NATURE AND ORIGIN OF AN ALUMINOUS VERMICULITIC WEATHERING PRODUCT IN ACID SOILS FROM UPLAND CATCHMENTS IN SCOTLAND

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Clay Minerals (1990) 25, 467-475 NATURE AND ORIGIN OF AN ALUMINOUS VERMICULITIC WEATHERING PRODUCT IN ACID SOILS FROM UPLAND CATCHMENTS IN SCOTLAND D. C. BAIN, A. MELLOR* AND M. J.

1 Clay Minerals (1990) 25, NATURE AND ORIGIN OF AN ALUMINOUS VERMICULITIC WEATHERING PRODUCT IN ACID SOILS FROM UPLAND CATCHMENTS IN SCOTLAND D. C. BAIN, A. MELLOR* AND M. J. WILSON The Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB9 2Q J, Scotland (Received 15 December 1989; revised 30 April 1990) A B S T R A C T: Vermiculitization of mica is one of the main weathering processes in soils from three upland catchments receiving various levels of acid deposition. Usually this process is manifested by the presence of interstratified mica-vermiculite with the interlayer space in the vermiculite often partially filled with polymeric hydroxyaluminium species. In one peaty podzol, regularly interstratified mica-vermiculite clearly develops at the expense of mica and is the dominant mineral in the Eh horizon. It was concentrated by chemical treatments to remove organic matter, free iron oxides, and any AI species in the interlayer, and the structural formula calculated from chemical analysis confirmed the dioctahedral character of both vermiculite and mica components, and indicated that the vermiculitic weathering product was formed from a dioctahedral mica. The degree of interlayering in the interstratified phases seemed to he ph-dependent with resultant implications for soil and freshwater acidification. There has been renewed interest in hydroxyaluminium interlayered clays in recent years in the context of acid deposition from the atmosphere, acidification of surface waters and the resulting damage to fish stocks. Many areas where there has been concern over recent acidification of surface waters are those with podzolic soils located in granitic terrain where the transformation of micas to vermiculite, which may or may not be hydroxyaluminium interlayered, has been identified as one of the main mineral weathering processes (Wilson, 1986). In 26 watersheds underlain by a variety of types of bedrock, April et al. (1986) found that an aluminous dioctahedral vermiculite likely derived from the weathering of mica (probably biotite) was the dominant clay mineral, and that fixation of hydroxyaluminium occurred mainly in the B horizons. Olson (1988) described two mountain watersheds in Virginia on contrasting bedrocks in which 14 A hydroxy-interlayer vermiculites provided a sink for A13+ and exerted a major control over A13+ concentrations in one watershed which was otherwise poorly buffered and highly sensitive to acidic deposition. Interlayered vermiculite and smectite have been widely reported in soils over a long period, as reviewed by Rich (1968), Barnhisel & Bertsch (1989) and Douglas (1989). These minerals are usually identified by a 14 A basal spacing which is unaffected by ethylene glycol and partially collapses on heating to a spacing intermediate between 14 and 10 A. Resistance to collapse upon KC1 treatment indicates that the interlayer material consists of non-exchangeable A1 polymers which have often be~n shown to be present (MacEwan, 1950; Brown, 1953; Rich & Obenshain, 1955), and their removal facilitates collapse and increases CEC (Frink, 1965). According to Rich (1968), interlayer formation is favoured by * Present address: The Polytechnic, Newcastle-upon-Tyne NE1 8ST, England The Macaulay Land Use Research Institute

