Characterisation of the hydroxy-interlayered vermiculite from the weathering of illite in Jiujiang red earth sediments.
The crystal-chemical properties of clay minerals are closely related to the physicochemical conditions prevailing when they form in soils and weathered zones. The degree of alteration of pre-existing clay minerals and the composition of the neo-formed clay mineral products reflect local climate and weathering (Banfield et al. 1999; Sheldon and Tabor 2009). Accordingly, clay mineralogy can be used as a proxy to characterise the evolution of the local environmental conditions in response to global climatic changes (Frouin et al. 2013). Vermiculite is most often a pedogenic clay mineral that can be formed by the transformation on weathering of illite or chlorite (Banfield and Murakami 1998; Bronger et al. 1998). This transformation is completed by the release of potassium (K) from the interlayer of illite or of magnesium (Mg) and iron (Fe) from that of chlorite during the weathering process (Aspandiar and Eggleton 2002; Bonifacio et al. 2009). Vermiculite may alter into hydroxy-interlayered vermiculite (HIV) by aluminium (Al)-intercalation in the interlayer in moderately acidic conditions (Rich 1968), and finally into smectite and/or kaolinite with increasing level of weathering (Vicente et al. 1997).
Hydroxy-interlayered vermiculite has been commonly identified in soils (Toksoy-Koksal et al. 2001; Bertrand et al. 2008; Shaw et al. 2010; Huang et al. 2011; Mavris et al. 2011; Darunsontaya et al. 2012; Falsone et al. 2012; Pal et al. 2012a). It is formed by the adsorption of polymers of hydroxyl-Al ions into the interlayer region of vermiculite in moderately acidic soils. The incorporation of Al hydroxide polymers in the interlayer region modifies the expansion or the collapse of the layers (Meunier 2007). Generally, HIV tends to show a 1,4-nm peak. The 1.4-nm peak in HIV is often overlapped by peaks of other clay minerals such as vermiculite, chlorite, illite-smectite mixed clays and hydroxy-interlayered smectite (HIS) in X-ray diffraction, which makes the identification of HIV difficult in samples of polymineralic composition (Y in et al. 2012). Since soil properties are closely related to the clay assemblage, the presence of HIV will strongly influence not only the cation exchange capacity (CEC) but also physical properties such as swelling and dispersive behaviour (Stem et al. 1991; Churchman et al. 1994; Shaw et al. 2010).
Unlike the loess-palaeosol sequence in north-western China, Quaternary sediments distributed in south China are mainly red earth sediments. Both red earth sediments and loess sediments have been considered the consequence of increasing aridity of the dust area in the north, coupled with intensified East Asia winter monsoon activity in association with uplift of the Tibet plateau (Liu 1985; Yang et al. 1991; Xiong et al. 2002). Previous studies of red earth sediments in the middle to lower Yangtze have focused on characteristics of grain size (Li et al. 1997; Li et al. 2001; Hu et al. 2005), chemical weathering (Xiong et al. 2000; Yang et al. 2004; Chen et al. 2008; Hong et al. 2010a), and environmental magnetism (Zhao and Yang 1995; Hu et al. 2003; Qiao et al. 2003; Zhang et al. 2007; Hu et al. 2009), with rare studies on the clay mineralogy of the sediments, especially on the mineralogy of HIV clays. The occurrence, thermal dehydration, genesis and climatic significance of HIV clays were investigated by previous researchers (Harris et al. 19926; Lin et al. 2002; Yin et al. 2013). However, X-ray diffraction analysis has been the only method of identification of HIV in most studies, and different XRD behaviours were shown by HIV clays from different regions (Huang et al. 2011; Pal et al. 2012a). Hence, systematic analyses are needed in order to obtain more information on the mineralogy of HIV clays.
Hydroxy-interlayered vermiculite clays occur abundantly in the upper portion of Jiujiang red earth sediments of south China, where the intercalated Al of HIV was considered to be present as hydroxy-Al hydroxides (Yin et al. 2013). The abundance of HIV in these soils provides an opportunity for understanding this clay species. The goal of this study was to characterise the mineralogy of HIV in Jiujiang red earth sediments using systematic analyses, with the aim of improving our understanding on its interlayer components and its formation process.
