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Mid-IR spectra (4000-400 cm -1 ) of synthetic calcic amphiboles in the tremolite-pargasite series and of various natural calcic amphiboles have been investigated. The pargasite substitution, a combination of the Tschermak (= [4] Al [6] Al [6] Mg -1  [4] Si -1 ) and edenite (= [4] Al [A] Na [4] Si -1  [A] □ -1 ) substitutions, causes the following features in the region 1200-600 cm -1 . (1) Weak  [4] Al-O stretching bands appear at 895 and 815 cm -1  that are distinct from the 955 and 925 cm -1  Si-O stretching bands in tremolite. (2) There is a reduction in the intensity and frequency of the Si-O-Si symmetric bending band (=“chain breathing” mode) at 750 cm -1  in tremolite, and there is an appearance of medium-strong composite bands having a weak shoulder on the high-frequency side near 690 cm -1 . These bands are assigned to Si-O-Al deformation bands. (3) Two OH-libration bands at 690 and 650 cm -1  become weak and broad composite bands from 720 to 610 cm -1 . And (4) because the intensity and frequency of the band at 640 cm -1  in tremolite is affected neither by deuteration nor by the pargasite substitution, this band is ascribed to O- [T2] Si-O bending. Even in pargasite, most T2 sites are occupied by Si, so that the O2- [T2] Si-O4 bending mode will be dominant in this amphibole. The same behavior occurs for the synthetic fluoro-gallian tremolite-pargasite series but with larger downward frequency shifts-Ga-O stretching bands appear at 880 and 780 cm -1 , and an Si-O-Ga bending band appears at 605 cm -1 . The major T-O-T and O-T-O deformation bands in synthetic amphiboles are readily apparent in natural calcic amphiboles whose compositions are near the tremolite-pargasite join.

Mid-IR bands of synthetic calcic amphiboles of tremolite-pargasite series and of natural calcic amphiboles