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

¹ Department of Environmental Changes, Graduate School of Social and Cultural Studies, Kyushu University, 4-2-1 Ropponmatsu, Chuo-ku, Fukuoka 810-8560, Japan
² Department of Geological Sciences and Environmental Studies, State University of New York at Binghamton, Binghamton, New York 13902-6000, U.S.A.
³ Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

Correspondence: ^* E-mail: kiyota{at}rc.kyushu-u.ac.jp

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.

Key Words: Mid-IR • calcic amphibole • tremolite • pargasite • deuteration