Materials and methods
The Jiujiang section (116[degrees]00'13.7"E, 29[degrees]42'40.27'N) is one of the typical red earth sections in the middle to lower reaches of the Yangtze River (Fig. 1). The section is 14 m thick, and is lithologically divided into three units. The upper unit (0-3.2 m) is pale yellow to yellow-brown and contains minor amounts of Fe-Mn films and Mn nodules. The middle unit (3.2-6.9 m) is reddish to red-brown, containing sporadically pale yellow to grey spots and small short veins. The lower unit is red-brown to dark-brown, characterised by well-developed, white net-like veins that are more common and are relatively longer and wider than those in the middle unit. The middle and lower units have experienced much stronger weathering than the upper one (Hao et al. 2010).
Optical stimulated luminescence dating of the sample at 0.8 m depth was 40.8 ([+ or -]4.9) ka, and the electron spin resonance dating of samples at depths of 6.3, 8.1, 10.9 and 13.8 m was 393 ([+ or -] 45), 452 (+ or -] 43), 592 ([+ or -] 77) and 685 ([+ or -] 65) ka, respectively. The oldest age of 685 ([+ or -] 65) ka for the lowest red earth layer of the section indicates that the red earth formed after the mid-Pleistocene (Hong et al. 2013). HIV occurs abundantly in the upper portion, in association with illite and kaolinite (Yin et al. 2013). A representative clay sample was collected from the upper unit (~0.9 m depth) for investigation of the mineralogy of the HIV.
The bulk sample was air-dried and then crushed manually to powder using an agate pestle and mortar. This powder sample was pre-treated with 30% [H.sub.2][O.sub.2] overnight to remove organic materials. The clay mineral fraction (<2 [micro]m) was obtained by sedimentation and centrifugation (Brown and Brindley 1980). The clay mineral phase was determined by X-ray diffraction (XRD) of the oriented air-dried, Mg-saturated, glycol-solvated and K-saturated samples. The Mg-saturated and K-saturated samples were obtained by applying Mg[Cl.sub.2] and KC1 solutions to air-dried mounts (Harris and White 2008). Expansion properties of the sample were determined by [Mg.sup.2+] saturation and glycol solvation (Velde and Meunier 2008). The Mgsaturated and glycol-solvated sample was prepared by treating the Mg-saturated sample in a sealed container with ethylene glycol at 70[degrees]C for 3 h (Hong et al. 2007). The air-dried K-saturated sample was analysed at 25C and then heated at 110[degrees]C, 450[degrees]C and 600[degrees]C, respectively. The XRD patterns of the clay samples were collected using a PANalytical (Almelo, The Netherlands) X'Pert PRO DY2198 diffractometer at the Laboratory of Geological Process and Mineral Resources, China University of Geosciences (Wuhan), which was operated at 40 kV and 40 mA with Ni-filtered Cu K[alpha] radiation. It was measured from 3[degrees] to 30[degrees]2[theta] at a scan rate of 4[degrees]2[theta] [min.sup.-1] and a step size of 0.02[degrees]2[theta].
Fourier-transform infrared analysis
Fourier-transform infrared (FTIR) spectroscopy was performed on KBr disks prepared by mixing 0.5 mg sample with 200 mg KBr and pressing at 10 kg [cm.sup.-2] (Madejova and Komadel 2001). The KBr disks were heated in a furnace overnight at 105[degrees]C to minimise the water adsorbed on KBr and the clay sample. The mid-infrared spectra (MIR) of the air-dried and heated samples (samples heated at 400[degrees]C and 600[degrees]C, respectively) were recorded on a Bruker (Billerica, MA, USA) Vertex 80 V Fourier-transform spectrometer, equipped with KBr beam splitter, and DTGS KBr detector for MIR measurements. For each sample, 64 scans in the 4000-3000 [cm.sup.-1] wavenumber ranges were recorded with a resolution of 4 [cm.sup.-1].
Differential scanning calorimetry (DSC) analysis was carried out on a NETZSCH (Selb, Germany) STA-409 thermal analyser. The endothermic and exothermic effects were recorded on the DSC curve and were used to determine the dehydroxylation temperatures of different clay species. About 10 mg of the ground clay sample was added to a corundum crucible for thermal analysis, and heated in air from ambient temperature to 1000C with a temperature gradient of 10[degrees]C [min.sup.-1].
