Closed Metastable Microsystem of Layering in Liesegang Reactions (On the Origin of Pisoliths and Oolites)

CLOSED METASTABLE MICROSYSTEM OF LAYERING IN LIESEGANG REACTIONS

(On the Origin of Pisoliths and Oolites)

A. N. KONDAKOV

(Presented by Prof. A. M. Kuzmin)

The mechanism of redistribution of components in a forming oolitic diffusion structure is concealed in the distinctive evolution of the course of a periodic reaction of the Liesegang type when a substance diffuses into a closed volume. Elements of redistribution are already revealed when one considers the structure of the next layer, separated from the medium that excites the diffusion flux by the preceding layer.

Several variants have been proposed for a mathematical treatment of a qualitative understanding of the nature of periodic precipitation [1, 2, 8]. They are aimed at establishing the absolute position of the next layer, proceeding from emission-wave concepts or from ideas about periodically arising conditions of supersaturation with respect to the component precipitating during counter-diffusion. The questions resolved from these positions include the distance between neighboring zones, the dependence of these distances, and also the width of the precipitate bands on the kinetic constants of crystallization (diffusion coefficients, rate of crystal growth, etc.). At the same time, essential aspects of the process of periodic precipitation have not been taken into account:

1) Since researchers were concerned chiefly with the kinetics of the diffusion field, they considered the process one-sidedly, mainly with respect to the component that precipitates. Hence, by the next layer was understood only the immediate layer of precipitate. The distribution of the other components participating in the reaction, as well as the quantitative distribution of the substance within the precipitate layer itself, was not taken into account. The midpoints of the precipitate bands, usually recorded in the interpretation of experimental data, served only as reference marks of the developing diffusion field.

2) In all cases, the medium in which the precipitate formed was regarded as chemically neutral. Its role was reduced only to creating the conditions under which the reaction takes place in the diffusion region.

3) Diffusion into a closed volume was not studied. (By a closed volume is understood a part of the medium separated from the enclosing mass by the membrane of the first layer. The direction of the diffusion front is normal to the surface of the first layer.)

In the structure of natural Liesegang formations, where diffusion of matter takes place in a closed volume (rings of hydrated iron oxide in rocks of the weathering crust, geodes, pisoliths of ferruginous and clayey-

of aluminous composition), A. M. Kuz’min [4] established the following regularities.

Rings of iron-oxide hydrate in rocks of the weathering crust are formed when ferruginous material diffuses from the walls of cracks toward the center of the body bounded by them, or into any other closed contour. Regardless of the initial geometric configuration of the front of the diffusion field, as the material moves inward they tend to create volumetric rings with a surface close to spherical. Within each ring an asymmetric (rhythmic, in A. M. Kuz’min’s terminology) distribution of ferruginous material is observed. Its asymmetry is expressed in a gradual increase in the concentration of ferruginous material in the ring in the direction toward the center of the contour, with a sharp fall in concentration in the frontal part of the ring.

In limonite geodes, alongside the phenomena noted, as the structure becomes saturated with iron within each ring in the direction toward the center, a transition is observed from highly hydrated iron-oxide hydrates to low-hydrated ones (goethite). In geodes of mature stages, toward the center there is both a general increase in the amount of iron and transitions to minerals with a denser packing of matter (hydrohematite and hematite). Within the rings, low-hydrated and anhydrous iron oxides tend toward the inner margin; but even if the ring is composed entirely of hematite, a border composed of brownish-yellow iron hydroxides is found in the outer part of the ring.