2 468 D.C. Bain et al. Y Fic. 1. Location of the catchments studied. 1: Loch Chon. 2: Kelty Water. 3: Allt a Mharcaidh. moderate ph ( ), frequent wetting and drying cycles, and low organic matter content. Similarly, Barnhisel & Bertsch (1989) reported widespread formation of interlayers in acid soils over a ph range of Vicente et al. (1977) considered aluminous hydroxy 2:1 minerals to be the most common clay minerals in the soils of Northern Europe or North America and speculated that this abundance in temperate regions is due to weathering of micas by organic acids in the soils. In a study of mineral weathering in upland catchments in Scotland being carried out under the auspices of SWAP (Surface Water Acidification Programme), vermiculitization of mica is one of the main processes, often accompanied by the precipitation of hydroxyaluminium in the interlayer space, particularly in B horizons (Mellor & Wilson, 1987). The vermiculite commonly occurs in interstratified mica-vermiculite, and the purpose of this paper is to describe the occurrence of this mineral, to characterize it by X-ray diffraction (XRD) and chemical analyses from which a structural formula is calculated and deductions made as to the origin of the mineral. MATERIALS AND METHODS Three Scottish upland catchments subject to different pollution loadings have been studied (Fig. 1). The two which receive high levels of acid deposition (Loch Chon and Kelty Water) are developed on quartz-mica-schist and the dominant soil types are humus iron podzols, peaty podzols, peaty gleys and peats. The site receiving an intermediate level of acid deposition at Allt a Mharcaidh in the Cairngorm Mountains is underlain by granite, and the main soil types are alpine podzols (weakly expressed podzols showing signs of cryoturbation), peaty podzols and peat. Nineteen soil profiles were studied. The clay fraction (<2/~m) was separated using standard sedimentation procedures from soils dispersed by ultrasonic vibration and end-over-end shaking. No chemical pretreatments were employed. XRD traces were obtained from oriented aggregates using a Philips 2 kw diffractometer

Aluminous vermiculitic weathering product 469 with Fe-filtered Co-Ko~ radiation, divergent, receiving and scatter slits of 1% 0.2 mm and 1 ~ respectively, and a scanning rate of 2 ~ 20 per min.

3 Aluminous vermiculitic weathering product 469 with Fe-filtered Co-Ko~ radiation, divergent, receiving and scatter slits of 1% 0.2 mm and 1 ~ respectively, and a scanning rate of 2 ~ 20 per min. An interlayered vermiculite was identified by a 14 ~, reflection at room temperature which was unaffected by ethylene glycol solvation, and, on saturation with K +, only partially collapsed on heating to 300~ Chemical analyses of the clays were obtained by X-ray fluorescence spectrometry using the methods of Norrish & Hutton (1969) for sample fusion and for corrections for interelement effects. The K0c lines were measured using a Philips PW 1404 spectrometer with a Rh tube and pulse height analysis settings appropriate to each element. Several chemical treatments were used to characterize particular minerals in some of the clay fractions: (i) 6 M HC1 at 95~ for 30 min to dissolve trioctahedral minerals; (ii) H202 to remove organic matter; (iii) sodium dithionite to remove free iron oxides; (iv) heating at 550~ followed by boiling in 0.5 M NaOH to remove interlayer A1 (similar to the treatment described by Hashimoto & Jackson, 1960). RESULTS AND DISCUSSION Characterization of interlayered vermiculite by XRD In many of the profiles studied from all three catchments, a broad XRD peak is often observed at ~ which is relatively unaffected by ethylene glycol solvation, or by heating after K-saturation (Fig. 2). This indicates the presence of interstratified micavermiculite with the interlayer space in the vermiculite filled, thereby preventing collapse on heating. This mineral is commonly present in small amounts in soils developed on granite and quartz-mica-schist (Fig. 2). In profiles where the mica-vermiculite is more abundant, it is clear that it is developing as a result of pedogenic processes. A good example can be seen in Fig. 3 which illustrates the clay mineralogy of a peaty gley developed on quartz-mica-schist. A small peak at about 12 A. in the Bg horizon (ph = 4-9 in water) can be seen to intensify into a strong 11.9 A peak (Fig. 3A) in the lower part of the Eg horizon (ph = 4.6). This peak is not significantly affected by ethylene glycol (Fig. 3B), and collapses only slightly to 11.3 A at 300~ (Fig. 3C) indicating that the interlayer space of the vermiculite is partially filled, in the upper part of the Eg horizon (ph = 4.0), the mineral has a spacing of 10.9 A which is not significantly affected by ethylene glycol, but collapses to 10.3 A at 300~ In this more acidic part of the horizon, there is only a small amount of interlayer material in the vermiculite. The best example of the weathering of mica to vermiculite occurs in a peaty podzol developed on quartz-mica-schist at Kelty Water. In the BC horizon, XRD peaks at 14-1, 9.9 and 7-09 ~ (Fig. 4A) are relatively unaffected by ethylene glycol or heating at 300~ (Fig. 4B), but the 7 ~, peak disappears and the 14 A peak intensifies at 550~ (Fig. 4C); therefore chlorite and mica are the main minerals in this horizon. Mica is present as a main component of the Bs and Eh horizons, but chlorite diminishes in amount and is absent in the Eh. The presence of interstratified mica-vermiculite in all horizons is revealed by the peaks at 11-9 A which are unaffected by ethylene glycol solvation after Mg saturation. The peak is weak and broad in the BC horizon but intensifies in the Bs and becomes dominant in the Eh where it is accompanied by a strong higher-order reflection at 24-2 A indicating that the interstratification is regular. The effect of heating at 300~ varies throughout the profile (Fig. 4B): in the BC horizon (ph = 4-8) there is a slight collapse to 11-2 A, a further collapse in the Bs horizon (ph = 4.5) to 10.9 A, and complete collapse to 10.0 A in the Eh (ph = 4-1). The degree of interlaying in the vermiculite therefore appears to decrease with

