Abstract

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.

You do not currently have access to this article.