Analogous relations have been established in oolites and pisoliths of weathering crusts and bauxite deposits. The oolitic structure is most clearly expressed in ferruginous pisoliths. In section, this structure appears as an alternation of rings enriched in iron compounds, separated from one another by concenters that are lighter and contain less iron. In pisoliths of mature stages of development, there is a clearly expressed general quantitative increase in the coloring pigment both in each concenter from the periphery toward the inner margin, and overall from the periphery of the system toward the center. The more lightly colored parts of the concenters increase in their cross sections toward the periphery, and in the same direction they become lighter; in the outer part they are composed of hydrargillite. Study of the chemical composition of pisoliths and of the rock mass cementing them permits one to speak of a sharp contradiction between them. The content of silica and alumina in ferruginous pisoliths is always lower than in the surrounding cement, whereas the iron content progressively increases; and the steady accumulation of iron in a favorable medium leads to the transformation of goethite into hematite, then into maghemite and magnetite. In aluminous pisoliths, alumina in the form of hydrargillite is concentrated at the periphery, while in the center of the pisolith it is transformed into diaspore.

The study of such formations allowed A. M. Kuz’min to assign them to structures that develop in weathering crusts according to the type of Liesegang rings, in cases where ferruginous solutions create closed contours—“bubbles.”

Diffusion of material inward into the contour proceeds periodically and rhythmically in the form of concenters. Within the system an active nature of the motion of matter arises. In this process several stages of this motion can be outlined; it leads to the fact that substances with relatively small molecular volumes in the concenters increase quantitatively toward the center, whereas substances with relatively large molecular volume in the concenters increase quantitatively in the opposite direction, and their maxima are reached in the peripheral part of the pisoliths. Such redistribution is carried out, according to the ideas of A. M. Kuz’min [4], by the destruction

of the primary substance of the rock (medium) to the ionic state, with its subsequent regular mineral distribution in the Liesegang structure or its removal beyond its limits.

The increase in the density of the substance in the direction toward the center of the system, both in the ring and in the structure as a whole, is caused by the growth of interaction among closely growing crystallites. Further absorption occurs by transition to minerals with an increasingly dense packing of the crystalline structure.

What has been set forth is the developmental tendency (according to A. M. Kuzmin [4]) of Liesegang structures unfolding in a closed volume under conditions of a progressive influx of substance from outside.

Distribution of reaction products in the next layer

To study the distribution of the products of a periodic reaction within a layer, we obtained rings of iron oxide hydrate and silver bichromate in a gelatin medium by the usual method. (By a ring is meant the entire region from the preceding layer to the frontal part of the precipitate ring.)

In the reaction of $\mathrm{FeCl}_3$ and $\mathrm{NaOH}$ in a gelatin medium, brown rings of $\mathrm{Fe(OH)}_3$ and crystals of $\mathrm{NaCl}$ are formed. Within the ring, both reaction products are arranged regularly. From the condition of supersaturation it follows that the component determining the structure of the periodic reaction is the substance with the lower dissociation constant (in our reaction, $\mathrm{Fe(OH)}_3$). Regardless of the direction of diffusion of $\mathrm{NaOH}$ from the center or into a closed volume, within the ring the $\mathrm{NaCl}$ crystals tend to concentrate in the opposite direction, while the $\mathrm{Fe(OH)}_3$ precipitate tends to concentrate in the direction coinciding with the motion of the diffusion front (Fig. 1). Analogous relationships between the reaction products in a ring are found in the periodic precipitation of silver bichromate. The following observation is noteworthy. In that part of the system where the ring structure has already disappeared and the precipitate acquires a generally diffuse character, very small spheroidal bodies of concentric-zonal structure arise (closed systems of the second order), sometimes enclosed by a single common shell (Fig. 2). The dark part of the rings in them is represented by reduced silver, the light part by potassium nitrate crystallites finely distributed in the gelatin mass. Structures of this type may serve as a model for the redistribution of iron and aluminum in the pisoliths of Kazakhstani bauxites.

In sections perpendicular to the layer, about 1 mm wide, photometry was carried out for quantitative comparison across sections of the precipitated mass of iron oxide hydrate and its distribution in the ring; a corresponding calculation was also made of the volume of the crystalline mass of sodium chloride and its distribution within the ring. The distribution of both light and heavy reaction products is asymmetric and confirms A. M. Kuzmin’s observations on natural objects (Fig. 3). Different sections in the ring have close or equal ratios of the corresponding quantities, and, consequently, the system is close to equilibrium (Table 1).

