SOME PROBLEMS IN THE IDENTIFICATION OF CLAY MINERALS IN MIXTURES BY X-RAY DIFFRA CTIO N L CHLORITE-KAOLINITE MIXTURES

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1 SOME PROBLEMS N THE DENTFCATON OF CLAY MNERALS N MXTURES BY X-RAY DFFRA CTO N L CHLORTE-KAOLNTE MXTURES By J. L. MARTN VVALD and M. RODRGUEZ GALLEGO Estaci6n Experimental del Zaidin and Departamento de Silicatos, Granada, Spain [Received 15th May, 1961] ABSTRACT The use of thermal and acid treatments, prior to X-ray diffraction examination, for the identification of kaolinite and chlorite in mixture is discussed. Thermal treatment, although useful in confirming the presence of chlorite, cannot give definite information on the presence of kaolinite, but acid treatment is conclusive in this respect. NTRODUCTON Among the problems encountered in this laboratory during the identification of clay minerals in soils and sediments, that of the identification of kaolinite in the presence of chlorite has been one of the most frequent, and a need has been felt for a reliable procedure for distinguishing between these minerals--especially in soil samples from the Vega de Granada. The mineralogical composition of these soils (Martin Vivaldi and Rodriguez Gallego, 1957; Rodriguez Gallego, 1960) is inherited from the parent material--the mica schist of the Sierra Nevada, containing chlorite, mica, and kaolinite. As the second order basal reflection for chlorite coincides with the first order for kaolinite it would be necessary to remove the chlorite, or to modify one mineral with respect to the other, before kaolinite can be identified in a mixture. At the time these soils were examined two solutions to this problem were proposed in the literature: (a) thermal treatment of the sample (MacEwan, 1948; Brindley and Robinson, 1951) and (b) acid treatment of the sample (Brindley and Robinson, 1951). The solubility of chlorite in acid has been studied in detail by Brindley and Youell (1951) and by Pask and Davies (1945). These two procedures have been checked and a preliminary note on the findings has beert published in a paper on some of the soils mentioned above (Martin Vivaldi and Rodriguez Gallego, 1957). Since that time a similar procedure has been used in the study of 288

2 DENTFCATON OF CLAY MNERALS N MXTURES several soils and sediments (Rodriguez Gallego, 1960; Sanchez Camazano, 1960) and a report of the methods used and the findings is given below. EXPERMENTAL Materials. The following samples were used: (a) Triassic chlorite from the Launas (Martin Vivaldi and MacEwan, 1957) containing some sericite and quartz as impurities, and (b) a kaolinite (Thomas Kaolin) from Georgia, U.S.A., containing a small proportion of montmorillonite as an impurity. Oriented aggregates of the samples were prepared with the < 2t~ fraction, separated using sodium carbonate-sodium polymetaphosphate solution. Methods. For the X-ray study a mm diameter Philips camera was used, so modified as to record spacing of the order of 20 A using Cu Ka radiation (Martin Vivaldi, Girela Vilchez and MacEwan, 1959). X-ray photographs were scanned with a Hilger Model H.451 microphotometer. As this instrument is of the filament type it was set slightly out of focus in order to avoid irregularities in the trace arising from the grain size of the emulsion. Thermal treatments consisted of heating the oriented aggregates of chlorite and kaolinite at 450~ 500~ and 550~ for 15 minutes; other samples were heated at the same temperatures for 2 hours. n the acid treatment an 0.5 g sample was refluxed with 30 ml 20 per cent. sulphuric acid for 30 minutes; sulphates were removed by washing and centrifugation, and oriented aggregates were prepared. RESULTS AND DSCUSSON Curves for d against optical density (calculated from the transparencies read on the Galvoscale Projector of the microphotomete O for untreated, acid treated, and thermally treated samples are shown in Fig. 1. Samples heated for periods of 2 hours are not included as the only difference is that heating at 500~ for 2 hours gives results identical with heating at 550~ for 15 minutes. The results show that thermal treatment does not help in the identification of kaolinite in the presence of chlorite, since the second order basal reflection for chlorite and the first order basal reflection for kaolinite disappear or weaken at the same temperature. The results for chlorite are in accordance with those obtained by Weiss and Rowland (1956) using the oscillating-heating method, and the results for kaolinite are in_