4 470 D. C. Bain et al. I 14.2 / / 11S / ]7" "5 / oj Loch Chon Allt a Mharcaidh Fro. 2. XRD traces of <2 ~m fractions from the Eh horizon of a peaty podzol developed on granite at Allt a Mharcaidh, and the Bs horizon of a peaty podzol developed on quartz-mica-schist in the Loch Chon catchment. A: room temperature; B: ethylene glycol; C: heated to 300~ D: heated to 550~ Peak spacings in ~. increasing acidity. The interstratified mineral resists treatment with HC1 suggesting that it is aluminous and dioctahedral in character. The presence of a small peak at 7 A on the XRD traces of the acid-treated clays (Fig. 4D) indicates that there is a small amount of kaolinite in all horizons. Subordinate amounts of quartz and plagioclase feldspar were detected at higher angles in the XRD traces for all the horizons. Chemical characterization In order to characterize chemically the regularly interstratified mica-vermiculite in the Eh horizon of the Kelty Water profile, it was concentrated by treating the clay with (i) H202 to remove any organic matter, (ii) sodium dithionite to remove any free iron oxides, and (iii) NaOH after heating at 550~ to remove any interlayer A1. The analyses of the untreated and treated clays are shown in Table 1. The analysis of the treated clay was corrected for 5%

5 Aluminous vermiculitic weathering product 471 quartz, albite and kaolinite, and for anatase by assuming that all the TiO2 was present in this form, and then normalized to 100%. The corrected figures represent the analysis of the interstratified mica-vermiculite plus the discrete mica also present. The high Na20 figure is due to exchangeable Na + from the sodium dithionite treatment. As the oxidation state of Fe is not known, the structural formula was calculated in two ways by assuming that (i) all the Fe is ferric, and (ii) all the Fe is ferrous. This resulted in the following formulae: (i) KI-30 Nao.47 (Si6.65A11.35) (A12.95Fe3+o.62Mgo.42) 020 (OH)4 (ii) K1.32 Nao.47 (Si6.75All.25) (A13.12Fee+o.63Mgo.43) 020 (OH)4 The sum of the octahedral cations is 3.99 if all the Fe is ferric, and 4.18 if the Fe is ferrous. Although the formulae do not represent the interstratified phase alone (discrete mica is also present in the residue), the fact that the number of octahedral cations is close to the theoretical value of 4.0 for a dioctahedral mineral indicates that both components of the interstratified phase are dioctahedral in character. Origin of the aluminous vermiculite The vermiculitization of di- and trioctahedral micas with concomitant interlayer alumination has already been reported as a common feature in Scottish soils due to "3 11.,' i Yi l' / ] Eg horizon 16-24cm Eg horizon 30-40cm 7.12 / C i / 2 Bg horizon 50-60cm Fro. 3. XRD traces of <2 ~tm fractions from a peaty gley developed on quartz-mica-schist in the Loch Chon catchment. A: room temperature; B: ethylene glycol; C: heated to 300~ Peak spacings in/~.

