M. L. Jackson

Gibbsite solubility and thermodynamic properties of hydroxy-aluminum ions in aqueous solution at 25°C

Solubility curves were determined for a synthetic gibbsite and a natural gibbsite (Minas Gerais, Brazil) from pH 4 to 9, in 0.2% gibbsite suspensions in 0.01 M NaNO3 that were buffered by low concentrations of non-complexing buffer agents. Equilibrium solubility was approached from oversaturation (in suspensions spiked with Al(NO3)3 solution), and also from undersaturation in some synthetic gibbsite suspensions. Mononuclear Al ion concentrations and pH values were periodically determined. Within 1 month or less, data from over-and undersaturated suspensions of synthetic gibbsite converged to describe an equilibrium solubility curve. A downward shift of the solubility curve, beginning at pH 6.7, indicates that a phase more stable than gibbsite controls Al solubility in alkaline systems. Extrapolation of the initial portion of the high-pH side of the synthetic gibbsite solubility curve provides the first unified equilibrium experimental model of Al ion speciation in waters from pH 4 to 9. The significant mononuclear ion species at equilibrium with gibbsite are Al3+, AlOH2+, Al(OH)+2 and Al(OH)−4, and their ion activity products are ∗K50 = 1.29 × 108, ∗Ks1 = 1.33 × 103, ∗Ks2 = 9.49 × 10−3 and ∗Ks4 = 8.94 × 10−15. The calculated standard Gibbs free energies of formation (ΔG°f) for the synthetic gibbsite and the A1OH2+, Al(OH)+2 and Al(OH)−4 ions are −276.0, −166.9, −216.5 and −313.5 kcal mol−1, respectively. These ΔG°f values are based on the recently revised ΔG°f value for Al3+ (−117.0 ± 0.3 kcal mol−1) and carry the same uncertainty. The ΔG°f of the natural gibbsite is −275.1 ± 0.4 kcal mol−, which suggests that a range of ΔG°f values can exist even for relatively simple natural minerals.

Chemical Weathering of Minerals in Soils

Publisher Summary As agriculture is intensified in particular areas, the proportion of nutrients supplied by weathering becomes relatively small, compared to that added as fertilizers. Even in areas of well-developed agriculture, both in temperate and tropical regions, the weathering release of nutrient elements is of major importance in soil fertility and crop production. It is found that the inherent fertility of soils is related to their mineral content. As the weathering stage advances, soils gradually change, first toward increased productivity and finally, to extremely low productivity. The release of the major and minor elements can be correlated with mineral composition of the soil. In backward areas, agriculture has been directed toward maximum use of the native mineral source of nutrients by systems such as patch agriculture and paddy culture. Even in areas of intensive agriculture, it is clear that the addition of ground rocks (such as ground limestone) and minerals to soils is a reversal of the weathering scheme, with extremely beneficial effects on crop productivity. The chapter examines the processes and products of chemical weathering of minerals in soils from the standpoint of each of these phases and highlights the relative stability of minerals; weathering sequences and indexes. The chapter also lists out the factors affecting the reaction rates of chemical weathering and their distribution frequency.

Interlayering of Expansible Layer Silicates in Soils by Chemical Weathering

Interlayering of 2: 1 layer silicates varies as a function of chemical weathering from the simple, homogeneous K or Na interlayers of micas to the heterogeneous systems of mica intercalated with expanded 2: 1 layer silicates. “Frayed edge” type of weathering at dislocation planes of mica is collated with K release and preferential cation-exchange uptake of K relative to Ca by such expansible layer silicate systems; mica islands maintain alignment of the silica sheet cavities, which facilitates recapture of lattice K. Intercalation of the expanded 2: 1 layer silicates with alumina interlayers appears to be a characteristic function of chemical weathering in soils, with the formation of 2: 1–2: 2 intergrades not only of 14 A spacing but also of swelling 18 A types that give small 12, 14, 18 A and higher spacing peaks (along with the 10 A peak) at 550°C. Interlayer precipitates appear to be characteristic of soil clays, contrasting with “pure” minerals of deposits developed in less “open” environments than those of soils. The “2: 2 lattice building” phenomenon in expansible 2: 1 layer silicates relates to layer charge density and crystal size, and frequently tends to inhibit the formation of free gibbsite in soil chemical weathering so long as there are expansible layer silicates present to become intercalated with aluminum hydroxide—a weathering phenomenon that may be called an “antigibbsite effect”. Accumulation of alumina (possibly with some iron, magnesium, and allophane) as interlayers in 2: 1 minerals of soils is seen as a genetic stage in the 2: 2 → 1: 1 weathering sequence through which kaolinite and halloysite develop in soils.

