PREPARATION AND PROPERTIES OF A BASIC LEAD CARBONATE-- MONTMORILLONITE COMPLEX

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1 Clay Minerals (1993) 28, This paper is dedicated to Professor Lisa Heller-Kallai on the occasion of her 65th birthday. PREPARATION AND PROPERTIES OF A BASIC LEAD CARBONATE-- MONTMORILLONITE COMPLEX S. TSUTSUMI, A. YAMAZAKI*,M. UEHARAAND R. OTSUKA* Institute of Earth Science, School of Education, Waseda University, Nishiwaseda, Shinjuku-ku, Tokyo , and *Department of Mineral Resources Engineering, School of Science and Engineering, Waseda University, hkubo, Shinjuku-ku, Tokyo 169, Japan (Received 30 September 1991; revised 27 January 1992) A B S T R A C T: A basic lead carbonate-montmorillonite complex was prepared by treating a natural montmorillonite hydrothermally for 120 h at 250~ with lead powder, dry ice and lead nitrate solution. The product is a non-swelling material showing well-outlined, hexagonal, thin plates <1/~m in size; the symmetry is pseudo-orthorhombic, a = 5.141(7) A, b = 9.005(5) A, c = (4) A, and Z = 2. The X-ray powder pattern is characterized by a 17.4 A reflection and an integral series to the 14th order. The TG-DTA curves of this 17 A-mineral showed one endotherm around 400~ accompanied by weight loss and two exthotherms at about 680 and 780~ By applying hightemperature X-ray diffractometry (XRD) and infrared (IR) spectroscopy, it was found that the endotherm is due to decomposition of carbonate hydroxide in the interlayer, while the two exthotherms are caused by crystallization of a hexagonal phase of PbA12Si208 and by the conversion of this phase into lead feldspar, respectively. The crystal structure of the 17 A-mineral was determined and refined as a 2:1 dioctahedral smectite interlayered with a hydrocerussite-like layer by a one-dimensional Fourier synthesis method. Hayase et al. (1978) studied a Pb-rich, clay-like material occurring in the oxidation zone of Pb-Zn-Cu deposits of the Cruz del Sur Mine, Argentina, and described this material as a new mineral species, surite. The mineral has the structure of a 2: l layer-silicate similar to dioctahedral smectite intercalated with a cerussite-like layer. Little is known about the genesis and crystal structure of this mineral, but synthesis of surite has been attempted in various ways for many years. In the course of the synthetic study, a product having a basal reflection of -17A (hereafter, abbreviated to 17 A-mineral) was obtained. The detailed study indicated that this material was a clay-inorganic complex, the structure of which was based on a basic lead carbonate intercalated with a 2:1 silicate layer of montmorillonite. This paper describes (i) the preparation of the 17 A-mineral; (ii) its properties which were characterized by X-ray powder diffractometry, transmission electron microscopy (TEM), thermal analysis and IR absorption spectroscopy; and (iii) its crystal structure as revealed by a one-dimensional Fourier synthesis method. Sample preparation EXPERIMENTAL The starting material was natural montmorillonite, purified from Aterazawa bentonite, Yamagata Pref., Japan (Kunimine Co., Ltd.). Impurities such as quartz and feldspar were The Mineralogical Society