In works [1, 2, 8], the medium in which the periodic reaction unfolds is regarded as neutral; however, this premise is erroneous. One should speak of stable and chemically active components of the medium in specific physicochemical processes at a given stage of layer formation. However, the stable components of the medium also actively participate in the general process, determining, as established by G. L. Pospelov and P. I. Kaushanskaya [5, 6, 7], the zonal development of sorb-

Fig. 1. Distribution of sodium chloride crystals and ferric hydroxide precipitate in annular layering (here and in the other figures, the arrow indicates the direction of motion of the diffusion field).

Fig. 2. Closed microsystems of the second order in the reaction of periodic precipitation of silver dichromate.

…sorption phenomena complicating the diffusion process. As the precipitate becomes compacted, displacement and chemical interaction with the formerly stable components of the medium begin, together with their redistribution within the ring according to the principle set forth above by G. L. Pospelov and P. I. Kauzhanskaya [6, 7]. The counter-motion of decomposition products of previously stable minerals, dissolved in the ionic state, has been established experimentally, and the process itself has been termed “adsorptive replacement.” However, the experimental procedure described in [5, 6, 7], in contrast to precipitation processes in a closed system, permits the free removal of reaction products in the direction opposite to the diffusion of the solutions. Water, like the other components, performs not only the functions of a transporting medium but also participates in hydrolysis processes. Thus, within the ring, during diffusion into a closed volume, a complex ordered physicochemical equilibrium arises, in which, ultimately, all components participate. The distribution of the components constituting the ring in the experiment is shown schematically in Fig. 4.

Fig. 3. Curves of the intensity of light absorption by a precipitate of ferric hydroxide hydrate (1) and of the distribution of sodium chloride crystals (2) in the section of a ring.

In the forming ring of a closed structure, under equilibrium conditions, because of geometrical features the specific density of particles moving toward the center is greater than the density of particles in the opposite diffusion direction. This phenomenon explains many morphological features of natural and artificial closed Liesegang systems, in particular the tendency, in an isotropic medium, to create precipitate rings with a surface close to spherical. The supersaturation zone is situated along the surface of equal specific density of the diffusing component.

Fig. 4. Diagram of the distribution of components under equilibrium conditions in a system of ring layering.

Since in the system conditions of dynamic equilibrium exist in the individual rings, the entire structure is also in a state of metastable equilibrium. An excess of substance during min—

radial transitions in the inner part of the rings is removed beyond the limits of the structure. With the progressive diffusion of matter into a closed system, ultimately, while the other factors are preserved, an exchange of matter takes place; the system lives, obeying its own internal laws. In the limit, all light reaction products may be displaced from the system, and the oolitic structure in the pisoliths disappears.

Such a process takes place, for example, in the geodes of the Liesegang deposit, where limonite cements oolitic-terrigenous material. The oolites in the outer loose part of the geodes retain an oolitic structure, whereas in the inner dense part they are composed of massive goethite.

Table 1

The interrelations between the closed Liesegang structure and the medium are a special and complex question. Undoubtedly, the mineralogical face of the Liesegang structure and the quantitative interrelations among the minerals are determined by the physicochemical conditions of the medium in which they arise. In exogenous mineral formation, the leading factors are the hydrogen-ion index and the oxidation-reduction potential; consequently, in the pH—Eh system it is possible to establish the regions of existence of different mineralogical compositions of oolitic structures. P. I. Kaushanskii [3] established that, when two reacting streams meet at the site of future precipitation, a sharp change in the oxidation-reduction potential occurs, i.e., prerequisites are created for the existence of a new mineral. The Liesegang structure is a polymineral system, and the minerals composing it exist under definite physicochemical conditions. The regular change in mineralogical composition within the limits of an oolitic structure testifies to a regular change in the physicochemical conditions arising in the developing Liesegang system, and to a definite tendency for these conditions to change as the structure matures.

A second, no less important, unstudied aspect remains the geochemistry of trace elements concentrated in pisoliths. The behavior of these elements in a regularly constructed Liesegang structure apparently obeys its internal physicochemical structure.

REFERENCES

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