Aqueous dissolution, solubilities and thermodynamic stabilities of common aluminosilicate clay minerals: Kaolinite and smectites

Abstract Determinations of the aqueous solubilities of kaolinite at pH 4, and of five smectite minerals in suspensions set between pH 5 and 8, were undertaken with mineral suspensions adjusted to approach equilibrium from over- and undersaturation. After 1,237 days, Dry Branch, Georgia kaolinite suspensions attained equilibrium solubility with respect to the kaolinite, for which K eq = (2.72 ± 0.35) × 10 7 . The experimentally determined Gibbs free energy of formation ( ΔG f ,298 0 ) for the kaolinite is −3,789.51 ± 6.60 kj mol −1 . Equilibrium solubilities could not be determined for the smectites because the composition of the solution phase in the smectite suspensions appeared to be controlled by the formation of gibbsite or amorphous aluminum hydroxide and not by the smectites, preventing attempts to determine valid ΔG f 0 values for these complex aluminosilicate clay minerals. Reported solubility-based ΔG f 0 determinations for smectites and other variable composition aluminosilicate clay minerals are shown to be invalid because of experimental deficiencies and of conceptual flaws arising from the nature of the minerals themselves. Because of the variable composition of smectites and similar minerals, it is concluded that reliable equilibrium solubilities and solubility-derived ΔG f 0 values can neither be rigorously determined by conventional experimental procedures, nor applied in equilibriabased models of smectite-water interactions.

Eolian Origin of Quartz in Soils of Hawaiian Islands and in Pacific Pelagic Sediments

The particle-size distribution and oxygen-isotopic composition of quartz isolated from Hawaiian soils, east-central Pacific sediments, and tropospheric dusts are remarkably uniform. A common origin and tropospheric transport from continental land masses is suggested.

EXPANDABLE PHYLLOSILICATE REACTIONS WITH LITHIUM ON HEATING

A method using Li saturation and heating to 250°C to differentiate montmorillonite from beidellite and nontronite has been developed. The test utilizes three washings with 3 M LiCl and two washings with 0.01 M LiCl in 90% methanol to prevent dispersion. An ’infinitely thick’ sample (6–8 mg/cm2) on a glass slide is used to avoid the effects of the reaction of a thin clay film with sodium of the slide when it is heated at 250°C. Solvation with glycerol rather than ethylene glycol is used, because all of the Li smectites studied expanded to some extent in ethylene glycol after the heating. The smectites included several montmorillonites, a nontronite, and saponites. The presence of interstratified montmorillonite and beidellite layers was clearly shown by the test for several smectite samples, including the so-called beidellites from Beidell, Colorado, and Chen-yuan, Taiwan, and several soil clays. The test thereby provides more mineralogical information than the often-used arbitrary dividing point between montmorillonite and beidellite at 50% tetrahedral charge. Heating the Li-saturated clays at 250°C caused substitution of 35 to 125 meq/100 g of nonexchangeable Li. These amounts exceeded the changes in cation-exchange capacity plus Li by 4 to 21 meq/100 g, except for the end-member beidellite from the Black Jack mine, Idaho. Fusion with LiNO3 at 300°C could not be used to differentiate between smectites instead of washing with LiCl solution and heating to 250°C, because fused montmorillonite subsequently expanded to 18 Å with glycerol. Large increases in nonexchangeable Li were caused by the fusion of smectites, a vermiculite, and two partially expanded micas.

Quartz isolation from rocks, sediments and soils for determination of oxygen isotopes composition

Abstract Fusion of ground samples of rocks, sediments, and soils with sodium pyrosulfate removed most of the minerals except quartz and feldspars. Treatment of the residue with hydrofluosilicic acid (H2SiF6) completely removed feldspars. Quartz thus isolated, with O 18 O 16 ratio unchanged, is suitable for isotopic analysis.

