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

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): T1 Si T1 Al in which Al occurs at the T1 site, and (M1M1M3)-OH- A (Na,K)- O3 (O 2 -,F-,Cl-); the component of (M1M1M3)-OH- A (Na,K): T1 Si T1 Si 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 Fe 2+ -poor actinolite, a weak shoulder on the principal A band at ~3669 cm -1 is assigned to the configuration M4 Ca M4 (Mg,Fe 2+ ,Mn 2+ ,Na):(MgMgMg)-OH- A ◻: T1 Si T1 Si (◻ = 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 M1 Fe 2+ M1 Mg M3 Mg-OH- A ◻, and the lower-frequency band B' to M1 Mg M1 Mg M3 Fe 2+ -OH A ◻; the higher-frequency band C' is assigned to M1 Fe 2+ M1 Fe 2+ M3 Mg-OH- A ◻ and the lower-frequency band C'' to M1 Mg M1 Fe 2+ M3 Fe 2+ -OH- A ◻. Some broad OH-stretching bands attributed to (M1M1M3)- OH- A ◻: T1 Si T1 Al are observed at 3640~3580 cm-1. In amphiboles of the tremolite-ferro-actinolite series that show a substantial B [(MgMgFe 2+ )-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- -(MgMgFe 3+ )-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-O 2 - configuration