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Charge-transfer in ferromagnesian silicates: The polarized electronic spectra of trioctahedral micas

Published online by Cambridge University Press:  05 July 2018

D. W. Robbins  and
R. G. J. Strens
D. W. Robbins
Affiliation:
Turner Bros. Asbestos Ltd., P.O. Box 40, Rochdale, Lancashire
R. G. J. Strens
Affiliation:
School of Physics, The University, Newcastle upon Tyne NE1 7RU
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Summary

Polarized spectra (2000 to 25000 Å) have been obtained for fifteen analysed trioctahedral micas covering a wide range of compositions, and including four phlogopites, nine biotites, and two lepidomelanes. Three main contributions to the absorption have been noted: charge transfer from oxygen to ferrous iron throughout the visible and near ultraviolet, charge transfer from ferrous to ferric ions at the red end of the visible spectrum and internal d-d transitions of ferrous ions in the near infrared.

Substitutions in the brucite layer (especially Ti) cause the O → Fe2+ band to broaden, and thus to encroach on the visible region for vibration directions in the plane of the cleavage flake. The transmission window between these bands is further blocked by the Fe2+ → Fe3+ charge-transfer band, which is polarized in the plane of the flake. The overall effect is to produce very strong absorption throughout the visible region for (β, γ) vibration directions, and relatively weak absorption for α, i.e. the observed very strong pleochroism of biotite.

Many features of the spectra of chlorite, amphiboles, and tourmaline, which contain octahedral ions in brucite-like sheets, strips, and fragments respectively, may be interpreted by analogy with biotite.

Two types of solid solution effect are described: substitutional broadening, which is responsible for the extension of the O → Fe2+ band into the visible region in biotites and aluminous ortho-pyroxenes, and substitutional intensification, which permits in solid solutions transitions that are forbidden by the conventional selection rules.

Type
Research Article
Information
Mineralogical Magazine , Volume 38 , Issue 297 , March 1972 , pp. 551 - 563
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1972

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Footnotes

1

To whom correspondence should be addressed.

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