James A. Kittrick

The mechanism of Zn2+ adsorption on calcite

The adsorption of Zn2+ on calcite (CaCO3(s)) was investigated from aqueous solutions in equilibrium with CaCO3(s) and undersaturated with respect to Zn5(OH)6(CO3)2(s)). Zinc adsorption occurred via exchange with Ca2+ in a surface-adsorbed layer on calcite. The validity of this exchange reaction was supported by adsorption isotherm and constant concentration experiments, where Ca2+(aq) was varied by systematically changing the pH and CO2(g). Greater adsorption of Zn2+ occurred at higher pH and CO2(g) levels, where Ca2+ activities were lowest. Sites available for Zn2+ sorption were less than 10% of Ca2+ sites on the calcite surface. Surface exchange of Zn2+ did not affect the solubility of calcite. Zinc sorption was apparently independent of surface charge, which suggested that the surface complex had covalent character. Desorption and isotopic exchange experiments indicated that the surface complex remained hydrated and labile as Zn2+ was rapidly exchangeable with Ca2+. Careful analysis of the adsorption data showed that Zn2+ and ZnOH+ were the sorbing species. The exchange reaction was generalized as a power exchange function: K = 0.62 = {(Ca2+)(Zn2+ + ZnOH+)}[ZnXCaX]1.69 Zinc adsorption on calcite was compared to and was consistent with that of Co2+, but Zn2+ was more strongly sorbed.