6 472 D.C. Bain et al "0 O ~ , S Horizon U 24,, ~ "7 ' L ! " 14.1 BC Horrzon D C B,J ~'*'~ A FIG. 4. XRD traces of <2 ffm fractions from a peaty podzol developed on quartz-mica-schist in the Kelty Water catchment. A: room temperature; B: heated to 300~ C: heated to 550~ D: treated with 6 M HCI at 95~ for 30 rain. Peak spacings in ~. pedogenic processes (Wilson et al., 1984). The increasing amount of interstratified micavermiculite towards the surface of the profiles described indicates that vermiculitization of mica takes place in these soils also, and from the structural formulae, it appears that it is dioctahedral vermiculite that is being formed from dioctahedral mica. Formation of dioctahedral hydroxy-interlayer vermiculite from dioctahedral mica was suggested by Brown (1953) and more recently was confirmed by transmission electron microscopy (from the nature of the electron diffraction patterns) by Farmer et al. (1988) in the clay and silt fraction from a brown podzolic soil on andesite. The only structural formulae available for natural hydroxy-interlayered minerals are approximations made by Kirkland & Hajek (1972) for minerals in some Alabama soils. As the average charge resulting from isomorphous substitution was 0.96 per half-cell which is similar to the charge for muscovite, this suggests that these interlayered materials had also formed from a dioctahedral precursor. The hydroxy-interlayered vermiculite described by Olson (1988) was also dioctahedral and was considered to have a pedogenic origin from

7 Aluminous vermiculitic weathering product 473 TABLE 1. Chemical analyses (wt%, ignited at 1000~ basis) of the <2 #m fraction from the Kelty Water 15 profile. Untreated Treated* Corrected? SiO TiO A Fe203~: MnO MgO CaO Na K~O P SO Total * treated with H202, sodium dithionite, and NaOH after heating at 550~ (see text).? corrected for the presence of 5% quartz, 5% kaolinite and 5.3% albite. $ total iron as Fe203. inherited dioctahedral mica. In the Scottish soils studied, the interstratified phase may be an intermediate stage in the formation of interlayered vermiculite. Implications for soil and fresh water acidification The soil processes occurring in the profiles studied in the Scottish catchments can be regarded under the general heading of podzolization during which aluminium in the O and E horizons is mobilized by organic complexation under conditions of low ph and transported downwards in the profile. Although there are numerous reports of hydroxyinterlayered materials being concentrated in surface horizons (Rich & Obenshain, 1955; Bryant & Dixon, 1963; Carlisle & Zelany, 1973; Harris et al., 1980; Dixon, 1989; Douglas, 1989), April et al. (1986) concluded that fixation of hydroxyaluminium interlayers in vermiculite occurred chiefly in the B horizon where organic complexes decompose and release A1, and ph increases, causing the formation of hydroxyaluminium precipitates. In a study of 59 pedons, Harris & Carlisle (1987) found that hydroxy-interlayered materials were significantly concentrated in Bh horizons. In the Scottish soils described, the amount of interstratified mica-vermiculite increases towards the surface of the profile with the result that there is more interlayer space available for adsorption of hydroxyaluminium in the upper horizons. The degree of interlayering as indicated by the degree of collapse of the vermiculite structure on heating seems to be phdependent. In the Kelty Water profile, on heating to 300~ the 11-9 ~ peak decreases to 11.2 A at ph = 4.8 (BC horizon), to 10.9 A at ph = 4.5 (Bs horizon) and completely collapses to 10.0 A at ph = 4.1 (Eh horizon). Therefore the amount of hydroxyaluminium species being leached down from the surface which is adsorbed by the vermiculite appears to depend on the ph. Conversely, if the ph of a horizon should decrease, because of, say,

474 D.C. Bain et al. acid precipitation, A1 from the interlayer will be mobilized and may find its way into stream water with potentially toxic consequences to fish.

