Abstract

Infrared OH-stretching bands in some heat-treated A-site occupied and IVAl-free (or nearly free) manganoan sodic-calcic and sodic amphiboles, manganoan magnesio-arfvedsonites and richterites, have been assigned. Two OH-stretching bands, (MgMgMg)-OH-A-O2−/F/Cl and (MgMgMg)-OH-□ (□ = vacancy) configurations, persist to high temperature. With increasing temperature, the OH-stretching band, A*, of the (MgMgMg)-OH-A-OH (A = A site cation) configuration shifts downward from 3730 to near 3700 cm−1 with formation of the (MgMgMg)-OH-A-O2− configuration; the repulsive interaction between the proton and the A cation is removed through dehydrogenation of OH at the O3 site, coupled with movement of the A cation toward the dehydrogenated side. In natural F-and Cl-bearing sodic-calcic and sodic amphiboles, two kinds of (MgMgMg)-OH-A stretching bands are observed at around 3730 and 3700 cm−1, in which A-site alkali ions move toward the F(Cl)substituted O3 site. In this manner, (MgMgMg)-OH-K and (MgMgMg)-OH-Na bands shift downward 20 cm−1 and 26–29 cm−1 by heat-treatment, respectively, reflecting the different size of the A-site cations.

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