3 290 a. L. MARTEN V1VALD AND M. RODRGUEZ GALLEGO agreement with those of Bradley (1953) in that rapid firing to 450~ causes little change in the intensity of the 7 A reflection. However, thermal treatment is useful in showing the presence of iron-rich chlorite which has a weak 14/~ reflection that strengthens on heating (Brindley and Robinson, 1951). Curves E and K (Fig. 1) show the complete absence of the 14/~ and 7A reflections for chlorite and the persistence of 7 A reflection D FG. 1--Microphotometer traces from X-ray diffraction photographs of chlorite and kaolinite after various treatments: A--oriented aggregate of chlorite; B, C and D--oriented aggregates of the same material heated at 450~ 500~ and 550~ for 15 minutes; E--oriented aggregate of the same material after acid treatment; F--oriented aggregate of kaolinite; G, H and/--oriented aggregates of the same material heated at 450~ 500~ and 550~ for 15 minutes; K--oriented aggregate of the same material after acid treatment. for kaolinite after the acid treatment described above. No exhaustive study has been made of the effect of acid treatment on other minerals of the chlorite and kaolinite groups and, although many chlorites dissolve in warm acid, it may be that some do not (Brindley and Robinson, 1951). Similarly, the method may not apply to all minerals of the kaolin group, as those containing iron would probably dissolve.

4 DENTFCATON OF CLAY MNERALS N MXTURES TABLE 1--X-ray photographs from an oriented aggregate of Sample No. 18 (Sanchez Camazano, 1960). 14"1 vw 10.1 ms 7'16 ms 4"98 w 3.54 ms 3'33 s 10.0 s 7.0 s 4-98 m 3.54 m 3-33 vs TABLE 2--X-ray photographs from an oriented aggregate of Sample No (Rodriguez Gallego, 1960) w 9.81 s 7-02 ms 5 '09 m 3"53 ms 3-29 m a(a) 9.82 s 7.07 mw 5.06 mw 4.33 vw 3.47 vw 3-25 s TABLe 3---X-ray photographs from an oriented aggregate of Sample No. 179 (Martin Vivaldi et al., 1957). d(,~) d(/~) 14"0 10'0 7'1 5"0 4"73 3 '54 3'33 S vs 9.99 ms 7-22 s 4"99 m -- 4 "08 ms 3.58 vs 3-33 W m W

5 292 J.L. MARTN VVALD AND M. RODRGUEZ GALLEGO Because of the low solubility of some chlorites in hydrochloric acid, Lucas, Camez and Millot (1959) believe that no systematic procedure for the characterization of kaolinite in the presence of chlorite is possible. Nevertheless, in the authors' laboratory acid treatment has been applied to hundreds of samples containing mixtures of chlorite and kaolinite (Martin Vivaldi and Rodriguez Gallego, 1957; Rodriguez Gallego, 1960; Sanchez Camazano, 1960; Martin Vivaldi et al., 1957) with consistent success, and it has been found to be very helpful and simple in routine work. The results in Tables 1, 2 and 3 illustrate some examples of its applicability. REFERENCES BRADLEY, W. F., Analyt. Chem., 25, 727. BRNDLEY, G. W., and ROBrNSON, K., X-ray dentification and Crystal Structures of Clay Minerals (G. W. Brindley, editor). Mineralogical Society, London, Chapter 6, p BRNDLEY, G. W., and YOUELL, R. F., Acta Cryst., 4, 495. LtrcAS, J., CAMEZ, T., and MmLOT, G., Bull. Carte gdol Als. Lorr., 12, No. 2, 21. MACEWAN, D. M. C., Verre Silic. ndustr., 12, 1. MARTN VVALD, J. L., FONTBOTE, J. M., RAUSELLS, J. A., and TRUYOLS, J., Estudios GeoL, 14, 305. MARTN VVALD, J. L., and MACEWAN, D. M. C., Clay Min. Bull., 3,177. MARTN V1VALD, J. L., GRELA VLCHEZ, F., and MACEWAN, D. M. C., Clay Min. Bull., 4, 110. MARTN VVALD, J. L., and RODRGUEZ GALLEGO, M., Mineralogia de los suelos de la Vega de Granada. Diputacion Prov. de Granada. PASK, J. A., and DAVES, L., Tech. Pap. Bur. Min., Wash., No. 664, 56 RODRGUEZ GALLEGO, M., Thesis, University of Granada. SANCHEZ CAMAZANO, M., Thesis, University of Salamanca. WESS, E.., and ROWLAND, R. A., Amer. Min., 41, 117.