2 14 S. Tsutsumi et al. removed by the hydraulic elutriation technique. Hydrothermal experiments were performed using a Morey-type bomb, containing a Teflon bottle --60 ml in volume. Initially, 360 mg of starting material and 1160 mg of lead powder (corresponding to meq/100 g of starting material) (Junsei Chemical Co., Ltd) were placed in the Teflon bottle, and then 35 ml of 0.5 N Pb(NO3)2 solution adjusted at ph = 1 by dilute nitric acid, were added. After the suspension had been stirred well, mg of dry ice was put in the bottle to provide CO2, and the bottle was sealed immediately. The mixture was hydrothermally treated for 120 h at 250~ in an electric furnace. The product was then centrifugally separated from the solution and washed five times with distilled water. Finally, impure products such as cerussite and hydrocerussite were removed by the hydraulic elutriation technique. Analytical methods X-ray powder diffraction was carried out with a Rigaku Geigerflex RAD-RC using graphite monochromatized Cu-K0lradiation (55 kv, 200 ma). Each reflection of the 17 ~- mineral was indexed and the unit-cell parameters were calculated using the program by Appleman & Evans (1973) modified and improved for a NEC PC9801 personal computer by the authors. High-temperature X-ray powder diffraction was carried out using a Rigaku Giegerflex RAD-IC with a high temperature attachment, using Ni-filtered Cu-Ko:radiation (40 kv, 20mA). In order to investigate the fundamental layer structure of the 17/~-mineral, interlayer materials were removed by the following acid treatment: the 17 A-mineral was treated with 0.75% HC1 at room temperature for five days and then washed with distilled water; the residue was then dispersed in 1 N NaC1 solution and stirred with a magnetic stirrer for 24 h after which the product was centrifugally separated from the solution, washed five times with distilled water, and the exchangeable cations in the interlayer were completely exchanged by Na ions. Transmission electron micrographs were taken with a Philips CM30 at an accelerating voltage of 300 kv. Chemical analysis was carried out by using an energy-dispersive analysis system (Tracor Northern TN5400 EDS) installed in an electron probe microanalyser (JEOL JXA733) using an accelerating voltage of 15 kv and a beam current of A. The CO2 content was determined by using a HITACHI Type 163 gas chromatograph equipped with a hightemperature dissolution attachment. The water content was obtained by subtracting the CO2 content from the weight loss between room temperature and 1000~ determined by thermogravimetry. The measured values of the elements, except CO2 and H20, were corrected by the ZAF correction method. The TG-DTA curves were recorded with a Rigaku Thermoflex 8076 at a heating rate of 10 K/min in static air, N2 and CO2 with a flow rate of 100 ml/min. The IR spectra were recorded with a JASCO FT-IR-8000 spectrometer, using the KBr disk method. XRD and EM Data RESULTS AND DISCUSSION The XRD data for the 17 A-mineral formed during the hydrothermal treatment are given in Table 1. Assuming that the symmetry of 17 ]k-mineral was pseudo-orthorhombic

3 Basic lead carbonate--montmorillonite complex 15 TABLE 1. X-ray powder diffraction data for the 17 A-mineral. dobs/a dcalc/a h k l 1/lo* * Relative intensities obtained by applying the slit correction. (/3 = 90.0~ the unit-cell parameters obtained were a = 5.141(7) A, b = 9.005(5) A and c = (4) A. In the diffraction pattern, the basal reflections corresponding to a spacing of 17-4 A up to the fourteenth order were observed, and the d-spacing of the 060 reflection was A. This suggests that the structure of the 17 A-mineral can be regarded as based on dioctahedral smectite. Treatment with ethylene glycol did not cause discernible change in the intensities and spacings of the overall peaks, and swelling character was not observed. The XRD pattern of the residue obtained with the series of acid treatments described showed a basal reflection corresponding to a spacing of A, clearly expanding to 17-2 A after treatment with ethylene glycol (Fig. 1). These properties agree with those of a Na form of montmorillonite. Hence, it is suggested that the structure of the 17 A-mineral can be regarded as based on the 2:1 silicate layer of dioctahedral smectite. Figure 2 shows well-outlined, hexagonal thin plates <1 /tm in size, which are very different in morphology from common montmorillonite, which consists of irregular-shaped or curled thin plates. The well-defined shape of the 17 A-mineral is similar to that of a basic lead carbonate (Cowley, 1956). Hence, the TEM observation suggests that the morphology of the 17 A-mineral is strongly controlled by that of the interlayer material. Chemical composition and structural formula From the chemical analysis listed in Table 2 of the 17 A-mineral and the residue after treatment with HCI, the chemical composition of the 17 A-mineral can be regarded as a 2:1 dioctahedral smectite interlayered with basic lead carbonate.

4 16 S. Tsutsumi et al. I do01= 17.40~ I~, J! il i, ~- Ca) I I--~ w ~L..I o-i I rl2 <~ dool= 12.,47A (b) doot= 17.2A (C) (Cu-Ka)/degree FIG. 1. X-ray diffraction patterns of (a) 17 A-mineral; (b) residue obtained from the 17 A-mineral powder pretreated with 0-75% HCI and Na-exchanged; (c) residue treated with ethylene glycol. FIG. 2. Transmission electron micrograph of the 17 A-mineral.