The clay sample was treated with 1/3 m sodium citrate in order to extract hydroxy-Al interlayered polycations of clay minerals (Tamura 1958). The clay fraction was treated with sodium citrate solution (30 mg clay + 25mL of 1/3 m sodium citrate) at 80[degrees]C for 12 h in a sealed polyethylene tube. The extracted solution was then separated from the clay fraction residue by centrifuging for 30 min, and the supernatant was transferred into a 10mL sealed polyethylene tube. The concentrations of Al and/or Fe in the supernatant were measured using an inductively coupled plasma-atomic emission spectrometer (ICP-AES). The clay fraction residue was made into the oriented, air-dried and glycol-solvated samples for XRD measurement of the Na-saturated sample.
The XRD patterns of the clay samples are shown in Fig. 2a. The air-dried sample exhibited a distinct 1.41-nm peak, which did not change after [Mg.sup.2+] saturation and glycol solvation. However, the intensity of the 1.41-nm peak decreased significantly after K+ saturation and drying at 25[degrees]C, indicating presence of vermiculite (Meunier 2007). The collapse of 1.41 nm to 1.0 nm and the increasing intensity of the 0.498-nm peak after [K.sup.+] saturation at 25[degrees]C indicated the dioctahedral nature of vermiculite (Brown and Brindley 1980). When the K-saturated sample was heated to 110[degrees]C, the 1.41-nm peak shifted to 1.38 nm, and it collapsed to 1.0 nm in association with the presence of a weak 1.25 nm peak when further heated to 450[degrees]C. On heating to 600[degrees]C, the spacing changed from 1.25 nm to 1.20 nm, with a notable increase in its intensity. Sequential heat treatment from 110C to 600C of the K-saturated samples resulted in partial collapse of the clay mineral, as inferred from the 001 spacing changing from 1.41 nm to 1.20 nm, which indicated the loss of hydroxy-interlayered materials in the interlayer region of clay minerals (Barnhisel and Bertsch 1989; Chiang et al. 1999). The collapsing behaviours on sequential heat treatments of K-saturated samples were consistent with those of HIV reported by Harris et al. (1992a). The 1.41-nm peak of the air-dried sample in the Jiujiang red earth sediments comprised overlapping peaks of both HIV and vermiculite. Illite--vermiculite or illite--HIV interstratified clays were characterised by the basal XRD peaks between 1.0 nm and 1.4 nm for an air-dried sample (Vanderaveroet et al. 2000). The XRD patterns for the Mg-saturated and K-saturated samples showed obvious peaks at 0.997, 0.498, and 0.333 nm; these showed no shift on the above treatments, indicating the presence of illite in the sample. Kaolinite was identified from its 0.715-nm and 0.357-nm peaks on Mg-saturated and K-saturated samples. Broad 0.715-nm peaks may indicate disorder in the stacking sequence of the kaolinite or the presence of randomly interstratified kaolinite-smectite (Brindley 1980; Churchman et al. 1994). Both the 0.715-nm and 0.357-nm peaks disappeared after heating to 600[degrees]C, indicating that the structure of kaolinite was destroyed at high temperature. The 1.4-nm peak disappeared after heating the K-saturated sample at 600[degrees]C, which indicated that there was no chlorite in the sample (Yeo et al. 1999).
Three main absorption bands were observed on the FTIR spectra in the 3700-3000 [cm.sup.-1] region of the air-dried sample. The strongest absorption band at ~3621 [cm.sup.-1] and the weaker one at -3697 [cm.sup.-1] were assigned to OH-stretching in the octahedral sheet of aluminous minerals such as illite or kaolinite (Farmer 1974). The broad bands at -3485 [cm.sup.-1] and ~3415 [cm.sup.-1] might be attributed to OH-stretching vibrations of hydroxides in the interlayer sheets of HIV, in good agreement with those observed for hydroxyl-Al vermiculite by Ahlrichs (1968). When the air-dried sample was heated to 400[degrees]C, the spectra showed peaks quite similar to the air-dried sample. However, on heating to 600[degrees]C, the adsorption band at ~3697 [cm.sup.-1] disappeared, which resulted from the decomposition of kaolinite. The intensity of the adsorption band at ~3621 [cm.sup.-1] was reduced significantly compared with the adsorption bands between ~3400 [cm.sup.-1] and ~3500 [cm.sup.-1] as the temperature increased, indicating the decrease in kaolinite and vermiculite layers. However, the adsorption bands at -3485 [cm.sup.-1] and ~3415 [cm.sup.-1] were well preserved, suggesting that parts of hydroxides still existed intact in the interlayer of vermiculite. When heating to 600[degrees]C, the relatively increased intensity of ~3485 [cm.sup.-1] and ~3415 [cm.sup.-1] peaks was associated with the collapse of vermiculite layers and decomposition of kaolinite (Fig. 3).
