Abstract

Infrared OH-stretching bands of calcic amphiboles in the magnesiohornblende-tschermakite/ferrotschermakite and edenite-pargasite/hastingsite series have been assigned by deuteration and heat treatment in air. Mössbauer spectra indicate that with increasing temperature for heat-treatment in air, Fe2+ at the M1 and M3 sites is first converted to Fe3+ through dehydrogenation, and then Fe2+ at the M2 site is oxidized at higher temperature. The quadrupole-splitting parameters of (oxidized) Fe3+ at the M1 and M3 sites and Fe3+ at the M2 site are much larger than in natural (= non-dehydro-genated) amphiboles, indicating that dehydrogenation of O3H causes large electric-field-gradients at the M1-3 sites. The intensity of absorption of Fe3+ at M2 decreases with heating temperature, which is consistent with the migration of Fe3+ at M2 to the M1 and/or M3 sites. The (MgMgAl)-OH band, designated K*T, occurs at ~3678 cm−1 in Fe2+-poor pargasitic amphiboles, and is assigned to the configuration (MgMgAl)-OH-A(Na,K): T1SiT1Al. Three (2–4) of the following four types of band systems occur with decreasing band frequency: (1) A*–D* bands at 3730–3675 cm−1, associated with (M1M1M3)-OH-A(Na,K):T1SiT1Si configurations; (2) A*T–D*T bands at 3725–3650 cm−1, associated with (M1M1M3)-OH-A(Na,K): T1SiT1Al configurations; (3) A–D bands at 3680–3620 cm−1 , associated with (M1M1M3)-OH-A□: T1SiT1Si (□ = vacancy) configurations; and (4) AT–DT bands at 3650–3580 cm−1 , associated with (M1M1M3)-OH-A□: T1SiT1Al configurations. In addition, A**T, E*T, and K**T bands ascribed to the configurations (MgMgMg)-OH-A(Na,K)-O3O2−:T1SiT1Al, (MgMgFe3+)-OH-A(Na,K)-O3O2−:T1SiT1Al, and (MgMgAl)-OH-A(Na,K)-O3O2−:T1SiT1Al are important constituents of the spectra of oxidized magnesiohornblende and pargasite. The high frequency bands, A*–D*, are particularly weak, indicating short-range order involving local association of the T1SiT1Al configuration with a locally occupied A-site.

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