Raman and FTIR spectroscopy of synthetic amphiboles; 1, The OH librational bands and the determination of the OH-F content of richterites via Raman spectroscopy

• Published in: Canadian mineralogist Vol. 59; no. 1; pp. 31 - 41
• Main Authors: Della Ventura, Giancarlo, Hawthorne, Frank C, Mihailova, Boriana, Sodo, Armida
• Format: Journal Article
• Language: English
• Published: Mineralogical Association of Canada 08-12-2020
• Subjects: alkali metals; amphibole group; chain silicates; clinoamphibole; crystal chemistry; fluorine; FTIR spectra; halogens; hydroxyl ion; infrared spectra; metals; Mineralogy; minor elements; potassic composition; Raman spectra; richterite; rubidium; silicates; spectra; synthetic materials
• ISSN: 0008-4476
• DOI: 10.3749/canmin.1900090

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.