CRYSTAL CHEMISTRY, OPTICAL SPECTRA AND COLOR OF BERYL. I. HELIODOR AND GOLDEN BERYL — TWO VARIETIES OF NATURAL YELLOW BERYL

UDC 549.091

https://doi.org/10.15407/mineraljournal.38.02.003

A.N. Platonov, V.M. Khomenko, M.N. Taran

M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of the NAS of Ukraine

34, Acad. Palladina Pr., Kyiv-142, Ukraine, 03680

E-mail: vladimir.khom@yahoo.com; m_taran@hotmail.com

CRYSTAL CHEMISTRY, OPTICAL SPECTRA AND COLOR OF BERYL. I. HELIODOR AND GOLDEN BERYL — TWO VARIETIES OF NATURAL YELLOW BERYL

Language: Russian

Mineralogical journal 2016,38 (2): 3-14

Abstract: On the basis of new experimental and analytical data, as well as analysis of previously published results, two varieties of natural yellow beryl, known in literature as heliodor and/or golden beryl, were differentiated. They are characterized by sharply different configuration of optical absorption spectra and related optical properties such as pleochroism. The spectra of type I are characterized by weak polarization of the charge-transfer band O2– → Fe3+ that causes weak pleochroism of the crystals. The near-infrared region is dominated by absorption band of the OAC [4] Fe2+ centered at 12 350 cm–1 (E ⊥ c), whereas the doublet at 11 900—10 300 cm–1 (E || c), caused by OAC [6] Fe2+, is very weak or absent. The authors propose to attribute this type of spectra to yellow beryl called "heliodor". When heated up to 400 °C, such beryl crystals usually became colorless. Optical absorption spectra of type II were attributed to the variety called "golden beryl". They demonstrate strong shift of the edge of CTB O2– → Fe3+ to longer wavelengths in the polarization E || c. Doublet of [6] Fe2+ in vicinity 11 900—10 300 cm–1 is a main feature of their spectra in this polarization in the near IR region. An absorption band at 12 350 cm–1 in the E ⊥ c — polarized spectra shows unusually low intensity. A distinguishing feature of the type II spectra is the presence of the absorption band with a maximum near 19 000—20 000 cm–1 that was never observed in any other types of beryl spectra. Crystals of this type are characterized by intense pleochroism; when heated to 400 °C, they acquire a blue color. It was concluded that differences between spectra of types I and II are caused by different patterns of distribution of Fe3+ ions among non-equivalent positions in the beryl structure: in crystals with spectra of type I Fe3+ ions substitute Al in the octahedral positions, whereas in the samples with spectra of type II Fe3+ occupies Be-tetrahedra. It was proposed to use the traditional names of yellow beryl — heliodor and golden beryl for these two specific spectroscopic and crystal-chemical varieties. These varieties represent end members of natural yellow low-temperature beryls, in which Fe2+ and Fe3+ ions in different proportions occupy regular octahedral, tetrahedral and interstitial positions of crystal structure.

Keywords: heliodor, golden beryl, optical absorption spectra, nature of color, crystal chemistry of Fe ions.

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