5 Basic lead carbonate--montmorillonite complex 17 TABLE 2. Chemical compositions and formulae of the 17/i-mineral (1), and residue obtained from the 17/i-mineral powder pretreated with 0-75% HCI and Na-exchanged (2). (1) (2) SiO A Fe MgO CaO trace trace Na20 trace 2-72 K20 trace trace PbO trace CO H2 O H Total Composition per formula unit containing O10(OH)2 Si Alte t 0" 19 0"05 Aloct 1"70 1 "65 Fe Mg Interlayer composition Ca - - Na K Pb CO 3 1"88 -- OH H An interlayer negative charge was assumed to be close to -0.37, obtained for the structural formula of the Na-exchanged residue after HC1 treatment corresponding to a Na form of montmorillonite, as shown in (2) of Table 2. Based on this assumption, the following structural formula was obtained for the 2:1 layer of the 17/i-mineral: (Si3.81Alo.19)(All.voFeo.15Mgo.15)Olo(OH)2 The interlayer negative charge from this formula is -0.34, very close to The amount of tetrahedral A1 in the residue was very small compared with that of the 17 A- mineral, as shown in Table 2. This was presumably because some A1 ions had been dissolved by acid treatment. Based upon the interlayer charge of 0.34, the amounts of Pb and CO3 in the interlayer were calculated as 2.90 and 1.88, respectively. If the basic lead carbonate in the interlayer has a composition similar to that of hydrocerussite (Pb3(CO3)2(OH)2) and Pb 2+ compensates for the negative layer charge of -0-34, then the chemical composition of the interlayer substance can be estimated as Pb2.90(CO3)t.82(OH)l.s2, in good agreement with the results of the chemical analysis in

6 18 S. Tsutsumi et al. Table 2. Excess 0.15H20 may be ascribed to adsorption water. Hence, the structural formula of the 17 A-mineral in a half unit-cell can be represented as follows: Pb2-90(CO3) 1.82(OH) 1.82(Si3.81A10.19)(Ala.70Feo. 15Mgo. 15)O10(OH)2 TG-DTA curves Figure 3 shows the TG-DTA curves of the 17 A-mineral in various atmospheres (a-c), the montmorillonite used as the starting material (f), basic lead carbonate (d), and the mixture of montmorillonite and basic lead carbonate, prepared so as to have the same composition as the 17 A-mineral (e). In static air, the DTA curve of the 17 A-mineral has an endothermic peak at 400~ and two exothermic peaks, one at -680~ and the other around 780~ These peak temperatures were almost unchanged in N2 gas flow, while the TG ~t I (c) ~ (d) (0 I I [ I 1 I I I I 1 O o C FIG. 3. TG-DTA curves. (a)-(c): 17 A-mineral; (d): basic lead carbonate; (e): mixed powder of basic lead carbonate and montmorillonite; (f): montmorillonite.

7 Basic lead carbonate--montmorillonite complex 19 ~ ~,r R T ~,~,,,,,,]! 275 ~ 3oo ~ ~I '~..~,,,~.,,,.~~.1 k~ 450 ~ p P p 0 ~ t ~,,.~,;~ p 675 C o k_~.~ ~,.~~ ~,,,~~,. ' L L ~LL L r il~l L o I [ I I I I (Cu-Ka)/degree FIG. 4. X-ray diffraction patterns of the 17 A-mineral at various temperatures. P: PbA12Si208; L: lead feldspar. endotherm around 400~ became slightly sharper and 15~ higher, and the temperature of the exotherm around 680~ lowered by 15~ in CO2 gas flow. As the endotherm around 400~ was accompanied by a loss in weight, this suggested that it was caused by decomposition of carbonate hydroxide situated in the interlayer. This endotherm, however, was nearly 50~ higher than the endotherm at 360~ due to decomposition of basic lead carbonate, as shown by curve (d) in Fig. 3.