Micaceous Occlusions in Kaolinite Observed by Ultramicrotomy and High Resolution Electron Microscopy

The layer structure of kaolinite from Twiggs, Georgia and fire-clay type kaolinite (Frantex B, from France), particle size separates 2–0·2 μm was studied by high resolution electron microscopy after embedment in Spurr low-viscosity Epoxy media and thin sectioning normal to the (001) planes by an ultramicrotome. Images of the (001) planes (viewed edge-on) of both kaolinites were spaced at 7 Å and generally aligned in parallel, with occasional bending into more widely spaced images of about 10 Å interval. Some of the 10 Å images converged to 7 Å at one or both ends, forming ellipse-shaped islands 80 to 130 Å thick and 300 to 500 Å long. The island areas and interleaved 10 Å layers between 7 Å layers may represent a residue of incomplete weathering of mica to kaolinite.The proportions of micaccous occlusions were too small and the layer sequences too irregular to be detected by X-ray diffraction. The lateral continuity of the layers through the 7-10-7 Å sequence in a kaolinite particle would partially interrupt or prevent expansion in dimethyl sulfoxide (DMSO) and other kaolinite intercalating media. Discrete mica particles were also observed with parallel images at 10 Å, as impurities in both kaolinites. The small K content of the chemical analyses of the kaolinite samples is accounted for as interlayer K, not only in discrete mica particles but also in the micaceous occlusions.

