A range of extractants commonly used in soil studies, both to clean up the crystalline clay minerals by dissolving poorly ordered phases, and to assist in the classification of soils, has been used to assess their efficiency in extracting inorganic forms of translocated Al, Fe and Si from a podzol Bs horizon. A 4 h extraction with ammonium oxalate pH 3 is the most efficient single procedure for these phases. It is concluded that satisfactory characterization of a podzol B horizon requires the use of two extractants: acid oxalate to assess the total amounts of Al, Fe and Si in non-crystalline weathering products, and pyrophosphate to assess the amounts in organic forms. Dithionite-citrate-bicarbonate gives a measure of total free iron oxides, but extracts only an ill-defined fraction of the allophaneimogolite complex.

The mineralogy of the clay fractions of two soil profiles representin~ the end-members of a catena developed on a glacial till derived from basic lavas has been determined. Particular attention has been given to the assessment of the nature of the amorphous inorganic material in the clay fraction of these soils. Chemical dissolution techniques were used and their effects on the clay fraction were followed by X-ray diffraction, differential thermal, infrared absorption, electron-optical and surface area measurements. The principal conclusion is that the soil clays are a continuum from completely disordered, through poorly ordered to well crystallized material.

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The reaction of sodium fluoride solution with soils and soil minerals at constant pH has been assessed as a possible single-value characteristic in the classification of soils, particularly those containing significant amounts of poorly ordered inorganic material. A suitable method involves reaction of the soil or clay at 25 "C with 0 . 8 5~ sodium fluoride at pH 6.8, the amount of OH-released after 25 min being taken as the single-value characteristic.There was a statistically significant correlation between the amount of OHreleased and the amount of alumina extracted with cold 5 per cent sodium carbonate solution from both freely drained and poorly drained Scottish soils. The multiple correlation coefficients were improved for freely drained soils by including the dithionite-extracted ferric oxide and the silica soluble in 5 per cent sodium carbonate solution. The procedure is not suitable for surface soils, because of interference by organic matter, or for carbonate-containing soils unless carbonate is removed.Introduction SOME studies of the reaction between fluoride solutions and clay minerals have indicated disruption of the mineral structure and the formation of new compounds such as cryolite (Semmens and Meggy, 1966) or soluble fluoroaluminate complexes (Huang and Jackson, 1965), whereas others suggest that only an adsorption reaction occurs Bower

ABSTRACT:Fromequilibriumsilicaconcentrationsoverimogoliteandboehmiteat 100 155~ the heat and free energy of the reaction (HO)3AI203SiOH + H20 ~ 2A1OOH + Si(OH)4 have been obtained: AH~ r (298.15 K)=38.6 +4.1, AG ~ (298.15 K)=26.8 + 1.1 kJ mo1-1, and hence the heat and free energy of formation of imogolite: AH~ (298.15 K)= -3189.6 _+4.1, AG~ (298.15 K) = -2926-7 _+ 1.1 kJ mol-l These results are consistent with observations indicating that imogolite, halloysite and gibbsite can co-exist in soils, but that imogolite is metastable relative to either halloysite or gibbsite in the long term. At temperatures above 25~ there is a widening range of silica concentrations in which imogolite is more stable than halloysite, although both are metastable relative to kaolinite.The conditions necessary for the formation and persistence of imogolite in soils and sediments are not yet well understood (Wada, t977). Although it is clearly a metastable phase, knowledge of its ideal thermodynamic stability relative to various combinations of gibbsite, boehmite, halloysite, kaolinite and quartz can still serve to distinguish possible from impossible reactions. Since equilibria involving silica and alumina at normal soil temperatures are exceedingly slowly achieved, equilibrium silica concentrations over imogolite and boehmite were measured at 100-155~ and calculated at 25~ by extrapolation. The results at higher temperature are also of significance in defining the conditions under which imogolite might be synthesized in the laboratory (Farmer et al., 1977;Farmer & Fraser, 1978). EXPERIMENTALImogolite was heated with water or silica solutions (prepared by passing a dilute sodium silicate solution through a cation exchanger in the acid form) for periods up to 100 days in Teflon vessels contained in sealed stainless steel bombs at equilibrium water-vapour pressures, and the silica contents of the supernatants determined colorimetrically by the molybdenum blue method after cooling the bombs and centrifuging. The solid phases present in the system were characterized by electron microscopy and electron diffraction. RESULTSOnly boehmite and imogolite were detected in systems heated to 100-140~ At 155~ traces of an incipient poorly crystalline layer silicate appeared after 68 days, in addition to boehmite and imogolite. At 175~ kaolinite and boehrnite were detected at 25 days, and

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The dominant source of oxalate-soluble Al in the B horizon of a brown podzolic soil developed on andesite appears to be hydroxyaluminium interlayers in a dioctahedral vermiculite in the clay and silt fractions, of which the latter contributes most to the total extractable Al and fluoride reactivity of the whole soil. Cold sodium carbonate solutions extract as much interlayer Al as does oxalate, indicating that this fraction of the interlayer material is highly reactive, and sharply distinguished from more resistant interlayers which are also present.

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