8 474 D.C. Bain et al. acid precipitation, A1 from the interlayer will be mobilized and may find its way into stream water with potentially toxic consequences to fish. It may be significant that the best example of the interstratified mica-vermiculite found in the three Scottish catchments is at Kelty Water where the degree of interlayering decreases towards the surface, and this is the only catchment with streams incapable of supporting fish. Hydroxyaluminium interlayered vermiculite may play a major role in the amounts of mobile A13+ in catchments (Olson, 1988). If soil-water residence time is sufficiently long, A13+ released by weathering may be taken up into the interlayer positions, whereas if water flushes quickly through the soil system, A13+ may be discharged into the stream system. Variations in the clay-mineral suites and particularly the distribution of interlayered expansible clays may play a key role in the chemical response and weathering processes occurring in catchments. Karathanasis (1988) has further suggested that the stability of hydroxyaluminium interlayered vermiculite and smectite is a function of dynamic processes reflected in a constant replenishment of the hydroxyaluminium interlayer in order to maintain a stable composition in a particular environment, and that these minerals could serve as good indicators of gradual changes in natural soil environments. CONCLUSIONS Vermiculitization of mica is one of the main weathering processes in three upland Scottish catchments receiving different levels of acid deposition. Interstratified mica-vermiculite forms in the soil profiles and the interlayer space in the vermiculite component is filled with various amounts of polymeric hydroxyaluminium as measured by the degree of collapse on heating. A regularly interstratified form of the mica-vermiculite is the dominant mineral in the clay fraction of one peaty podzol. The mineral was concentrated by chemical treatments, and from the chemical analysis of the residue, an approximate structural formula was calculated which confirmed the dioctahedral character of the vermiculite indicated by its resistance to HC1 treatment, and suggested that the vermiculite weathering product is formed from a dioctahedral mica. The degree of interlayering seems to be ph-dependent with hydroxyaluminium being removed from the interlayers at ph below 4-3. Consequently the occurrence of interlayered vermiculite has implications for soil and water acidification in that A13+ may be adsorbed into the interlayer space, or released from the interlayer space depending on the ph conditions. ACKNOWLEDGMENTS This work was partially funded from the Surface Water Acidification Programme, administered by the Royal Society, the Norwegian Academy of Science and Letters, and the Royal Swedish Academy of Sciences. Thanks are due to Ms. L. Thomson and Miss R. Perry for technical assistance. REFERENCES APRIL R.H., HLUCHY M.M. & NEWTON R.M. (1986) The nature of vermiculite in Adirondack soils and till. Clays Clay Miner. 34, BARNHISEL R.I. 8r BERTSCH P.M. (1989) Chlorites and hydroxy-interlayered vermiculite and smectite. Pp in: Minerals in Soil Environments (J.B. Dixon & S.B. Weed, editors). Soil Sci. Soc. Am., Madison, Wisconsin. BROWN G. (1953) The dioctahedral analogue of vermiculite. Clay Miner. Bull. 2, BRYANT J.P. & DIXON J.B. (1963) Clay mineralogy and weathering of a red-yellow podzolic soil from quartz mica schist in the Alabama piedmont. Clays Clay Miner. 12,

Aluminous vermiculitic weathering product 475 CARLISLE V.W. ZELAZNY L.W. (1973) Mineralogy of selected Florida Paleudults. Proc. Soil Crop Sci. Soc. Fla. 33, 136-139. DOUGLAS L.S. (1989) Vermiculites.

