Robert M. Garrels

Origin and Classification of Chemical Sediments in Terms of pH and Oxidation-Reduction Potentials

Chemical sediments of marine origin are divided into three main classes, representing deposition in normal marine open-circulation environments, restricted humid (euxinic) environments, and restricted arid (evaporite) environments. The characteristics of these environments are briefly reviewed, and the position is taken that the hydrogen-ion concentration, pH, and the oxidation-reduction potential, Eh, afford two basic controls which largely determine the kinds of chemical end-members produced by both inorganic and biochemical reactions. Within the framework of these controls, the depositions of calcium carbonate, iron minerals, manganese minerals, phosphates, evaporites, and organic matter are shown in their relation to variations in pH and Eh of the environment. Certain of the end-members depend mainly upon one or the other of the two controls, and some depend upon both. A classification of chemical sediments is proposed which shows their relation to pH and Eh and serves to indicate genetic relations among the chemical end-members. Occurrences of chemical end-members are summarized on a pH-Eh graph, which shows the range of these values which normally occurs in the environments discussed. Observed mineral associations in typical chemical sediments are listed in support of the theoretical treatment. The influence of postdepositional changes on the original mineral associations is also pointed out. The writers conclude that the environment of deposition of many ancient chemical sediments can be reconstructed in terms of its essential physicochemical characteristics from study of the mineral assemblages among the chemical end-members present in the sediment.

Evaluation of irreversible reactions in geochemical processes involving minerals and aqueous solutions—II. Applications

Abstract Equilibrium relations among common rock-forming minerals and aqueous solutions over a range of temperatures and pressures are known experimentally for a number of systems and can be calculated for others. This information permits prediction of the mass transfer involved in chemical reactions characteristic of geochemical processes. Calculations of this kind are used to examine various chemical and geologic implications of irreversibility in idealized models of weathering, evaporative concentration, diagenesis, hydrothermal rock alteration, and ore deposition.

Observations on the Solubility of Skeletal Carbonates in Aqueous Solutions

Carbonate skeletal materials of marine organisms exhibit a wide range of solubilities in aqueous solutions. In most cases, the dissolution of the carbonate mineral is irreversible and therefore the material can have no true equilibrium solubility. Relative solubilities have been measured in distilled water and in sea water. The least soluble mineral appears to be calcite with low magnesium content; the most soluble is calcite containing 20 to 30 percent MgCO3 in solid solution. Aragonite has an intermediate solubility.

Silica in Sea Water: Control by Silica Minerals

Silicate minerals typical of those carried in the suspended load of streams release silica to silica-deficient sea water and abstract silica from silicaenriched sea water. Experimental rates of release and uptake permit the conclusion that the suspended solids carried into the oceans by streams are a major control of the concentration of silica in the ocean.

Geochemical compositions of some Precambrian shales from the Canadian Shield

Forty-three elements or constituents have been analysed on eight composites representing 406 samples of Archean (> 2.5 Ga) shale and nine composites of 396 samples of Aphebian (1.6–2.5 Ga) shale. These samples were obtained from 54 localities in, or flanking, the Superior Province of the Canadian Shield. The composition of the Archean samples reflects their origin as products of the rapid deposition of detritus from terrane that was dominantly volcanic and mafic in composition. By contrast, the Aphebian samples are highly mature and were derived from more “granitic” sources. The C/S ratio for Aphebian shales is close to 0.36, similar to that of modern sediments, which is consistent with the fixing of S by sulphide-reducing bacteria. Only three of the Archean composites contain significant amounts of C and S and for these samples the elements are closely correlated in the ratio 1 : 1. δ34S values for the Archean composites are close to zero, suggesting a volcanic source. It appears that within Archean greenstone belts the environments most favourable to organic growth and preservation were close to sulphur springs, where sulphur-oxidizing bacteria fixed CO2. These C- and S-rich Archean samples are also enriched in Cu, Zn, As, Ag, Sn, Sb, Hg and Pb, further suggestive of deposition near volcanic springs. The Aphebian samples have a whole-rock average δ34S of + 12‰, quite different from the negative values characteristic of the Phanerozoic and Recent sediments. There is a remarkable peak of Hg in shales of this age. Sedimentary differentiation taking place during Aphebian time has produced both Fe-poor and Fe-rich sediments, including Superior-type iron formation.

