福建寿山溪蛋石的矿物学和谱学研究

doi:10.3964/j.issn.1000-0593(2019)02-0543-05

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Abstract Xidan Stone, which is a kind of mountain stones scattered across the Yueyang Stream and weathering product from the famous Shoushan Furong Stone, is one of the well-known varieties of Shoushan stone. Washed into the stream by rain, blocks of Xidan Stone have been impacted by the water and river sand for several years to form a round pebble appearance. Because it is easy to be sculptured, Xidan Stone is widely praised by modern sculptors. In order to analyze this stone from different aspects including mineral components, spectroscopic features, chemical composition and color origin, the systematic mineralogical and spectroscopic studies were conducted on the yellow Xidan stone samples from Shoushan rivulet of Fujian Province, with the help of standard gemological methods, X-Ray powder diffraction, infrared absorption spectroscopy, laser Raman spectroscopy and Electron probe microanalysis. The gemological testing results inferred that Xidan stone samples are pebble-shaped which contain light yellow substrate and rough weathered skin. Examined under magnification, the Xidan stone samples have tiny black dot-like inclusions and white grey mineral component. The Xidan stone samples have an average relative density of 2.8 by hydrostatic weighing method and a Moh’s hardness below three. According to the testing results of XRD, the major constituent mineral of Xidan stone is pyrophyllite, which is in the type of monoclinic pyrophyllite (2M type). The characteristic feature is the three diffraction peaks of 4.44 4.44Å (020), 4.24Å ( $\100dpi \inline \bar{1}$ 12) and 4.17Å(111) between 2θ=19° and 22°. The two diffraction peaks ( $\100dpi \inline \bar{1}$ 12) and (111) lie closed to each other, therefore a diffraction shoulder appears on the right side of the ( $\100dpi \inline \bar{1}$ 12) diffraction peak (2θ=21.06°). Another characteristic feature is the 3.06Å (003) strong peak (2θ=29.05°) between 2θ=28° and 31°. Infrared spectroscopy is an effective method to tell the mineral composition of weathering skin parts as well as the substrate parts of Xidan stone samples. The FTIR spectrum shows that these two parts share the same mineral component of pyrophyllite. In fingerprint region, the main absorption bands are 1 122，1 068，1 052，949，853，835，812，541 and 484 cm^-1. Infrared absorption band at 1 122，1 068 and 1 052 cm^-1 are induced with Si—O symmetric stretching vibration and Si—O—Si antisymmetric stretching vibration. Infrared absorption band at 949 cm^-1 is induced with Al-OH in-plane bending vibration. Mountain-like infrared absorption bands at 853，835 and 812 cm^-1 are induced with Al—OH out-of-plane bending vibration. Infrared absorption peaks at 541 and 484 cm^-1 are induced with Si—O—Al stretching vibration and Si—O bending vibration. In high frequency region, the acute infrared absorption peak at 3 675 cm^-1 is induced with Al—OH stretching vibration indicating the highly ordered structure of Xidan Stone samples. Laser Raman spectroscopy is an effective and non-destructive way to analyze the inclusions. LRM testing confirmed that the black inclusions are composed by hematite and the white grey mineral component is diaspore. Raman peaks at 224，291，409，494 and 1 315 cm^-1 are typicalfeatures of hematite. Raman peaks at 448，499，667，707，788 and 1 194 cm^-1 correspond to the typical features of diaspore. In addition, Raman spectrum of substrate parts of samples shows the characteristic peaks of pyrophyllite at 111，194 and 261 cm^-1, which are induced with O—H stretching vibration. Based on the unit price of mineral balance principle molecule and the total number of positive charges, the average crystal structural formula of Xidan Stone is (Al_1.98Na_0.02Cr_0.01)[(Si_3.98Al_0.02)O₁₀](OH)₂. Thedata of EPMA testing tell that Xidan Stone samples have stable chemical composition. Samples mainly contain Si(64.88%) and Al(27.55%). Given that the Xidan Stone samples contain fewer Fe(0.02%) but more Cr(0.2%), Fe as well as Crmightcause the light yellow of stream-egg stones.

Key words：Xidan Stone; Pyrophyllite; FTIR spectra; Raman spectra; Fujian shoushan

Received: 2017-12-06 Accepted: 2018-04-19

ZTFLH:

P575.4

Corresponding Authors: CHEN Tao E-mail: summerjewelry@163.com

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2019)02-0543-05 OR https://www.gpxygpfx.com/EN/Y2019/V39/I02/543