesized and characterized amphiboles in the systems richterite–fluoro-richterite, potassic-richterite–potassic-fluoro-richterite, rubidium-richterite–rubidium-fluoro-richterite, and potassic-richterite–deuterated potassic-richterite. The NIR spectra of Na-, K-, and Rb-richterites have peaks at 4325, 4210 cm–1, and 3735–3730 cm–1. All three peaks decrease in intensity with increasing F content, indicating that the higher-energy peaks are combination bands involving the principal OH-stretch at 3735–3730 cm–1 and two OH-libration modes, the frequencies of which can be calculated from the relation ωcombination ≈ ωOH + ωlibration; these are 590 and 475 cm–1, respectively. The FTIR spectra of the richterite–fluoro-richterite and potassic-richterite–potassic-fluoro-richterite series show decreasing intensity and eventual disappearance of the band at ∼600 cm–1 with increasing F in accord with the assignment of this band as due to OH-libration. Raman spectra of the potassic-richterite–deuterated potassic-richterite amphiboles in the low-energy region show the disappearance of two peaks at 585 and 473 cm–1 that we have assigned to OH-libration modes. Collectively, these spectroscopic results indicate that there are two OH-libration modes at ∼590 and ∼475 cm–1 in these synthetic richterites and, by implication, in all monoclinic OH-bearing amphiboles with filled A sites. Similarly to FTIR, the relative intensities of the stretching modes associated with OH-A-OH and OH-A-F local configurations in the Raman spectra provide a reliable estimation of the F content in these A site-filled amphiboles.

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