北红玛瑙的颜色成因及光谱学特征研究

doi:10.3964/j.issn.1000-0593(2020)08-2531-07

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Abstract The Beihong agate is commonly known as agate with the translucent and yellowish-red appearance and is widely produced in the southern section of Xing’an Mountains. In this paper, the phase composition, chemical composition and spectroscopic characteristics of Beihong agate and its yellow and white control samples were investigated by polarizing microscope observation, X-ray powder diffraction, Raman spectra, UV-VIS absorption spectra and whole rock chemical analysis. The results show that Beihong agate is composed mainly by α-quartz, with moganite, goethite as the minor mineral composition, a very small amount of hematite can also exist in Beihong agate. The yellowish-red appearance of Beihong agate is related to goethite and a very small amount of hematite, which is different from the coloration mechanism of Nanghong agate by hematite. Based on distribution pattern, goethite and hematite can be divided into scattered and disseminated forms. The size of goethite and hematite with scattered distribution is about 10 μm, but they possess amorphous morphology, it is presumed that the scattered minerals were the aggregates of goethite and hematite crystal at the submicron scale. In addition, the disseminated goethite and hematite exhibit invisible pattern, which are similar to the scattered minerals and goethite and hematite crystal that are all sub-micron in size, but they do not aggregate to form microscopic scattered aggregates. On the whole, the content of goethite and hematite in Beihong agate is higher than that in the yellow control sample, as the content of goethite and hematite increases, the hue of Beihong agate changes from yellowish-red to red. The Beihong agate exhibited ⁶A₁→⁴E, ⁶A₁→⁴E⁴A₁, 2(⁶A₁)→2(⁴T₁)(4G), ⁶A₁→⁴T₂(4G) electron transitions of goethite and ⁶A₁→⁴E⁴A₁, ⁶A₁→⁴T₂ electron transitions of hematite and the charge transfer between O^2- and Fe³⁺, the synergy of electron transition in the act of the crystal field and charge transfer produce significant yellowish-red color. In the first derivative spectra of UV-VIS absorption spectra, the minimum value position of Beihong agate, yellow and light-yellow contrast samples are 555～556, 530 and 502 nm, respectively. As the red tone of agate decreases gradually, the position of the minimum value in the visible range of the first derivative spectrum decreases gradually. It can be used to characterize red to yellow hue of agate, which is also significant for the variety identification and color grade of quartz jades.

Key words：Agate; Color and genesis; Raman spectra; UV-Vis absorption spectra; First derivative spectra

Received: 2019-07-13 Accepted: 2019-11-19

ZTFLH:

P574

Corresponding Authors: HE Xue-mei E-mail: hexuemei3127@126.com

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2020)08-2531-07 OR https://www.gpxygpfx.com/EN/Y2020/V40/I08/2531