NOTE EVALUATION OF SELECTIVE DISSOLUTION EXTRACTANTS IN SOIL CHEMISTRY AND MINERALOGY BY DIFFERENTIAL X-RAY DIFFRACTION

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1 Clay Minerals ( 1985) 20, NOTE EVALUATION OF SELECTIVE DISSOLUTION EXTRACTANTS IN SOIL CHEMISTRY AND MINERALOGY BY DIFFERENTIAL X-RAY DIFFRACTION Selective dissolution analysis is widely used to separate various soil minerals (e.g. oxides and oxyhydroxides of A1 and Fe, allophanes, phyllosilicates) from each other. Although a wide variety of reagents has been used for some of these determinations, few rigorous comparative studies have been attempted. Too often, reagents used to extract particular soil components are evaluated using geological or other specimens that may bear little resemblance to soil minerals formed by pedogenic processes; the investigations of Borggaard (1982) and Chao & Zhou (1983) are two recent examples of such an approach. Insufficient use has been made of difference infrared spectra (Wada & Greenland, 1970) obtained from soil samples. Ideally, soil extractants should be tested on phases actually present in a wide range of soils. In soils, crystals of minerals may differ considerably in size, order and degree of isomorphous substitution. Consequently any empirical selective dissolution technique that involves exposing soils to a fixed concentration of a particular reagent, under defined conditions of time and temperature, is unlikely consistently to completely extract only the component it was intended to remove yet never attack other phases. Even poorly-ordered minerals like ferrihydrite may vary in crystallinity, and thus in the time taken to dissolve in an extracting reagent (Schwertmann et al., 1982). The recent development of a differential X-ray diffraction (DXRD) technique (Schulze, 1981; Schwertmann et al., 1982) now makes it possible to test and evaluate most selective dissolution reagents using only soil samples. In DXRD, an X-ray diffraction (XRD) pattern of a sample from which all or part of some mineral component(s) has been removed by selective dissolution is computer-subtracted from the pattern obtained before its removal. A resistant soil mineral such as quartz may be used as an internal standard to calculate the scale factor used to adjust the pattern being subtracted, in order to compensate for any increase in intensity of the residual components (Campbell & Schwertmann, 1984). If the sample does not contain any suitable internal standard a-alumina may be added (Bryant et al., 1983). Use of an internal standard removes any possibility of creating spurious peaks through the application of an incorrect scale factor. If the surface of the sample is not flush with the top of the sample holder in both the treated and untreated samples, maxima of sharp peaks (such as those of quartz) will be displaced, and may not coincide when subtracted. This produces characteristic 'blips' in the DXRD pattern. This problem is minimized by the use of a diffraction control unit (such as the Philips PW 1710 used to obtain the patterns shown in Fig. 1) that can search a selected peak for maximum intensity, and then use that position to calibrate the 20 scale The Mineralogical Society

2 516 Note UT-DCB UT-OX Q f ( P P P Q Q Q QP P~ OX UT ,A... i l, i i I,h i I 7 b ' 5' 0 30 ' ' 10 ' Q2O FIG. 1. XRDvT, XRDox, DXRDvr ox and DXRDuTmc B patterns for 2Csr horizon clay from a Haplaquept. (Note: the vertical scale (intensity = counts sec -1) has been exaggerated by a factor of four3 A, allophane; f, feldspar; P, phybosilicate; Q, quartz. Sample preparation and treatment, and instrument parameters, were as listed in Campbell & Schwertmann (1984), except that the samples in Fig. I were scanned from 5 ~ to 75 ~ 20 using Fe-Kc~ radiation (50 kv, 40 ma). All DXRD diagrams were smoothed as described in the above paper. Fig. 1 shows four diffraction patterns for the clay (<2 ~tm) fraction of the 2Cs~ horizon of a Katrine silt loam (Haplaquept, or 'Spodic Ochraquult') samples at Bealey Spur, Canterbury, New Zealand (Young, 1980). IR spectra of this clay indicate that it contains the proto-imogolite form of allophane (Young et al, 1980). Tile XRD pattern of the untreated (UT) clay shows the presence of quartz, feldspar and phyllosilicates. Broad bands at 3.40 and 2.25.A, and an increase in low-angle scattering, indicate allophane (Yoshinaga & Aomine, 1962; Brown, 1980). Oxalate (OX) dissolution (Schwertmann, 1959, 1964) reduced low-angle scattering and the intensity of the bands at 3.40 and 2.25 A, and increased the intensity of the quartz, feldspar and phyllosilicate peaks. DXRD patterns clearly show that both oxalate and dithionite-citrate-bicarbonate (DCB) (Mehra & Jackson, 1960) dissolve allophane. Both patterns show a broad peak at 15 A with smaller peaks at 3.40 and 2.25.A. Further broad peaks are just discernible at 4.40 and 2.25 A. The relative areas of the peaks at 4.40 and 2.25 ~. suggest that oxalate is the more effective reagent for dissolving allophane. The amounts of aluminium dissolved by DCB

