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

摘要

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 fluorogallian 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.