The thermal behaviour of the clay fraction is shown in Fig. 4. The dehydration process of the clay minerals was characterised by a distinct endothermic effect on heating to 100[degrees]C, and the dehydroxylation was characterised by an endothermic effect between 400[degrees]C and 450[degrees]C. The endothermic effects between 60[degrees]C and 80[degrees]C were assigned to adsorbed water over the surface (Egli et al. 2007) or water in the interlayer of clay minerals (Reichenbach and Beyer 1994). The endothermic dehydroxylation effects between 400[degrees]C and 450[degrees]C were attributed to the dehydroxylation of HIV (Harris et al. 19926). However, dehydroxylation of HIV could not be differentiated from that of kaolinite in this temperature range. The obvious exothermic effects at ~768[degrees]C suggested the formation of new phase (Fig. 4).
Measurable Al, Mg, Ca, Fe and K contents were determined in the sodium citrate extracts using 1CP-AES. The amount of Al released was 0.19 cmol/kg, and that of Fe was 0.003 cmol/kg. However, no Mg, Ca and K. were detected in the extract. Far more Al than Fe was extracted, as indicated by the low Fe : Al ratio. No Si was detected in the extracted solution, suggesting that most of Al and Fe came from the interlayer region of clay minerals. The absence of Mg, Ca and K in the extract indicated that those simple cations could not be exchanged by Na within the interlayer of illite, vermiculite and HIV. When saturated with sodium citrate, no obvious change in peaks was observed in the XRD patterns compared with those of the air-dried sample. When solvated with glycol and [Na.sup.+], the 1.41-nm peak showed no shift in the XRD patterns (Fig. 2b), indicating that the amount of Al and/or Fe removed by sodium citrate did not change the expandability of 1,4-nm clay minerals in the Jiujiang red earth sediments.
The 1,4-nm clay species in soils
Although structurally hydroxy-interlayered minerals (HIMS) are rather like chlorites, the main difference is that their interlayer space is not completely filled by the interlayer hydroxide polymers. HIMS all have the same (001) reflection centred at ~1.4 nm, and the different behaviours of these under glycol solvation, [K.sup.+]-saturation and heat treatment are attributed to different types and proportions of Al ion complexes in their interlayer region (Mcunier 2007; Velde and Meunier 2008). When there is a higher proportion of Al-polymers in the interlayer region, the expanded peak near 1.4 nm can withstand higher temperatures. Bamhisel and Bertsch (1989) characterise an HIM that retains its ~1.4nm peak upon heating at 500[degrees]C as a pedogenic chlorite, whereas Velde and Meunier (2008) describe it as a dioctahedral aluminous soil chlorite. Furthermore, interlayered Al-polymers of HIMs with a high proportion of these interlayers cannot be exchanged by [K.sup.+] and other alkali ions, resulting in lower CECs in soils that are rich in these minerals (Meunier 2007; Velde and Meunier 2008). HIV and HIS can be considered to form by adsorption and polymerisation of Al-polymers in the interlayer region of vermiculite and smectite, respectively (Meunier 2007). The basal spacings of HIMs and hydrated forms of vermiculite and smectite range from 1.4 nm to 1.5 nm. HIV does not show any change in its basal spacing upon [K.sup.