KAOLINS: SOURCES OF DIFFERENCES IN CATION-EXCHANGE CAPACITIES AND CESIUM RETENTION

Seven kaolins from Georgia (southeastern U.S.A.), ranging from high to low commercial grade, were characterized by X-ray powder diffraction and chemical techniques to establish that the variation in quality was caused by impurities. The Ca and Cs cation-exchange capacities (CEC) varied from 2.67 to 8.17 and from 3.29 to 8.77 meq/100g, respectively. Selective dissolution and correlation analyses strongly indicated that expandable 2:1 minerals, particularly smectite (1.2-5.9%), were responsible for most of the observed variations in Ca CEC (r = 0.85*). The external surface CEC of kaolinite ranged from 0 to 1 meq/ 100 g. The positive significant correlation (r = 0.90**) between the Ca CEC and the K-mica content (03.9%) suggested that Ca CEC may be related to the degree of mica weathering through an expandable mineral stage.The Cs-retention capacity (0.19–1.14 meq/100 g) was closely related to Cs-measured vermiculite content (r = 0.80*), and this content plus specific surface (R = 0.93**) or mica content (R = 0.86*). The Cs retention appeared to be primarily related to the presence of interlayer wedges at mica/vermiculite XY interfaces.РезюмеСемь каолинов из Джорджии (юго-восток США), представленные в диапазоне от высоко- до низкокачественных коммерческих марок, были охарактеризованы порошковым методом рентгеноструктурного анализа и химическими анализами, чтобы доказать что изменения качества вызываются примесями. Катионно-обменные способности (КОС) Са и Cs изменялись от 2,67 до 8,17 и от 3,29 до 8,77 М.ЭК./100 г соответственно.Селективные анализы растворения и корреляции убедительно доказали, что расширяющиеся минералы 2:1, особенно смектит (1,22–5,9%), обусловили большинство замеченных изменений КОС Са (г = 0,85*). Наружная поверхностная КОС каолинита колеблется от 0 до 1 М.ЭК./ЮО г. Существенная положительная корреляция (г = 0,90**) между КОС Са и содержанием К-слюды (02–3,9%) указывает на то, что КОС Са может быть связана со степенью выветривания слюды в течение фазы расширения минерала.Способность удержания Cs (0,192–1,14 М.ЭК./ЮО г) тесно связана с измеренным по Cs содержанием вермикулита (г = 0,80*) и этим содержанием плюс удельной поверхностью (R = 0,93**) или содержанием слюды (R = 0,86*). Удержание Cs, по-видимому, связано в основном с присутствием межслойных клиньев в промежутках XY слюда/вермикулит. [N.R.]ResümeeSieben Kaoline aus Georgia (südöstliche USA), die von hoch- bis niedrig-qualitativ reichen, wurden mittels Röntgenpulverdiffraktometrie und chemischen Methoden charakterisiert, um festzustellen, daß die Qualitätsunterschiede durch Verunreinigungen hervorgerufen werden. Die Ca- und Cs-Kationenaustauschkapazitäten, (CEC), variierten von 2,67 bis 8,17 bzw. von 3,29 bis 8,77 mÄq/100 g. Selektive Auflösungs- und Korrelationsanalysen zeigen sehr stark an, daß quellfähige 2:1 Minerale, vor allem Smektit (1,2–5,9%), für die meisten der beobachteten Variationen in der Ca CEC (r = 0,85*) verantwortlich sind. Die CEC der äußeren Oberfläche von Kaolinit reicht von 0–1 mÅq/100 g. Die positive beachtliche Korrelation (r = 0,90**) zwischen der Ca CEC und dem Gehalt an K-Glimmer (0–3,9%) deutet daraufhin, daß die Ca CEC im Zusammenhang stehen könnte mit dem Ausmaß, in dem die Glimmerverwitterung durch ein Stadium eines quellfähigen Minerales geht.Die Cs-Retentionskapazität (0,19–0,14 mÄq/100 g) stand in engem Zusammenhang mit dem Cs-gemes- senen Vermiculitgehalt (r = 0,80*) und mit diesem Gehalt plus spezifischer Oberfläche (R = 0,93**) bzw. Glimmergehalt (R = 0,86*). Die Cs-Retention scheint vor allem mit der Anwesenheit von Zwischenschichtkeilen an den Glimmer/Vermikulit XY Grenzflächen im Zusammenhang zu stehen. [U.W.]RésuméSept kaolins de Géorgie (Sud Est des E.U.) s’étageant de grade commercial haut à bas ont été caractérisés par diffraction poudrée aux rayons-X et par des techniques chimiques pour établir que la variation en quantité était causée par des impuretés. Les capacités d’échange de cations Ca et Cs (CEC) ont varié de 2,67 à 8,17 et de 3,29 à 8,77 meq/100 g, respectivement. Les analyses de dissolution sélective et de corrélation ont intensément indiqué que les minéraux expansibles 2:1, la smectite en particulier, (1,25,9%), étaient responsables pour la plupart des variations observées dans Ca CEC (r = 0,85*). La surface externe CEC de la kaolinite s’étageait de 0 a 1 meq/100 g. La corrélation positive significative (r = 0,90**) entre Ca CEC et le contenu en mica-K (0–3,9%) suggère que Ca CEC peut être apparenté au degré d’altération du mica à travers un stage minéral expansible. La capacité de retention-Cs (0,19–1,14 meq/100 g) était apparentée de près au contenu en vermiculite mesuré Cs (r = 0,80*), et ce contenu plus la surface spécifique(R = 0,93**) ou le contenu en mica (R = 0,86*). Larétention de Cs semblaitètre principalement apparentée à la présence de parties interfeuillets aux interfaces mica/vermiculite XY. [D.J.]

Frequency Distribution of Clay Minerals in Major Great Soil Groups as Related to the Factors of Soil Formation

The frequency distribution or relative abundance of minerals in soils varies with the five principal classes of factors that govern soil formation. The characteristics of the minerals of the parent material, the time factor, climatic factors, relief factors and biotic factors each can be shown to have important independent effects on clay mineralogy of soils under proper circumstances. The soil parent material exerts a control over the frequency distribution of minerals in soils by introduction of the clay minerals into the soil directly, by controlling the course of chemical weathering in the soil through the relative susceptibility of its minerals to weathering, by furnishing abundant divalent metallic cations, by impediment of drainage, or by acceleration of leaching when highly permeable. The time factor is conspicuous as long times give an advanced degree of weathering even in temperate climates. Climate is important, since highly weathered materials inevitably occur as a result of intense leaching in warm tropical and equatorial climates. Relief is important in concentrating leaching water and metallic cations, in affecting oxidation or reduction. The biotic factor affects minerals conspicuously where an A0, horizon develops and resulting cheluviation moves R2O3 out of the A2 horizon.