Mid-IR bands of synthetic calcic amphiboles of tremolite-pargasite series and of natural calcie 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 (=(AlAlMg-1Si-1)-Al-[4]-Al-[6]-Mg-[6]-Si-[4]) and edenite (=(AlNaSi-1[A])-Al-[4]-Na-[A]-Si-[4]square(1)) substitutions, causes the following features in the region 1200-600 cm(-1). (1) Weak Al-[4]-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-Si-[T2]-O bending. Even in pargasite, most T2 sites are occupied by Si, so that the O2-Si-[T2]-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.