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Optical spectroscopic study of tuhualite and a re-examination of the beryl, cordierite, and osumilite spectra

Taran, Michael N. and Rossman, George R. (2001) Optical spectroscopic study of tuhualite and a re-examination of the beryl, cordierite, and osumilite spectra. American Mineralogist, 89 (9). pp. 973-980. ISSN 0003-004X.

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Polarized optical absorption spectra of a rare Fe^(2+), Fe^(3+)-bearing silicate mineral, tuhualite, (Na,K)_2 Fe^(2+)_2 Fe^(3+)_2 Si_(12)O_(30), were measured at room temperature in the range 350–4000 nm (ca. 28500–2500 cm^(−1)). The spectra display a number of strongly pleochroic absorption bands in the visible and NIR range, which are attributed to ferric and ferrous ions distributed over octahedral and tetrahedral sites in the structure. No absorption bands caused by H_2O or OH stretching vibrations have been observed in the 3000 to 4000 cm^(−1) range. A narrow, weak absorption line at ~422 nm (γ ≈ β) is attributed to the ^6A_(1g) → ^4A_(1g),^4E_g spin-forbidden transition of six-coordinated Fe^(3+) ions that occupy the octahedral Fe2 position of the tuhualite structure. A broad, intense band at ~573 nm (γ > β >> α) is assigned to a Fe^(2+)/Fe^(3+) intervalence charge transfer transition (IVCT) between tetrahedral Fe^(2+) and octahedral Fe^(3+). Together with the high-energy absorption edge, the band at 573 nm causes the intense violet color and spectacular pleochroism (γ > β >> α) of tuhualite. An intense band at ~1040 nm (γ > β >> α) and a much weaker band ~ 2150 nm (α) are attributed to spin-allowed dd transitions of Fe^(2+) in the strongly distorted tetrahedral Fe1 sites. On the basis of the data obtained for tuhualite, optical spectra of the structurally related minerals beryl, cordierite, and osumilite are discussed and re-assigned. The spectra are re-interpreted on the basis of the distribution of Fe^(2+) in these minerals over structural octahedral and tetrahedral positions.

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Rossman, George R.0000-0002-4571-6884
Additional Information:© 2001 American Mineralogist. Manuscript received October 26, 2000. Manuscript accepted April 24, 2001. Manuscript handled by Celia Merzbacher. Gordon Brown (Stanford University) provided the tuhualite crystals from Hutton’s original collection. Alexander Lebedev (deceased) provided the synthetic beryls used in this study. Charles A. Geiger (Kiel, Germany) provided a helpful and thorough review that improved the quality of the paper. This project was funded in part by the National Science Foundation (U.S.A.) grant NSF EAR-9804871.
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Issue or Number:9
Record Number:CaltechAUTHORS:20130417-103911255
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Official Citation:Michael N. Taran and George R. Rossman Optical spectroscopic study of tuhualite and a re-examination of the beryl, cordierite, and osumilite spectra American Mineralogist, September, v. 86, p. 973-980
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:37985
Deposited By: Ruth Sustaita
Deposited On:17 Apr 2013 19:23
Last Modified:03 Oct 2019 04:53

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