The Gemological, Mineralogical, and Spectral Characteristics of Indian Longdan Stone
ZHOU Wu-bang1, QIN Dong-mei2, WANG Hao-tian1, CHEN Tao1, WANG Chao-wen1*
1. Gemmological Institute，China University of Geosciences (Wuhan)，Wuhan 430074，China
2. Art and Communication College，Wuhan University of Engineering Science，Wuhan 430200，China
Abstract：Indian Longdan stone was introduced into China as a carving stone because of its similar colors and materials to Qingtian Longdan stone. However, the spectral, mineral compositional, and structural characteristics of Indian Longdan stone are still unclear. In this paper, the gemmological characteristics, color genesis and spectral characteristics of a representative sample were studied by polarizing microscopy, X-ray Diffractometer (XRD), Scanning Electron Microscope (SEM), Electron Probe Micro Analyzer (EPMA) and Fourier Infrared Spectrometer (FTIR). Indian Longdan stone is characterized by “white meat and red heart”. The polarizing microscope shows that the illite is a cryptocrystalline lepidoblastic texture. Obvious red-brown spotted materials are aggregating in the “red heart” area, presenting as dense to loose from the central red area to the edge yellow-green area, consistent with the color change. XRD results show that the light yellow-green “white meat” and “red heart” diffraction patterns are the same, showing three strong diffraction peaks at 10.00，4.99 and 3.33 Å. Clear peaks can be observed at 2.86，2.99，3.20，3.49 and 3.73 Å and no other mineral phasecan be detected, indicating that the sample is pure 2M1 type illite. The full width at half maximum of XRD at 10.00 Å peak is 0.092°Δ two theta, indicating that the order and crystallinity of illite are well. EPMA further confirms that the Indian Longdan stone is mainly illite, with an average cation content of 0.824 p. u. f. and a structural iron content of 0.05%～0.08%. SEM backscattering composition images show that the red-brown patchy material has obvious bright contrast but generally shows similar morphology of illite. The energy spectrum analyses show that the average content of iron in this area is 0.48% wt, which is one order of magnitude higher than the content of structural iron in illite, indicating that the red-brown patchy iron-bearing material may be the chromogenic material of Indian Longdan stone. SEM observation reveals KCl crystals with cubic morphology, indicating that the illite may be directly crystallized in K-rich fluid. The results of FTIR show that the samples have an OH stretching vibration peak at 3 630 cm-1 and an Al—O vibration peak in tetrahedron at 830 cm-1. The absorption peak at 756 cm-1 is related to the substitution of Al in tetrahedral coordination instead of Si, which is characteristic of Si—O—Al vibration in the tetrahedron. The absorption peaks of OH stretching vibration near 3 625 cm-1 and at 825 and 750 cm-1 double fingerprints are the characteristic infrared absorption peaks of illite minerals, which confirms that the main mineral of Indian Longdan stone is illite. The study of Indian Longdan stone enriches the understanding of the gemmological and spectral characteristics of carved stone material, and the infrared spectral characteristics can be used as the identification basis for rapid, nondestructive testing of carved stone samples.
Key words：Indian Longdan Stone; Illite; The iron material; Color genesis; Spectral characteristics
周武邦，秦冬梅，王浩天，陈 涛，王朝文. 印度龙蛋石的宝石矿物学及谱学研究[J]. 光谱学与光谱分析, 2023, 43(06): 1895-1899.
ZHOU Wu-bang, QIN Dong-mei, WANG Hao-tian, CHEN Tao, WANG Chao-wen. The Gemological, Mineralogical, and Spectral Characteristics of Indian Longdan Stone. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2023, 43(06): 1895-1899.
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