STUDIES ON THE BASALTIC SOILS OF NORTHERN IRELAND v. CATION‐EXCHANGE CAPACITIES AND MINERALOGY OF THE SILT SEPARATES (2–20μ)

Abstract

X-ray and chemical studies of silt separates (2-zop) from basaltic soils were made to determine the origin of the rather high cation-exchange capacities (C.E.C.) found for these separates. The silts as isolated by the usual method of mechanical analysis consisted in the main of quartz and feldspars with variable amounts of clay minerals which occurred as aggregates. These aggregates were of two types:(a) Composed of clay-size particles, i.e. < zp cemented or bound into silt-size aggregates by 'free' sesquioxides of which the basaltic soils have a high content. The aggregates were disrupted by reduction and the clay particles subsequently released. (b) 'Pseudo-aggregates' which were shown to be individual particles of siltsize dimensions and having a flaky appearance. These particles were either vermiculitic (under good drainage conditions) or montmorillonitic (under poor drainage conditions), and it is suggested that they form intermediate products in the weathering of primary-rock minerals to clay minerals. Whilst the major contributor to the C.E.C. of the silts was undoubtedly the 'pseudo-aggregates' and to a lesser extent the true clay aggregates, most of the silt separates had a much higher C.E.C. than implied by their mineralogical composition.IN the study of the physical and chemical properties of soils, the main emphasis is generally laced on the mineral nature of the colloid fraction, exchange. Recent investigations (McConaghy and McAleese, 1957 ; McAleese and McConaghy, 19 7,1958) show the need to focus attention mining the potential fertility level of these soils. These studies have shown that it is not possible to account for the high total cation-exchange capacities (C.E.C.) (of the order 30-5om.e. per cent.) of the basaltic soils from chemical and mineralogical considerations of the clay separates only (McAleese and Mitchell, 1958). Silt separates as isolated by the ordinary International Method of mechanical analysis are shown to contribute considerably to the total C.E.C. of the soils (McAleese and McConaghy, 1957), and in some cases these contributions are greater than those for the corresponding clay se arates. which is regarded as t K e seat of the physico-chemical activity, e.g. cation on the mineral composition o f5 the coarser separates, e.g. silt, in deterThe present communication records t x e results of mineral0 of the soils and the contri E utions by the silts to these values. chemical studies of silt se arates (2-2op) in relation to the

Mineralogy and ion exchange properties of the particle size fractions of some Brazilian soils in tropical humid areas

“…There was hardly any variation within the profiles, except for the soil derived from silty shale and mica schist, whose values tended to increase in the subsurface. Our results for the silt fraction confirmed those obtained by McAleese and Mitchell (1958), Tedrow (1966), Morrás (1995) and Asadu et al (1997), who all found similar, or even higher, ECEC values for the silt and the sand fractions. We found that the ECEC values of the coarse soil fractions were highly correlated with the mineralogical composition of the samples which in turn was related to the parent material, something which we will discuss in more detail later in the text.…”

“…Weathering may modify the surface area of feldspar (especially for hydrated feldspar) which is an important source of electrical charges (McAleese and Mitchell, 1958). Feldspars may be transformed directly to gibbsite under severe weathering conditions, bypassing the previous intermediary stage of silica dissolution (Huang, 1989).…”

“…The importance of the silt and sand fractions usually lies in their effect on the physical properties of a soil, although some research (McAleese and McConaghy, 1957;McAleese and Mitchell, 1958;Tedrow, 1966) has shown that the coarse fraction of fine earth is not chemically inactive. The cation exchange capacity (CEC) of a soil depends on both the nature and the amount of cation exchangers present, but the use of clay content alone as an estimator of the mineral CEC of soils has been considered inadequate by various authors (Curtin and Smillie, 1976;Mendonça and Rowell, 1996) and it is plausible that some of the silt and sand fractions may also contribute to CEC.…”

Soil Components

“…The results of McALEESE and MITCHELL [1958] are in aeeordanee with this general outline. Vermieulite has been found in freely drained soils on basalt; illite was predominant in the elay fraetion of moderately drained soils; and a remarkable portion of the elay fraetion eonsisted of newly formed montmorillonite under poor drainage eonditions.…”

“…While c1ay vermieulites form in well-drained profiles, montmorillonites develop where gleying symptoms are evident and to an extent that is related to the degree of waterlogging , VAN DER MERWE and HEYSTEK [1955], McALEESE and MITCHELL [1958], MILLOT and CAMEZ [1963]). studied the influenee of parent material on c1ay minerai eomposition and eonc1uded that, although c1ay vermieulites may develop from many different parent material types (see also WALKER [1950], BARSHAD [1960]), montmorillonites or c1ay vermieulites predominate in soils derived from basie igneous roeks, whereas they are less important eonstituents or absent from soils derived from acid igneous and from various sedimentary and metamorphie roeks.…”