+]-saturation at 25[degrees]C (Bamhisel and Bertsch 1989; Harris et al. 1992a, 19926), but its basal spacing does change upon heating >300[degrees]C (Douglas 1989). The progressive but incomplete collapse of the (001) reflection of HIV towards 1.0 nm after progressive heating has been reported in many previous investigations (Bamhisel and Bertsch 1989; Harris et al. 1992a, 19926; Lin et al. 2002). For example, Harris et al. (1992a, 19926) found that the basal spacing of an HIV from Florida collapsed to 1.38 nm and 1.15nm on heating to 300[degrees]C and 550[degrees]C, respectively; Lin et al. (2002) and Shaw el al. (2010) found similar effects on heating specimens of HIV. By contrast, Pal et al. (2012b) found that a hydroxy-interlayered mineral showed a peak at 1.38-1.36 nm instead of 1.42nm (true chlorite) after the [K.sup.+]-saturated sample was heated at 550[degrees]C, suggesting that the extent of interlayering was quite high. The authors characterised this particular instance of HIV as a 'pseudo chlorite', which they identified with a pedogenic chlorite. The basal spacing of HIS also collapses towards 1.0 nm upon [K.sup.+]-saturation and progressive heating (Meunier 2007; Velde and Meunier 2008), and although it may expand slightly upon [Mg.sup.2+]-saturation and glycol solvation, its complete expansion to 1.7 nm is generally inhibited (Meunier 2007; Velde and Meunier 2008). Lacking in Al-polymers in the interlayer region, vermiculite and smectite display a greater expandability and contractibility than their hydroxy-interlayered counterparts. Vermiculite does not expand beyond -1.6 nm after [Mg.sup.2+]-saturation and glycol solvation and contracts to close to 1.0 nm in the K-saturated and air-dried state, whereas smectite is expected to expand to 1.7 nm in the glycol-solvated state (Meunier 2007; Hong et al. 20106; Velde and Wang et al. 2011) and collapse to 1.0 nm after [K.sup.+] saturation and/or heat treatment >300[degrees]C (Meunier 2007; Furquim et al. 2008; Perez-Maqueda et al. 2012). Hence, the combination of [K.sup.+] saturation, glycol solvation and heat treatment is an extremely effective way to distinguish HIV from HIS, true chlorite, vermiculite and smectite.
Interlayer components of the HIV
The XRD results indicated that the clay assemblages of the Jiujiang sediments are mainly illite, kaolinite, vermiculite and HIV, with minor illite--vermiculite interstratified minerals and trace interstratified kaolinite minerals. Partial collapse of the 001 spacing from 1.41 nm to 1.20 nm with increasing temperature indicated hydroxy-interlayered materials in the interlayer region of the HIM present in these sediments. No change in the 1,4-nm reflection on [Mg.sup.2+] saturation and glycol solvation identifies vermiculite, not smectite, in its basic layers (Fig. 2a); hence, its characterisation as HIV, rather than HIS.
The FTIR spectra of the clay fractions also indicated the clay assemblage of illite, kaolinite and vermiculitic minerals (including HIV), consistent with the results of XRD. HIV clay was characterised by adsorption bands at ~3485 [cm.sup.-1] and ~3415 [cm.sup.-1], which did not change on heating. The FTIR spectra also suggested that kaolinite was completely destroyed after heating to 600[degrees]C, whereas illite was still present, and the broad peak at ~3621 [cm.sup.-1] suggested that illite crystallinity decreased on heating to 600[degrees]C, in good agreement with the results of XRD analysis (Fig. 3).
Thermal behaviour of the clay fractions was characterised by the DSC results (Fig. 4). Dehydroxylation of the HIV occurred at >400[degrees]C, in a good agreement with results reported by Harris et al. (19926). Change of 001 spacing front 1.41 nm to 1.25nm on heating to 450[degrees]C also indicated dehydroxylation of HIV clay species (Fig. 2a), consistent with the DSC results. The dehydroxylation of kaolinite usually occurs at >500[degrees]C (Guggenheim 2001), and the relatively lower dehydroxylation temperature of kaolinite in Jiujiang soil probably suggests that kaolinite was interlaycred, possibly with vermiculite (Wada and Kakuto 1983).