9 Aluminous vermiculitic weathering product 475 CARLISLE V.W. ZELAZNY L.W. (1973) Mineralogy of selected Florida Paleudults. Proc. Soil Crop Sci. Soc. Fla. 33, DOUGLAS L.S. (1989) Vermiculites. Pp. 635~674 in: Minerals in Soil Environments (J.B. Dixon & S.B. Weed, editors). Soil Sci. Soc. Am., Madison, Wisconsin. DIXON J.B. (1989) Kaolinite and serpentine group materials. Pp in: Minerals in Soil Environments (J.B. Dixon & S.B. Weed, editors). Soil Sci. Soc. Am., Madison, Wisconsin. FARMER V.C., SMITH B.F.L., WILSON M.J., LOVELAND P.J. & PAYTON R.W. (1988) Readily-extractable hydroxyaluminium interlayers in clay and silt-sized vermiculite. Clay Miner. 23, FRINK C.R. (1965) Characterization of aluminum interlayers in soil clays. Soil Sci. Soc. Am. Proc. 29, HARRIS W.G. & CARLISLE V.W. (1987) Clay mineralogical relationships in Florida Haplaquods. Soil Sci. Soc. Am. J. 51, HARRIS W.G., IYENGAR S.S., ZELAZNY L.W., PARKER J.C., LIETZKE D.A. & EOMONDS W.J. (1980) Mineralogy of a chronosequence formed in New River alluvium. Soil Sci. Soc. Am. J. 44, HASHIMOTO I. & JACKSON M.L. (1960) Rapid dissolution of allophane and kaolinite-halloysite after dehydration. Clays Clay Miner. 7, KARAT~ANASlS A.D. (1988) Compositional and solubility relationships between aluminum-hydroxyinterlayered soil-smectites and vermiculites. Soil Sci. Soc. Am. J. 52, KIRKLAND D.L. & HAJEK B.F. (1972) Formula derivation of Al-interlayered vermiculite in selected soil clays. Soil Sci. 114, MAcEWAN D.M.C. (1950) Some notes on the recording and interpretation of X-ray diagrams of soil clays. J. Soil Sci. 1, MELLOR A. & WILSON M.J. (1987) Processes and products of mineral weathering in soils of the SWAP catchments in Scotland and Norway. Proc. SWAP Mid Term Review Conf. Roy. Soc. Lond NORRISH K. & HUTrON J.T. (1969) An accurate X-ray spectrographic method for the analysis of a wide range of geological samples. Geochim. Cosmochim. Acta 33, OLSON C.G. (1988) Clay-mineral contribution to the weathering mechanisms in two contrasting watersheds. J. Soil Sci. 39, RICH C.I. (1968) Hydroxyinterlayers in expansible layer silicates. Clays Clay Miner. 16, RICH C.I. & OBENSHAIN S.S. (1955) Chemical and clay mineral properties of a red-yellow podzolic soil derived from muscovite schist. Soil Sci. Soc. Am. Proc. 19, VIEENTE M.A., RAZZAGHE M. & ROBERT M. (1977) Formation of aluminium hydroxy vermiculite (intergrade) and smectite from mica under acidic conditions. Clay Miner. 12, WILSON M.J. (1986) Mineral weathering processes in podzolic soils on granitic materials and their implications for surface water acidification. J. Geol. Soc. Lond. 143, 691~o97. WILSON M.J., BAIN D.C. & DUTHIE D.M.L. (1984) The soil clays of Great Britain: II. Scotland. Clay Miner. 19,

MINERAL CONTENT AND DISTRIBUTION AS INDEXES OF WEATHERING IN THE OMEGA AND AHMEEK SOILS OF NORTHERN WISCONSIN

MINERAL CONTENT AND DISTRIBUTION AS INDEXES OF WEATHERING IN THE OMEGA AND AHMEEK SOILS OF NORTHERN WISCONSIN By L. D. WHITTIG 1 AND M. L. JACKSON University of Wisconsin, Madison, Wisconsin ABSTRACT Quantitative