Spectral Characteristics of Light Yellow Feldspar From Guyang, Inner Mongolia
LI Xuan1, CHEN Quan-li1, 2*, ZHENG Xiao-hua2
1. Gemmological Institute，China University of Geosciences （Wuhan），Wuhan 430074，China
2. Gemmological Institute，West Yunnan University of Applied Sciences, Tengchong 679118, China
Abstract：Inner Mongolia can mine up to 100 tons of yellow to colorless feldspar yearly, with good transparency and concentrated distribution. It can be treated to meet the market demand. Guyang feldspar can be used as a gem resource with great development prospects. In this paper, the basic gemological characteristics, chemical composition and vibration spectral characteristics of feldspar in Guyang County, Inner Mongolia are systematically studied by using a series of testing technologies such as laser Raman spectroscopy, X-ray fluorescence spectroscopy, Fourier transforms infrared spectroscopy, electron probe and conventional gemological testing methods. The results show that the crystal form of feldspar raw stone samples in this area is mostly gravel, the refractive index is 1.555~1.570, the birefringence is 0.008~0.010, and the density is 2.65~2.68 g·cm-3. The UV fluorescence characteristics of the samples show that they are inert under long waves (365 nm) and short waves (254 nm). X-ray fluorescence spectrometer analysis shows that all samples contain a certain amount of Al, Si and Ca, and a small amount of Ti, Fe, Mn, Mg and Sr. According to the chemical molecular formula calculation and the proportion of end groups of feldspar according to the test results of electron microprobe, this kind of sample belongs to medium feldspar. The infrared absorption peak of feldspar is mainly between 1 200~400 cm-1. From albite to anorthite, it increases with the grade of feldspar. In the infrared absorption spectrum, the absorption peaks of 590 and 650 cm-1 are shifted to the low wavenumber range, to 575 cm-1± and 624 cm-1± respectively. The absorption peaks of Guyang middle feldspar studied in this paper are located at 578 and 632 cm-1 respectively, which is in line with the characteristics of the infrared spectrum changes of feldspar sequence and belongs to the infrared absorption spectrum characteristics of typical middle feldspar. The Raman peaks of this kind of feldspar are composed of seven main Raman peaks: 102, 186, 290, 489, 516, 572 and 800 cm-1. The 102, 186 and 290 cm-1 peaks below 450 cm-1 are caused by the vibration between metal cations and oxygen ([M—O]). The splitting degree of the Raman peaks at 290 cm-1 and 490 cm-1 can indicate the order of Al/Si in silicate minerals. The Raman peaks at 489, 516 and 572 cm-1 belong to the bending vibration spectrum of O—Si(AL)—O and the antisymmetric stretching of Si OBR Si (AL). The comparative analysis with feldspar from other producing areas can be used as one of the identification basis. Based on the above analysis, this feldspar’s composition and main structure are analyzed and discussed.
Key words：Guyang feldspar; Electron probe; Infrared spectrum; Raman spectroscopy