Silicates: Reactivity with Sea Water

Silicate minerals rapidly release silica to sea water. Fine-grained, 1-gram samples were placed in 200 milliliters of sea water, and the silica content of the water was measured intermittently for 6 months. At the end of 10 days and of 6 months the silica concentrations with various minerals, in parts per million, were, respectively: kaolinite, 0.6, 2.2; chlorite, 2.5, 2.4; illite, 1.8, 2.6; muscovite, 1.8, 3.9; montmorillonite A, 7, 10.5; montmorillonite B, 10, 21. Except for ion exchange phenomena, it generally has been assumed that these minerals do not react significantly with sea water, or, if they do, they react at geologically slow rates. The rates observed indicate that the ocean must be looked upon as a chemical system with a rapid response to added detrital silicates.

A quantitative model for the sedimentary rock cycle

Abstract A steady-state quantitative model for the sedimentary rock cycle is presented. The cycling of 11 major elements through the oceanic, atmospheric, biospheric, and rock reservoirs is shown. Flux rates are based on the estimated average geologic rates of transfer; the total flux of material through the oceans is about 1 3 that of today. The model is consistent with current estimates of the chemical composition of the average dissolved and suspended loads of streams, with the present-day composition of the oceans, that of the average sedimentary rock, and that of the average composition of precipitation. The mean residence time of the major elements in the cycle is about 400 million years; estimates are given for the cycling times of the individual elements.

MONTMORILLONITE/ILLITE STABILITY DIAGRAMS

Chemical activity diagrarns, prepared to illustrate the properties expected if mixed-layer mont-morillonite/illite is regarded as a solid solution, are compared to those derived from a treatment of these materials as a mixture of two phases. If the system is a solid solution, the coexisting aqueous solution should range from higher dissolved silica contents in the presence of kaolinite and a montmorillonite end member to lower dissolved silica in the presence of kaolinite and an illitic end member. Silica concentration in the aqueous solution might vary by a factor of as much as six. If the system is two phase, the silica content of a solution in equilibrium with kaolinite and both phases would be fixed at a given T and P, as would a solution equilibrated with both phases and K-feldspar. In the absence of a third phase, silica in equilibrium with both phases should be nearly constant, but increase with increasing ratio of K+/H+ in solution. Available data on coexisting aqueous solutions apparently are more nearly consistent with two phases than with a solid solution.РезюмеДиаграммы химической активности, подготовленные для показания ожидаемых свойств в случае, когда смешано-слойный монтмориллонит/иллит считается как твердый раствор, сравнивались с диаграммами полученными для этих материалов, рассматриваемых как смесь двух фаз. Если эта система рассматривается как твердый раствор, ожидается, что сосуществующий водный раствор имеет большие содержания растворенного кремнезема в присутствии каолинита и монтморилло-нитового конечного члена и меньшие содержания в присутствии каолинита и иллитового конечного члена. Концентрация кремнезема в водных растворах может изменяться даже в шесть раз. Если эта система рассматривается как смесь двух фаз, тогда содержание кремнезема в растворе в состоянии равновесия с каолинитом и двумя фазами будет неизменяющимся при данных температуре и давлении, так как раствор в равновесии с двумя фазами и К-фельдшпатом. В отсутствии третьей фазы, содержание кремнезема в равновесии с двумя фазами будет почти постоянное, но увеличивается с увеличением отношения К+/Н+ в растворе. Доступные данные по сосуществующим водным pacrl ворам, по-видимому, являются более согласующимися в случае двух фаз, чем в случае твердого раствора. [E.G.]ResümeeChemische Aktivitätsdiagramme, die hergestellt wurden, um die Eigenschaften, die man erwartet, wenn man Montmorillonit/Illit-Wechsellagerungen als feste Lösungen betrachtet, zu beschreiben, werden mit denen vergleichen, die man erhält, wenn man diese Substanzen als eine Mischung aus zwei Phasen behandelt. Wenn das System eine feste Lösung ist, dann sollte die koexistierende wässrige Lösung von höheren gelösten SiO2-Gehalten in der Gegenwart von Kaolinit und einem Montmorillonit-Endglied bis zu niedrigeren gelösten SiO2-Gehalten in der Gegenwart von Kaolinit und einem illitischen Endglied reichen. Die SiO2-Konzentration in der wässrigen Lösung kann bis zu einem Faktor von sechs variieren. Wenn das System als aus zwei Phasen bestehend betrachtet wird, dann wäre der SiO2-Gehalt einer Lösung im Gleichgewicht mit Kaolinit, und beide Phasen wären bei einem gegebenen T und P fixiert, wie auch eine Lösung mit beiden Phasen und K-Feldspat ins Gleichgewicht gebracht wäre. In Abwesenheit einer dritten Phase sollte das SiO2, das im Gleichgewicht mit beiden Phasen ist, nahezu konstant sein, aber mit wachsendem K+/H+-Verhältnis in der Lösung ansteigen. Die zur Verfügung stehenden Daten über koexistierende wässrige Lösungen stimmen offensichtlich eher mit der Annahme von zwei Phasen als mit der Annahme einer festen Lösung überein. [U.W.]RésuméDes diagrammes d’activité chimique, préparés pour illustrer les propriétés attendues si une montmorillonite/illite à couches mélangées est considérée comme une solution solide, sont comparés à ceux dérivés d’un traitement de ces matériaux en tant que mélange de deux phases. Si le système est une solution solide, la solution aqueuse coexistante devrait s’étager de contenus en silice dissoute plus élevés en la présence de kaolinite et d’un membre final de montmorillonite, à des contenus en silice dissoute moins élevés en présence de kaolinite et de l’illite comme membre final. La concentration en silice dans la solution aqueuse peut varier d’un facteur aussi élevé que six. Si le système est à deux phases, le contenu en silice d’une solution en équilibre avec la kaolinite et les deux phases serait fixé à des T et P données, comme le serait une solution équilibrée avec les deux phases et du feldspar-K. En l’absence d’une troisième phase, la silice en équilibre avec les deux phases devrait être presque constante, mais devrait augmenter proportionnellement à la proportion croissante de K+/H+ en solution. Les données sur les solutions coexistantes sont apparemment plus consistantes avec deux phases qu’avec une solution solide. [D.J.]