SOIL MINERALS IN THE AI~O3-SiO2-H20 SYSTEM AND A THEORY OF THEIR FORMATION*

An attempt has been made to assemble the best thermodynamic information currently available for soil minerals in the Al2O3-SiO2-H2O system at 25°C and 1 atm. Montmorillonite is included by considering its aluminum silicate phase. Diagrams are presented so that the stability of the minerals can be visualized in relation to the ionic environment. Although the Al2O3-SiO2-H2O system is a very simple one compared to soils and sediments, the stability diagrams depict a mineral stability sequence and mineral pair associations that are in good agreement with natural relations.According to the stability diagram, mineral pairs that can form in intimate association are gibbsite-kaolinite, kaolinite-montmorillonite, and montmorillonite-amorphous silica. Forbidden pairs are amorphous silica-kaolinite, amorphous silica-gibbsite, and montmorillonite-gibbsite. The formation of intimate mixtures of three or more of these minerals is also forbidden. The stability diagrams predict ion activity relationships that are in reasonable agreement with those obtained from soils and sediments.Amorphous silica probably limits high silica levels, with montmorillonite also forming at high silica levels. Kaolinite forms at intermediate and gibbsite at low silica levels. These minerals in turn probably control the activity of aluminum ions at a level appropriate to the pH. The formation of gibbsite, kaolinite, montmorillonite and amorphous silica appears to be controlled by a combination of kinetics and equilibria. That is, the kinetic dissolution of unstable silicates appears to control the H4SiO4 level. The new mineral(s) most stable at that H4SiO4 level appear to precipitate in response to solution equilibria.RésuméOn a tenté de rassembler toutes les informations thermodynamiques actuellement disponibles sur les minéraux terrestres dans le système Al2O3-SiO2-H2O a 25°C et 1 atm. La montmorillonite y est incluse compte tenu de sa phase de silicate d’aluminum. Des diagrammes sont présentés de façon à ce que la stabilité des minéraux puisse être représentée en relation avec l’environnement ionique. Bien que le système Al2O3-SiO2-H2O soit très simple par comparaison aux sols et sédiments, les diagrammes de stabilité décrivent une sequence de stabilité minérale et des associations minérales paires qui sont en accord avec les relations naturelles.Selon le diagramme de stabilité, les paires minérales qui peuvent se former en association intime sont gibbsite-kaolinites, kaolinites-montmorillonites. et silice amorphe montmorillonite. Les paires interdites sont les silices amorphes kaolinites. silices amorphes gibbsites, kaolinites montmorillonites, et silices amorphes montmorillonites. La formation de mélanges intimes de 3 ou plus de ces minéraux est également interdite.Le diagramme de stabilité prédit des rapports d’activité ionique en accord raisonable avec ceux obtenus à partir des sols et des sédiments. Les silices amorphes limitent probablement des niveaux de silice élevés, avec de la montmorillonite se formant également à ces memes niveaux de silice. Les kaolinites se forment à des niveaux intermédiaires et les gibbsites à des niveaux de silice très bas. A leur tour, ces minéraux contrôlent probablement les activités des ions d’aluminum à un niveau approprié au pH. La formation de gibbsite, de kaolinite, de montmorillonite et de silice amorphe paraît être contrôlée par une combinaison de cynétique et d’équilibre. C’est à dire que la dissolution cynétique de silices instables semble contrôler le niveau H4SiO4. Le nouveau minéral le plus stable à ce niveau H4SiO4, semble précipiter en réponse à la solution d’équilibre.KurzreferatEs wurde versucht, die besten derzeit erhältlichen thermodynamischen Daten für Bodenminerale im Al2O3-SiO2-H2O System bei 25°C und 1 Atmosphäre zu vereinigen. Montmorillonit wurde unter Berücksichtigung seiner Aluminiumsilikatphase miteingeschlossen. Aus den beigefügten Kurvenbildern lässt sich die Beständigkeit des Minerals in Bezug auf die lonenumgebung beurteilen. Obwohl das Al2O3-SiO2-H2O System im Vergleich mit Böden und Ablagerungen ein sehr einfaches ist, zeigen die Beständigkeitskurven eine Mineralbeständigkeitsfolge und Mineral-paarungen, die gut mit natürlichen Verhältnissen übereinstimmen.Gemäss den Beständigkeitskurven können folgende Mineralpaare in enger Association geformt werden: Gibbsit-Kaolinit, Kaolinit-Montmorillonit und Montmorillonit-amorphe Kieselsäure. Verboten sind Paare von amorpher Kieselsäure-Kaolinit, amorpher Kieselsäure-Gibbsit und Mont-morillonit-Gibbsit. Die Bildung enger Mischungen von drei oder mehr dieser Minerale ist ebenfalls ausgeschlossen. Die Beständigkeitskurven weisen auf Ionenaktivitätsbeziehungen hin, die recht gut mit den aus Boden- und Ablagerungsproben erhaltenen übereinstimmen.Amorphe Kieselsäure schliesst wahrscheinlich hohe Kieselsäureniveaus aus während Montmorillonite auch bei hohen Kieselsäureniveaus geformt werden. Kaolinit bildet sich bei mittleren, und Gibbsit bei niedrigen Kieselsäureniveaus. Diese Minerale bestimmen wahrscheinlich die Aktivität der Aluminiumionen a einem dem pH entsprechenden Niveau. Die Bildung von Gibbsit, Kaolinit. Montmorillonit und amorpher Kieselsäure scheint durch eine Kombination von Kinetik und Gleichuf gewichten bestimmt zu werden, d.h. die kinetische Auflösung unbeständiger Silikate scheint das H4SiO4 Niveau zu bestimmen. Die neuen auf diesem H4SiO4 Niveau beständigsten Minerale scheinen durch die Lösungsgleichgewichte zur Ausfällung gebracht zu werden.РезюмеСделана попытка собрать наиболее достоверные термодинамические данные для минералов почв, образующихся в системе Аl2О3-SiO2-Н2О при 25°С и 1 атм. Монтмориллонит включен в связи с его алюмосиликатной составной частью. Диаграммы даны так, чтобы можно было составить представление о стабильности минералов в зависимости от их ионного окружения. Хотя система Аl2О3-SiO2-Н2О очень проста в сравнении с почвами и осадками, диаграммы эти изображают и последовательность стабильности и парные ассоциации минералов в хорошем согласии с природными соотношениями.В соответствии с диаграммами стабильности, пары минералов, образующих тесные ассоциации, таковы: гиббсит—каолинит, каолинит—монтмориллонит, монтмориллонит—аморфный кремнезем. К запрещенным парам относятся: аморфный кремнезем—каолинит, аморфный кремнезем—гиббсит и монтмориллонит—гиббсит. Образование тонких смесей из трех (или более) минералов также запретно. Диаграммы стабильности позволяют предсказать соотношения активности ионов, удовлетворительно согласующиеся с найденными при изучении почв и осадков.Аморфный кремнезем, вероятно, ограничивает верхний предел активности кремнезема, причем монтмориллонит образуется также при высокой активности кремнекислоты. Образование каолинита происходит при промежуточных, а образование гиббсита—при низких уровнях активности кремнезема. Эти минералы, вероятно, в свою очередь контролируют активность ионов алюминия в соответствии со значением рН. Образование гиббсита, каолинита, монтмориллонита и аморфного кремнезема, как кажется, контролируется совместным влиянием и кинетики и равновесия, т.е. кинетика растворения неустойчивых силикатов, по-видимому, контролирует уровень активности Н4SiO4; новый минерал или минералы, которые наиболее устойчивы при этом уровне активности Н4SiO4, повидимому, осаждаются в соответствии с равновесными отношениями в растоворе.

Solubility and surface spectroscopy of zinc precipitates on calcite

The Sorption and precipitation of Zn was investigated in equilibrium calcite (CaCO3(s)) suspensions at Zn concentrations that approached and exceeded the solubility of known zinc carbonate solids. Surface-enhanced precipitation was not observed and CaCO3(s) did not nucleate Zn solids when the aqueous ion activity product (IAP) was below the equilibrium IAP of the least soluble, kinetically viable Zn phase. The CaCO3(s) surface was not requisite for Zn precipitation. When CaCO3(s) was present the precipitate formed a surface coating or discrete Zn particles bound to the surface. X-ray photoelectron spectroscopy, X-ray diffraction, and energy dispersive X-ray spectrometry of both Zn-treated CaCO3(s) and isolated Zn particles implied, but did not confirm, that the precipitate was hydrozincite [Zn5(OH)6(CO3)2(S)] or its hydrated form. Zinc-treated calcite maintained Zn concentrations that agreed with the solubility of a synthetic hydrozincite measured by Schindler et al. (1969). Direct measurements showed that the equilibrium solubility of the precipitate exceeded that of a natural hydrozincite (log IAP = a5Zn2+a2CO2−3a6H+ = 6.41 ± 0.51) and was less than a natural smithsonite (log IAP = aZn2+aCO2−3 = −10.53 ± 0.10). It is suggested that a poorly ordered form of hydrozincite that regulates Zn activities at higher levels than well-crystallized natural varieties readily precipitates in CaCO3(aq)CaCO3(s) systems. The environmental significance of this phase is unknown.