3 Note 517 L L L L L G UT-OX H H H H H 0X L L G P L G HG O p L ph ph H L H G H H G UT ~, i i I, K t i I,,, 8b 6b e FIG. 2. XRDuT, XRDox and DXRDuT_ox patterns for B horizon clay from a Placaquept. (Note: a constant vertical scale has been used.) A, Allophane; G, goethite; H, hematite; L, lepidocrocite; P, phyllosilicate; Q, quartz. and oxalate were 5.2 and 11.5% respectively. The sample contains 11.3% total carbon and it is likely that oxalate is also removing some A1 from organic complexes. The small negative peak near 7 A in the DXRDvT_OX diffraction trace is caused by a slight increase in preferred orientation of basal phyllosilicate spacings following oxalate dissolution. Fig. 2 shows diffraction patterns from the clay (<2 #m) fraction isolated from the 10 cm of the B horizon lying immediately below the placic horizon (at 50 cm) of a Swampy silt loam (Placaquept) developed on basalt near Dunedin, New Zealand (Staff, New Zealand Soil Bureau, 1968). The XRDuT pattern shows the presence of goethite, lepidocrocite and hematite, together with quartz (from loess) and phyllosilicates. The reduction in intensity of the lepidocrocite (020) peak (at 6.27 A) relative to the goethite (110) and phyllosilicate (020) peaks (at 4.17 and 4.48 A respectively) following oxalate treatment, which dissolved 5.0% by weight of Fe from the sample, shows oxalate has dissolved some lepidocrocite. This dissolution is shown even more clearly in the DXRDuT_ox pattern, which apart from a small goethite (110) peak, and some minor misalignment of the quartz (101) peak, contains only peaks from lepidocrocite. The flat baseline indicates that little, if any, poorly-ordered material was present in the sample and dissolved by the oxalate. The reduction in intensity of the lepidocrocite (020) peak following oxalate dissolution indicates that about 45% of that mineral had dissolved. Comparison of the intensities of the (020) peak in the DXRD~T_OX and XRDuT patterns shows 50% of the lepidocrocite was oxalate-soluble under the experimental conditions used.