The ICP-AES result showed that the components extracted by sodium citrate solution included Al and Fe. Generally, Fe occurs as a simple exchangeable ion in the interlayer of clay minerals, whereas the intercalated Al is probably present as hydroxy-Al hydroxides in the interlayer of clay minerals such as smectite and vermiculite from acid soils. The XRD results suggested that partial extraction of Al and Fe from the interlayer by sodium citrate exerted no obvious influence on the intensity of the 1.4-nm reflection (Fig. 2b). In addition, the removal of hydroxy-Al from HIV did not change its expandability after glycol solvation (Fig. 2b). Na+ saturation did not remove any other simple ions except of Fe from the interlayer region of vermiculite. More collapse of the 1.4-nm reflection occurred with [K.sup.+] saturation than with [Na.sup.+] saturation, consistent with more Al exchanged by KCl solution relative to that by sodium citrate solution (Yin et al. 2013). The collapse was attributed to the exchange of the interlayer ions by [K.sup.+], as suggested by the presence of Al, Mg, and Ca in KCl-extracted solution. The extracted Al, Mg, and Ca by KCl solution measured 2.3, 4.2 and 4.3 emol [kg.sup.-1]. respectively.
Therefore, HIV of the Jiujiang red earth sediments is a dioctahedral, Al-rich mineral in which the negative layer charge is compensated by Al polymers exchanged into the interlayer region (Kirkland and Hajek 1972). Hydroxy intercalation may occur following the formation of vermiculite or after further weathering to smectite, and hydroxy-Al cations may be removed by complex formation and chelation with organic matter. The presence of HIV was attributable to conditions favourable for hydroxy interlayer fonnation (Pai et al 2007). Meunier (2007) thought that Al ions at first might occur as monomers with a 6-fold coordination state [Al[(OH).sub.2]. [[([H.sub.2]0).sub.4]].sup.+] in place of alkali cations inside the interlayer zone of natural HIV, and Al polymers were formed by the polymerisation reactions among the Al monomers. Finally, discontinuous, gibbsite-like structures were built in the interlayer of HIV. However, just how gibbsite-like structures are organised inside the interlayer zones of HIV has yet to be determined (Meunier 2007). More advanced analyses are urgently needed in order to solve this problem.
Clay minerals in the Jiujiang red earth sediments were dominated by illite, kaolinite, vermiculite and HIV, with minor illite vermiculite or illite--HIV interstratified minerals and trace interstratified kaolinite minerals. HIV was characterised by partial collapse of a 1.4-nm XRD peak on heating to 600[degrees]C, which indicated the loss of hydroxy-interlayered materials in the interlayer region of HIV. HIV exhibited two FTIR adsorption bands at ~3485 [cm.sup.-1] and ~3415 [cm.sup.-1], respectively, attributed to OH-stretching vibrations of hydroxides in the interlayer sheets of HIV, and these two bands did not change on heating. Dehydroxylation of hydroxides in the interlayer region of HIV occurred at 400[degrees]C, and further dehydroxylation took place at ~600[degrees]C. Exchangeable cations in HIV clays were composed of mainly Al and minor Fe. These indicated that Al was probably present as hydroxy-Al hydroxides, which partially occupied the interlayer region of HIV.
This work was supported by the Natural Science Foundation of China (41272053 and 41072030), the Specialised Research Fund for the Doctoral Program of Higher Education of China (20110145110001) and the Fundamental Research Funds for the Central Universities (CUGL140805). The authors wish to thank Y. S. Gu for the sample preparation, Dr L. Y. Tie for the ICP-AES analyses, Dr W. Y. Cheng for the DSC analyses, and J. S. Yu for the XRD analyses, and especially to Professor D. B. Singh, the Editor, and the anonymous reviewers for their insightful reviews, valuable comments and suggestions.
Received 16 January 2013, accepted 8 April 2014, published online 14 August 2014
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Ke Yin (A,E), Hanlie Hong (A,B), Gordon Jock Churchman (C), Zhaohui, Li (D), Wen Han (A), and Chaowen Wang (A)
(A) Faculty of Earth Sciences, China University of Geosciences, Wuhan, Hubei, 430074, China.
(B) Key Laboratory of Geobiology and Environmental Geology, the Ministry of Education, China University of Geosciences, Wuhan, Hubei, 430074, China.
(C) School of Agriculture, Food and Wine, Waite Campus, The University of Adelaide, SA 5005, Australia.
(D) Geosciences Department, University of Wisconsin--Parkside, Kenosha, WI 53141-2000, USA.
(E) Corresponding author. Email: firstname.lastname@example.org
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|Author:||Yin, Ke; Hong, Hanlie; Churchman, Gordon Jock; Li, Zhaohui; Han, Wen; Wang, Chaowen|
|Date:||Sep 1, 2014|
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