Microscopic Laser Raman and Infrared Spectroscopic Study of Tengchongite

Tengchongite is a uranyl molybdate uranium mineral and it was found and named by Chen 1985.No more scholars studied on tengchongite after Chen’s work, The identification of this mineral has only been confirmed by single crystal X-ray diffraction. In the paper, micro laser Raman spectroscopy and infrared spectroscopy are used to identify the spectroscopy properties of tengchongite. The studies fill in the gaps of more than 30 years in terms of the molecular spectroscopy research of tengchongite. The mineral has an ideal model of Ca(UO2)6(MoO4)2O5·12H2O its bands attributed to the vibrating units are clearly identified in the Raman spectrum. Symmetric stretching modes at 812 cm and 839 cm are assigned to ν1 (UO2), The ν3 antisymmetric stretching modes of (UO2) are observed at 896 cm, Symmetric stretching modes at 419 and 440 cmare assigned to ν2 (UO2). Symmetric stretching modes at 919cm are assigned to ν1(MoO4), The ν3 antisymmetric stretching modes of (MoO4) are observed at 752 cm, the in-plane ν2(MoO4)and the out-of-plane ν4(MoO4) bending modes are at 169 cmand254 cm. IR spectrum of tengchongite shows the major uranyl band at 858 cmand 693 cm, Mo-O bonds are observed at about 985 cm and 780 cm, and H2O groups are present with a wide range peaks from 3100 cm to 3500 cm and 1647 cm, and the band at 1432.4 cm is probably due to the stretching vibration hydroxyl (OH), therefore, tengchongite contains may include hydroxyl and its chemical formula needs to be modified .

tengchongite has been done since then. Zhang et al.(1995) and Peter C et al. (1999) cited the research results of tengchongite in their monographs [3,4]. However, the X-ray diffraction of single tengchongite crystal, and the microscopic laser Raman and infrared spectroscopy of the mineral have not been carried out yet, which leads to a lack of spectroscopy data of the tengchongite. Laser Raman and infrared spectroscopy are effective techniques for identification of minerals. In this paper, the spectroscopy of the tengchongite was supplemented.

Samples
The tengchongite sample was from the exhibition specimen in the China Nuclear Geology Museum. Before the analysis of microscopic laser-Raman and IR spectroscopy, the sample was firstly analyzed for more accurate chemical composition by EDX method.

SEM and EDX Analysis
Secondary electronic images and back-scattered electronic images were obtained from TESCAN VEGA3 scanning electronic microscope. The sample size is <5 mm. The equipment parameters are set as that: high voltage is 20 KV, beam intensity is15μA,and working distance is15 mm. Chemical composition was analyzed by EDAX TEAM energy dispersive spectrometer with the following parameters: single point acquisition time of 200μs,inputCPS more than 20000,and dead time less than 30s.

Raman microprobe spectroscopy
The crystals of tengchongite were placed on the stage of an Olympus BX-41microscope that is equipped with10×to 100× objectives and a part of HORIBA evolution laser Raman microscope system, as well as an electronic cooled CCD detector, an illuminant system and a filter system. The Raman spectra were excited by 532nmand 785nmYAG laser at a resolution of 1 cm -1 with the following parameters: 100× objectives, scanning range between 100 and 4000cm -1 ,grating of 1800gr/mm, gaining a single point in 8 seconds, accumulating for 4 times. Spectra were calibrated using the 520.7 cm -1 line of a silicon wafer. Data process and spectral manipulation such as smoothing, peak analyzing and baseline correcting were performed using the software Labspec 6 by Horiba Scientific.

.Mid-IR spectroscopy
Mid-infrared spectra were obtained using BRUKER LUMOS Micro-FTIR with in reflection model. We obtained the spectra of tengchongite after 64 scans with a scanning range between 4000 and 640 cm -1 and a resolution of 4 cm -1 . Data process and spectral manipulation such as smoothing, baseline adjustment, normalization and band component analysis were performed using the special software OPTU 7.5 as an accessory of the instrument.  1432.4 cm -1 is probably due to the stretching vibration hydroxyl (OH -1 ),which indicates that there may be hydroxyl in tengchongite, and the original chemical formula of tenchongite needs to be modified. The band at 985cm -1 was attributed to ν 1 (MoO 2 ) 2+ stretching vibration and that at 780.6 cm -1 toν 3 (MoO 2 ) 2+ .The band at 905.6 cm -1 was attributed to ν 3 (UO 2 ) 2+ and that at 644.1 cm -1 toν 1 (UO 2 ) 2+ . (c) The very broad infrared bands at 3100~3500 cm-1 are assigned to the stretching vibrations of water. H 2 O groups give bands in the regions 1600-1650 cm -1 (δH 2 O) and 3100~3500 cm -1 (ν OH), and the band at 1432.4 cm -1 is probably due to the stretching vibration hydroxyl (OH -1 ), which indicates that there may be hydroxyl in tengchongite. The original chemical formula of tengchongite may be inaccurate, and further research should be studied.