8 20 S. Tsutsumi et al ?, I I I 200 o C I I Fro. 5. Values of the 001 reflections for the 17 A-mineral as a function of temperature. The TG-DTA curves of the mixture of montmorillonite and basic lead carbonate showed several endotherms accompanied by weight loss (curve (e), Fig. 3), Of these, two endotherms, near 100~ and around 680~ were due to dehydration of montmorillonite (curve (f), Fig. 3). In addition, a two-step endotherm followed by a sharp endotherm around 350~ were caused by decomposition of basic lead carbonate (curve (d), Fig. 3). The TG-DTA curves of the 17 k-mineral were quite different from those of the mixture, despite their similar chemical compositions. This indicates that the 17,~-mineral is not a simple mechanical mixture of montmorillonite and basic lead carbonate, but has the abovementioned interlayered structure. High-temperature X-ray powder diffractometry Figures 4 and 5 show the high-temperature XRD patterns of the 17 A-mineral, and d(001) as a function of temperature, respectively. Heating below 275~ did not cause appreciable changes in the XRD patterns. The basal spacing began to contract due to the release of both H20 and CO2 from the interlayer at 300~ and d(001) decreased to -15 at 325~ The 001 reflection of this heated product at 350~ did not expand with ethylene glycol treatment. All of the reflections began to weaken at 450~ and disappeared at 600~ A hexagonal phase of PbAlzSi2Os (Sorrell, 1962; JCPDS ; P in Fig. 4) crystallized at 675~ and the intensities reached the maximum at 700~ The reflections of lead feldspar (PbA12Si208, L in Fig. 4) appeared at 725~ and those of the hexagonal phase disappeared at 750~ These results indicate that the exotherms around 680 and 780~ were caused by the crystallization of a hexagonal phase of PbA12SizOs, and by the conversion of this phase into lead feldspar, respectively. Infrared absorption spectra Figure 6 shows the Fourier-transformed IR absorption spectra of the 17 A-mineral heated at the temperatures indicated for 30 min. The absorption bands at 1385, 855 and 685 cm -1 were assigned to v3, v2 and v4 of CO3, and the bands at 1000, 525 and 470 cm ~ to the 2:1 layer-silicates. Further, the 3620 cm 1 band was assigned to an OH group of the layer-silicate, while the 3560 cm 1 band was assigned to that of the hydrocerussite-like layer. The IR spectrum of the 17 k-mineral showed no discernible changes in intensities and wave numbers between room temperature and 200~ The 3560 cm 1 band

9 Basic lead carbonate--montmorillonite complex 21 became broad at 300~ and disappeared at 400~ whereas the band assigned to CO3 became weak and broad at 400~ and disappeared completely at 600~ suggesting that the basic lead carbonate layer of the 17 A-mineral dehydrated prior to the decomposition of carbonate. This thermal behaviour agrees well with those obtained by several workers (Kotama et al., 1969; White, 1974; Ball & Casson, 1977; Flemming et al., 1984) for basic lead carbonate. The bands due to the 2:1 silicate layer became broad at 600~ at which temperature the overall X-ray reflections disappeared (Fig. 4), showing distintegration of the 2:1 layer. 400 ~ IIIllllll I I I I I t t i"'-. ~ I I L Wave Number/cm- 1 FIc. 6. Infrared absorption spectra for the 17 A-mineral at various temperatures.

10 22 S. Tsutsumi et al. Structural formula and structure model The properties of the 17 A-mineral suggest that the mineral can be regarded as composed of a 2:1 layer of a dioctahedral smectite with a hydrocerussite-like interlayer. Based on this assumption, the crystal structure of the 17 A-mineral was examined by the one-dimensional Fourier synthesis method. Assuming that the symmetry of basic lead carbonate is hexagonal, the cell parameters are considered to be a = 9.06 A and c = 8.27 A (1/3 unit-cell of hydrocerussite, Cowley, 1956; Katz & Reed, 1957). As d(001) of the 17 A-mineral is 17.4 A and the thickness of the 2:1 layer along the c-axis is approximately 9.5 A, it is considered that a reasonable model of the crystal structure is a hydrocerussite-like layer intercalating with a 2:1 layer of montmorillonite in the same c-axis direction. On the basis of the above model, X-ray integrated intensities of the basal reflections from the first to the twelfth order were calculated in different cases where z parameters of ion planes and also ion contents in each plane were sightly different. Figure 7 shows the best fit between the observed and calculated one-dimensional electron density distribution curves which were obtained by a Fourier synthesis method, and the refined structural model of the 17 A-mineral is also shown with atomic parameters. Table 3 shows the comparison of F(00i)obs with F(00l)cal c and gives R = 0.126, revealing that the structure model is reasonable. The Pb planes occur at two levels: one is located in the middle of the basic lead carbonate layer (z = 8-70), and the other is between the CO3 and OH planes (z = 5.94). The distribution of Pb ions between the inner and outer Pb planes is obtained as Pb*0.91Pbl.99(OH)l.82(CO3)l.82, where Pb* is defined as Pb ions on the inner plane (z = 8.70). The ideal structural formula of a basic lead carbonate is Pb*Pb2(OH)2(CO3)2. Hence, the interlayer cationic charge of about for the 17 A-mineral is produced from Pb ions on the outer plane (z = 5.94). The z parameters shown in Fig. 7 indicate that the distance between the basal oxygen plane of the 2:1 silicate layer and CO3 plane is A. This distance seems to be short o.< o 2... ) ( ~:: _... ~ 2.68 ~ - ~ ~ ~.... L o o iii o o o o "o o'o L" 8.70 ELECTRON DENSITY ~ o.~o o.~o o~o o~ o.-o C03 I J o OH 9 Pb AI,Fe,Mg O,OH Si,A! O C03 Fro. 7. One-dimensional Fourier synthesis of electron density normal to ab plane (left)--solid line calculated, dotted line observed--and structural model (right) for the 17 A-mineral.