4 518 Note Although use of the DXRD technique has so far been restricted to identification of iron oxide minerals, it is clearly capable of much wider application in soil chemistry and mineralogy. DXRD provides a simple method for determining which, if any, inorganic phases are removed from soil clay samples by selective dissolution reagents. It may also be used to determine if reagents used to extract organically-bound aluminium and iron are also dissolving inorganic phases, as long as the amounts dissolved are sufficient to be detected in the DXRD diagram. Sodium pyrophosphate is commonly used to extract aluminium and iron from organic complexes (McKeague et al., 1971), but there is evidence that in some soils it is also extracting non-organic forms (Higashi et al., 1981). DXRD would readily show if the reagent was capable of attacking crystalline phases in soils and could, if the amounts dissolved were sufficient, show the solution of poorly-ordered inorganic phases such as allophane and ferrihydrite. DXRD, together with difference IR spectroscopy, should be adopted as the most suitable means available for evaluating reagents for use in selective dissolution analysis. Institut fur Bodernkunde, Technische Universitiit Mfinchen, 8050 Freising-Weihenstephan. FRG. Received 21 January 1985; revised 3 July A. S. CAMPBELL* U. SCHWERTMANN REFERENCES BORGGAARD O.K. (1982) Selective extraction of amorphous iron oxides by EDTA from selected silicates and mixtures of amorphous and crystalline iron oxides. Clay Miner. 17, BROWN G. (1980) Associated minerals. Pp l0 in: Crystal Structures of Clay Minerals and their X-ray Identification (G. W. Brindley & G. Brown, editors). Mineralogical Society, London. BRYANT R.B., CUR1 N., ROTH C.B. & FRANZMEIER D.P. (1983) Use of an internal standard with differential X-ray diffraction analysis for iron oxides. Soil Sci. Soc. Am. J. 47, CAMPBELL A.S. SCHWERTMANN U. (1984) Iron oxide mineralogy of placic horizons. J. Soil Sci. 35, CHAO T.T. & LIvl ZHOU (1983) Extraction techniques for selective dissolution of amorphous iron oxides from soils and sediments. Soil Sci. Soc. Am. J. 47, HIGASHI T., DE CONINCK F. & GELAUDE F. (1981) Characterization of some spodic horizons of the Campine (Belgium) with dithionite-citrate, pyrophosphate and sodium hydroxide-tetraborate. Geoderma 25, McKEAGUE J.A., BRYDON J.E. & MILES N.M. (1971) Differentiation of forms of extractable iron and aluminium in soils. Soil Sci. Soc. Am. Proc. 35, MEHRA O.P. & JACKSON M.L. (1960) Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. Clays Clay Miner. 7, SCHWERTMANN U. (1959) Die fraktionierte Extraktion der freien Eisenoxide in B6den, ihre mineralogischen Formen und ihre Entstehungsweisen. Z. Pflanzenerndhr., Dfing., Bodenkunde 84, SCHWERTMANN U. (1964) Differenzierung der Eisenoxide des Bodens durch photochemische Extraktion mit saurer Ammoniumoxalat-L6sung. Z. Pflanzenerndhr., Dfing., Bodenkunde 105, SCHWERTMANN U., SCHULZE D.G. & MURAD E. (1982) Identification of ferrihydrite in soils by dissolution kinetics, differential X-ray diffraction and M6ssbauer spectroscopy. Soil Sci. Soc. Am. J. 46, SCHULZE D.G. (198 l) Identification of soil iron oxide minerals by differential X-ray diffraction. Soil Sci. Soc. Am. J. 45, * Permanent address: Department of Soil Science, Lincoln College, Canterbury, New Zealand.

5 Note 519 STAFF, NEW ZEALAND SOIL BUREAU, DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH, DEPARTMENT OF AGRICULTURE & FOREST SERVICE (1968) Hygrous to Hydrous Brown Granular Loams and Clays. Pp in: General Survey of the Soils of South Island, New Zealand. N.Z. Soil Bureau Bulletin 27. Government Printer, Wellington. WADA K. (~ GREENLAND D.J. (1970) Selective dissolution and differential infrared spectroscopy for characterization of 'amorphous' constituents in soil clays. Clay Miner. 8, YOSHINAGA N. (~ AOMINE S. (1962) Allophane in some Ando soils. Soil Sci. andpl. Nutrition 8, YOUNG A.W. (1980) A Catena of Soils on Bealey Spur. Pp in: Guide Book for Tour 2, South Island. Soils with Variable Charge Conference, Palmerston North, New Zealand. Government Printer, Wellington. YOUNG A.W., CAMPBELL A.S. (~ WALKER T.W. (1980) Allophane isolated from a podzol developed on non-vitric parent material. Nature 284,