Fine structure of infrared OH-stretching bands in natural and heat-treated amphiboles of the tremolite-ferro-actinolite series

Abstract

Fine structure in the principal OH-stretching bands of amphiboles of the tremolite-ferro-actinolite series have been examined. In samples with partly filled A sites, a broad (composite) band is observed at 3725~3680 cm^-1 and is assigned to two types of configurations: (M1M1M3)-OH-^A( Na,K):^T1Si ^T1Al in which Al occurs at the T1 site, and (M1M1M3)-OH-^A(Na,K)-^O3(O²-,F-,Cl-); the component of (M1M1M3)-OH-^A(Na,K):^T1Si ^T1Si configuration is small, because Na and K at the A site are locally associated with Al at an adjacent T1 site. In tremolite, manganoan tremolite, and Fe²⁺-poor actinolite, a weak shoulder on the principal A band at ~3669 cm^-1 is assigned to the configuration ^M4Ca^M4(Mg,Fe²⁺,Mn²⁺,Na):(MgMgMg)-OH-^A◻: ^T1Si ^T1Si (◻ = vacancy). Fine structure in the principal bands B (B' and B'') and C (C' and C'') are also observed: the higher-frequency band B'' is assigned to ^M1Fe^2+M1Mg ^M3Mg-OH-^A◻, and the lower-frequency band B' to ^M1Mg^M1Mg^M3Fe²⁺-OH^A◻; the higher-frequency band C' is assigned to ^M1Fe^2+M1Fe^2+M3Mg-OH-^A◻ and the lower-frequency band C'' to ^M1Mg ^M1Fe^2+M3Fe²⁺-OH-^A◻. Some broad OH-stretching bands attributed to (M1M1M3)- OH-^A◻:^T1Si ^T1Al are observed at 3640~3580 cm-1. In amphiboles of the tremolite-ferro-actinolite series that show a substantial B [(MgMgFe²⁺)-OH] band, a new OH-stretching band (at around 3641 cm^-1), E, appears near the principal C band (at around 3643 cm^-1) on heat treatment. The shape of band E is similar to that of the original band B, and its local configuration is O^2--(MgMgFe³⁺)-OH-. A weak and broad band A** appears at ~3690 cm^-1 on heat treatment of some Na-bearing actinolites, and is ascribed to the (MgMgMg)-OH-ANa-O²- configuration