11 Basic lead carbonate--montmorillonite complex 23 TABLE 3. Observed and calculated structural factors of 00l reflections for the 17/~-mineral. 00l dool/ A l/ Io IF]obs Fcalc* * obtained from the final z parameters (Fig. 7) considering the van der Waals radius of these molecules, but is slightly long compared with the ionic radius. Because the peak of either an electron density or Fourier wave is discernible at z = 4, ions of either H + or H30 + may lie between the CO3 plane and the basal oxygen plane of the 2:1 silicate layer. According to Hayase et al. (1978), sufite is monoclinic, a = 5.22 A, b = 8.97/k, c = 16-3/k, fl = 96.1~ the chemical formula is Pb(Pb,Ca)l.17(CO3)2(A1,Fe,Mg)2(Si3.67A10.33)O10 (OH)2. Furthermore, the crystal structure of surite is based upon a cerussite-like layer intercalated with a 2:1 silicate layer similar to dioctahedral smectite as deduced from the one-dimensional Fourier synthesis method. This suggests that the interlayer material of the 17/k-mineral has a quite different structure from that of surite, and these two species are quite distinct from each other. ACKNOWLEDGMENTS The authors would like to thank Dr. A. Tsunashima, National Institute for Resources and Environment, and Prof. S. Yamanaka, Hiroshima University, for their helpful comments. We are also indebted to Dr. M. Shiraishi, National Research Institute for Resources and Environment, for permission to use the Institute's CM30 transmission electron microscope. This study was partially supported by a Waseda University Grant for Special Research Projects (63B-5). REFERENCES APPLEMAN A. & EVANS H.T. (1973) Indexing and Least-Square Refinement of Powder Diffraction Data. P.B. Rep. No , National Technical Information Services, Springfield. BALL M.C. & CASSON M.J. (1977) Thermal studies on lead(ii) salts-ii, J. Inorg. Nucl. Chem. 39, COWLEY J.M. (1956) Electron-diffraction study of the structure of basic lead carbonate, 2PbCO3.Pb(OH)2. Acta Cryst. 9, FLEMMIN~ N.J., LOPATA V.L, SANIPEW B.L. & TAYLOR P. (1984) Thermal decomposition of basic lead carbonates: A comparison of hydrocerussite and plumbonacraite. Thermochim. Acta. 81, 1-8. HAYASE K., DRISTAS J.A., TSUTSUM1 S., OTSUKA R., TANABE S., SUDO T. & NISHIYAMA T. (1978) Surite, a new Pbrich layer silicate mineral. Am. Miner. 63, KATZ G. & REED L. (1957) The unit cell and space group of basic lead carbonate. Acta Cryst. 10, 142.

12 24 S. Tsutsumi et al. KOTAMA R., NISHI Y. & KANO M. (1969) Thermal decomposition of lead hydroxide carbonate. Nippon Kagaku Zasshi, 90, SORRELL C.A. (1962) Solid state formation of barium, strontium and lead feldspars in clay-sulfate mixture. Am. Miner. 47, WHITE W.B. (1974) Carbonate. Pp in: The Infrared Spectra of Minerals (V.C. Farmer, editor), Mineralogical Society, London.