DIFFUSION OF IONS THROUGH INTERGRANULAR SPACES IN WATER-SATURATED ROCKS

One of the mechanisms of transfer of material through water-saturated rocks is solute diffusion from places of high concentration to places of lower concentration. The distance of movement through rocks of given concentration fronts has been studied experimentally as a function of time, temperature, and concentration of the reservoir. The accord of the experimental work with general diffusion theory has made it possible to present a nondimensional equation for the movement of diffusing ions through rocks. This equation makes it possible to calculate the distance of movement of selected concentration fronts over a wide range of solution composition. The rate of advance of a given concentration front is independent of the permeability or porosity of the limestones studied, but the amount of material transferred depends upon the porosity in the direction of diffusion. Methods of measuring this porosity are discussed, and values determined for several limestones. Solution of several sample problems indicates that solute diffusion through intergranular spaces in rocks is a more effective geologic process, even at low temperatures, than is generally believed.

Prediction of Gibbs energies of formation—I. Relationships among Gibbs energies of formation of hydroxides, oxides and aqueous ions

Abstract A parameter called ΔO2− is defined as the difference between the Gibbs energy of formation from the elements of a crystalline oxide and the Gibbs energy of formation from the elements of its cation in aqueous solution (per O2− in the oxide). Another parameter called Δ hydroxide is defined as the difference between the Gibbs energies of formation of an hydroxide and its constituent oxides, H2O oxide being considered as ice. A linear equation for 32 hydroxides, using these two parameters is: Δ hydroxide = −0.210 (ΔO2− cation − ΔO2− H+). This relation can also be expressed in terms of solution energies. The solution energy of an hydroxide is a linear function of the solution energy of the corresponding oxide. The equation above can be used to predict values for Gibbs energies of formation of hydroxides, oxides or aqueous cations and to test the consistency of existing Gibbs energy values.