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Assignment of infrared OH-stretching bands in calcic amphiboles through deuteration and heat treatment

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, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

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

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): ^T1Si^T1Al. 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):^T1Si^T1Si configurations; (2) A*_T–D*_T bands at 3725–3650 cm^–1, associated with (M1M1M3)-OH-^A(Na,K): ^T1Si^T1Al configurations; (3) A–D bands at 3680–3620 cm^–1 , associated with (M1M1M3)-OH-^A: ^T1Si^T1Si ( = vacancy) configurations; and (4) A_T–D_T bands at 3650–3580 cm^–1 , associated with (M1M1M3)-OH-^A: ^T1Si^T1Al configurations. In addition, A**_T, E*_T, and K**_T bands ascribed to the configurations (MgMgMg)-OH-^A(Na,K)-^O3O^2–:T1Si^T1Al, (MgMgFe³⁺)-OH-^A(Na,K)-^O3O^2–:T1Si^T1Al, and (MgMgAl)-OH-^A(Na,K)-^O3O^2–:T1Si^T1Al 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 ^T1Si^T1Al configuration with a locally occupied A-site.

Key Words: Infrared OH-stretching spectra • tetrahedral Al • calcic amphibole • pargasite • deuteration • heat-treatment

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