变色石榴石的紫外-可见吸收光谱与3D荧光光谱研究

doi:10.3964/j.issn.1000-0593(2020)07-2148-05

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Abstract Color-change garnet is a special variety in the garnet group, usually belongs to pyrope-spessartine solid-solution series and contains a trace of Cr and Fe. Few people have studied it in China, and no color-change garnets with obvious red fluorescence under ultraviolet radiation have been reported at home and abroad. A color-change garnet with red fluorescence from the Umba region of Tanzania was studied by chemical and spectral tests. Laser ablation inductively coupled plasma mass spectrometry was used to study sample’s chemical properties. The result shows that this color-change garnet belongs to the pyrope-spessartine solid-solution series and contains a certain amount of grossular end-member, with some trace elements of Cr, V, Fe. The average composition of it is Prp_46.28Sps_38.40Grs_13.57Alm_2.33Uvt_0.35. In ultraviolet-visible absorption spectrum, sharp absorption peaks at 409, 422, 430 and 486 nm are assigned to the spin-forbidden transition of Mn²⁺, and additional peaks at 459, 503 nm is caused by Fe²⁺, the broad absorption band peaked at 571 nm in the yellow region which is responsible for the color-change effect is attributed to the spin-allowed transition of Cr³⁺ and V³⁺ together. Two absorption maxima in violet-blue and yellow-orange regions and comparably equal transmittance of green and red light cause the color change effect, garnet appears greenish-yellow in day light and becomes purplish-pink under incandescent light. The emission peak at 690 nm in the fluorescence spectrum is ascribed to the ²E_g→⁴A_2g multiplicity-transition of Cr³⁺. And red fluorescence is most excited by violet light (400～440 nm) and yellow light (500～560 nm), that correspond to absorption bands caused by two spin-allowed transitions of Cr³⁺, ⁴A_2g→⁴T_1g and ⁴A_2g→⁴T_2g respectively. Compared with other color-changing garnets in previous studies, this sample contains lower Fe²⁺. It is speculated that high Cr and low Fe makes the color-change garnet can be excited to red fluorescence under ultraviolet radiation. The detailed spectra study of color-change garnet with red fluorescence and discussionon of its color-change effect and fluorescence mechanism provide spectral data and theoretical basis for further research on special garnets’ fluorescence in the future.

Key words：Color-change garnet; Pyrope-spessartine; Red fluorescence; Fluorescence mechanism

Received: 2019-06-21 Accepted: 2019-10-30

ZTFLH:

P575.4

Corresponding Authors: Andy Hsitien Shen E-mail: ahshen@foxmail.com

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2020)07-2148-05 OR https://www.gpxygpfx.com/EN/Y2020/V40/I07/2148