Illite equilibria in solutions: I. Phase relationships in the system K2OAl2O3SiO2H2O between 25 and 250°C

Natural illite from Marblehead, Wisconsin (MH), USA, has been equilibrated with 0.2 and 2.0M KCl/KOH and KCl/HCl solutions in the presence of excess kaolinite or microcline and quartz or amorphous silica at temperatures between 25 and 250°C and Pv = PH2O- Reversibility of univariant equilibria was demonstrated by approach from high and low aK+aH+ and from silica under- and super-saturation. Solutions were separated after experiments using immiscible displacement techniques. Isothermal, isobaric log ak+/aH+ vs. log aSiO2,aq diagrams have been constructed denning possible stability fields for kaolinite, microcline, gibbsite (or boehmite or diaspore), muscovite, and four illitic phases. Assuming an R+2-free stoichiometry, K-content per half cell, estimated from the slopes of univariant lines, are 0.29, 0.50, 0.69, and 0.85 K; these phases are compositional analogs of smectite (S), mixed-layer illite I/S (i.e., IS, ISII) and illite (I), respectively. Illitization reactions are strongly affected by temperature and porewater chemistry. At quartz saturation, direct conversion of smectite or kaolinite to endmember illite can occur at high pH; at low pH, these reactions are unlikely inasmuch as K+ requirements exceed concentrations observed in most natural pore waters. In silica-supersaturated solutions, illitization reactions proceed through crystallization of intermediate phases with compositions between smectite and endmember illite (I).

Muscovite dissolution at 25°C: implications for illite/smectite-kaolinite stability relations.

Matt igod and Kittr ick (1979) conducted solubility experiments in aqueous solutions at 25~ and 1 a tm using muscovite-gibbsite mixtures. Their experiments yielded solute activities (aKJam and amsio 4) that were inconsistent with the dissolution reaction based on the chemical composi t ion of the muscovite starting materials, Ko.9(A12)(Si3.1Alo.9)Olo(OH)2. Precipitation of a new, nonmuscovi te phase was considered to be unlikely because the solutions were not supersaturated with respect to other phases in the system K20-A1203SiO2-H20 and none were detected. Furthermore, assuming that muscovite and gibbsite participated in the equilibrium, precipitat ion of a new phase would have required isothermal and isobaric invariance rather than the observed univariance. Therefore, a new dissolution reaction was derived in which the muscovite composition was adjusted to satisfy the solute activity data, the presumption being that muscovite of a new composition, Ko.sl(A12)(Si3.19mlo.sl)Olo(OH)2, had crystallized and was the solubility-controlling phase. Although a change in muscovite composition was inferred, it could not be verified directly by X-ray powder diffraction, infrared spectroscopy, or electron microscoPY. In a later study (Rosenberg et aL, 1984), statistical analysis was used to confirm the conclusion of Mattigod and Kittr ick (1979) that solution composit ions were not controlled by the bulk composit ion o f the starting materials. The composi t ion of the solubilitycontrolling phase resulting from the apparent incongruent dissolution of muscovite, however, could not be detected by analytical electron microscopy. Compositional subgroups within muscovite crystals, probably at crystal edges, were invoked to explain these results. The apparent crystallization of a mica-like phase having a composi t ion o f 0.81 K per O~o(OH)2 at 25~ is of great interest inasmuch as all estimates of the composi t ion of natural, end-member illite (I) range between 0.8 K and 0.9 K per O~0(OH)2 (Weaver, 1965; Srodofi and Eberl, 1984; Eberl and Srodofi, 1988; Inoue et aL, 1988). Evidence from experimental studies leads to similar conclusions. Hemley et aL (1980) observed that muscovite is somewhat K-deficient in the relatively acid environment represented by muscovitekaolinite equilibrium. Solution equilibration studies at this phase boundary have suggested composit ions of 0.9 K (Sass et al., 1987) and 0.85 K (Aja, 1989) per

Stability diagram of CaNaK-plagioclase with authigenic minerals

Abstract Conventional stability diagrams are limited essentially to the equilibria of aqueous solutions with pure end-member minerals. These diagrams extend to natural minerals containing isomorphous ion substitutes and to solid solutions if moles of ion charge are employed instead of moles. Stability diagrams derived from moles of ion charge are, in the strictest sense, limited to closed systems but natural solutions and solids tend to be open systems. Because natural solutions are not at complete equilibrium with solids, however, and because thermodynamic data are uncertain, stability diagrams derived from moles of ion charge in solution may be as applicable to natural conditions as diagrams based on moles.