热处理绿黄色绿柱石光谱特征与颜色成因探究

doi:10.3964/j.issn.1000-0593(2020)12-3795-06

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Abstract Beryl group belongs to the hexagonal system and contains a series of beryllium-aluminosilicate minerals. Beryl gems normally show various colors, due to the occurrence of different trace elements. Greenish-yellow beryl and aquamarine are usually colored by iron ions in different valence states. Fe³⁺ and Fe²⁺ may occupy tetrahedral position (Fe²⁺₄), octahedral position (Fe³⁺₆) and crystalline channel([Fe₂(OH)₄]²⁺). Light blue aquamarine can be obtained by heating treatment of greenish-yellow beryl: (1) in 300～550 ℃ step heating-treatment experiments, the color of samples changes successively from light green, pale yellowish-green, through light yellow, pale blue to light blue; (2) the color turns into pale blue in 550～600 ℃; (3) desirable color modification occurs in 500～550 ℃. Mid FT-IR, UV-Vis and EPR spectroscopic analyses were conducted on beryl samples before and after heating treatment. The mid-FT-IR spectra of heated and unheated samples show similar molecular vibration spectroscopy characteristics. The absorption peaks in the region of 1 250～600 cm^-1 which correlate to the typical vibration of [Si₆O₁₈] group shift to the direction of low wave number, along with the reduction of absorption intensity, indicating changes in Fe³⁺-O^2- charge transfer and the bending vibration of the structural water during heating: (1) the weakening of Fe³⁺-O^2- charge transfer was caused by the reduction of Fe³⁺ in the structural channel; (2) the weakening of bending vibration of structural water was caused by the water loss in heating treatment. The UV-Vis spectra show that same characteristics in both reductive and oxidative environment. However, there are some changes indicating the weakening of Fe³⁺forbidden transition which leads to the disappearance of yellow color compared to unheated samples: (1) the absorption peak which attributed to O^2-→Fe³⁺ charge transfer shifts towards the ultraviolet-light region and forms 322 nm absorption; (2) ⁶A_1g→⁴T_2g transfer shifts towards violet-light region and form 370 nm absorption; (3) ⁶A_1g→⁴E_g+⁴A_1g transfer shifts towards red-light region and form 427 nm absorption. Also, a wide absorption band near 823 nm was enhanced and shifted towards the visible-light region indicating a content increase of Fe²⁺ in structural channels which led the appearance of blue color. The ESR spectrum of natural greenish yellow beryl shows that Fe³⁺, Mn²⁺and OH radical exist both in Al—O octahedron and structural channels. Following conclusions can be drawn from combined UV-Vis spectra: (1) Fe³⁺in structural channels has been reduced to Fe²⁺ in heating treatment; (2) the forbidden transition of Fe³⁺in Al—O octahedron is weakened during heating treatment; (3) the blue color acquired from heating treatment is caused by a combination of the above two changes in beryl crystals.

Key words：Greenish-yellow beryl; Heating treatment; Spectroscopic characteristics

Received: 2019-09-29 Accepted: 2020-02-04

ZTFLH:

P575.4

Corresponding Authors: YUE Su-wei E-mail: yuesuwei@gcu.edu.cn

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2020)12-3795-06 OR https://www.gpxygpfx.com/EN/Y2020/V40/I12/3795