ISSN 1069-3513, Izvestiya, Physics of the Solid Earth, 2009, Vol. 45, No. 11, pp. 952–963. © Pleiades Publishing, Ltd., 2009. Original Russian Text © V.Yu. Traskin, 2009, published in Fizika Zemli, 2009, No. 11, pp. 22–33.
Rehbinder Effect in Tectonophysics V. Yu. Traskin Laboratory of physico-chemical mechanics, Department of Chemistry, Moscow State University, Vorob’evy gory, Moscow, 117234 Russia e-mail:
[email protected] Received May 15, 2009
Abstract—The modern concepts regarding a number of phenomena, joined together by a common name “Rehbinder effect” and consisting of a change in the mechanical properties of solids as a result of their reversible physicochemical interaction with the medium (physical adsorption, low-energy chemisorption, wetting), which leads to the decrease of their surface energy, are considered. The different manifestations of the effect, such as the strength reduction, the intergranular destruction, the facilitation of plastic flow, studied on the rocks (in the laboratory experiments and at the places of natural occurrence) and on model objects, are examined. Special attention is given to the analysis of thermodynamic, kinetic and structural factors, which determine the possibility, the form, and the degree of manifestation of the Rehbinder effect, and considerations about its place among other mechanisms of fluid–rock interaction are also formulated. The main conclusion, which follows from the consideration of data accumulated at the present time, is the high probability that the Rehbinder effect in many cases plays a determining role in reducing the macro-strength and increasing the deformability of rock massifs. PACS numbers: 91.45.-c DOI: 10.1134/S1069351309110032
INTRODUCTION In 1928, P.A. Rehbinder “… was set the task of studying the effect of the surface energy of crystal (calcite, rock salt, gypsum, mica) on its mechanical and other properties, reducing the surface tension of the face by the introduction into the environment of surface-active agents” [Rehbinder, 1979a]. In the same year, speaking at the Sixth Congress of Russian Physicists, he reported the obtained results: it turned out that the force, necessary for the splitting of crystals along the cleavage, is reduced by several times, if trace amounts of surface-active agents, capable of being adsorbed on the surface of the fracture during its formation, are present in the medium. This effect, named after its founder, for a long time was also known as the adsorption reduction of strength. However, already long ago these two definitions ceased to be synonyms, since, during 80 years of the study of the Rehbinder effect on different materials over a wide range of conditions it became clear that the adsorptive reduction of strength is only one of the forms of the manifestation of the effect, which have, despite all their differences, a common physicochemical nature. At present, it is commonly supposed that the Rehbinder effect is a change in the mechanical properties of solids as a result of any action (in the overwhelming majority of cases, a reversible physicochemical interaction with the medium in the process of deformation or destruction), which leads to the reduction in their surface energy. In this case, several substantially different forms of the manifestation of the effect can be observed.
I. A small, by several tens of mJ/m2, reduction in the free surface energy can promote a plastic flow of the solid being deformed. This effect is explained by the decrease of the potential barrier, caused by the formation of a new surface and that impedes the emergence of dislocations on the crystal surface. II. If the liquid phase appreciably (by a factor of two or by several times) reduces the surface energy of a solid, the effect can manifest itself in a sharp drop in the strength and the appearance of brittleness. The catastrophic fracture of plates of zinc or duralumin, which are easily broken by hand after deposition on their surface of a small drop of liquid gallium (practically not dissolving, but only wetting the solid metal!), serves as a well-known working example. III. With an even larger reduction in the surface energy, the penetration of liquids into polycrystalline materials, which is accompanied by intergranular destruction in the absence of external mechanical loads, becomes possible. In exceptional cases, even single crystals can spontaneously be dispersed. The resultant heterophase systems with liquid intergranular interlayers usually manifest an increased deformability under the action of stresses (the effective viscosity of rock salt at room temperature drops by ten orders of magnitude with the wetting of intergranular boundaries); however, in this case, deformation occurs by the dissolution–reprecipitation mechanism, in contrast to point I. The possibility, the form, and the intensity of the processes of the adsorption action of a medium on the mechanical properties of solids are determined by a number of factors, which can be divided in three groups [Shchukin, 2007]:
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(1) the chemical nature of the medium and a solid, i.e., the nature of the forces, which act between the molecules (atoms) of both phases and especially at the interface of these phases; (2) the real (defective) structure of a solid, determined by the number and nature of defects, including grain sizes, the energy spectrum of the intergranular boundaries, the dislocation density, the presence and the dimensions of incipient microcracks, pores, etc.; (3) the conditions of carrying out the deformation and destruction of a solid, including the temperature, the deformation rate, the kind and the intensity of the stress state (i.e., the method of application and the value of the external mechanical loads), the quantity and the phase state of the medium, and the duration of its contact with a solid body. Brief mention should be made of the question regarding the place of the Rehbinder effect among the other modes of the interaction of fluids with the solid phases (the contact under unwettability conditions, the chemical reactions, the Ioffe effect, etc.). In this case, the determining role belongs to the intensity of interphase interactions, whose quantitative measure can be serve by the mixing energy of components A and B, which characterizes the deviation of the system from ideality [Rehbinder, 1979b]: (1) U0 = z[UAB – 1/2(UAA + UBB)], (where z is the coordination number, since the values in the right hand side refer to a separate bond, and U0 to an atom or to a molecule). With U0 kT, the components practically do not interact between themselves, the liquid phase does not wet the solid phase, it does not reduce its surface energy, and the Rehbinder effect is not observed. The opposite case is also unfavorable for its manifestation: when U0 < 0, |U0| kT, an intensive dissolution or chemical interaction occurs, the equilibrium interface is absent and the promotion of destruction can occur in those cases, when the liquid simply “corrodes” a solid body, reducing its section. For this, as a rule, a large volume of the active medium or the appreciable porosity of the solid material, which provides the accessibility of interparticle contacts for the aggressive liquid, are required. The Rehbinder effect is most probable in the intermediate case |U0| ≈ kT, when the liquid and solid phases possess a related chemical nature and a thermodynamically stable two-phase system with a low value of free interfacial energy at the interface is formed. Then, a break or rearrangement of the B–B bond in the solid phase is accompanied by their replacement by almost equivalent A–B bonds, and the absence of strong intercomponent interactions provides the mobility of component A and the easy transition of its atoms or molecules from one breaking bond to another one. It should be emphasized that while the condition U0 kT serves as a reliable criterion of the impossibility of the Rehbinder effect, the opposition of the second and third interaction modes is somewhat artificial and makes sense only in the most well-expressed cases. There is a vast group of processes, which take place under the combined (simultaneous or consecutive) influence of mechanical, physicochemical and purely chemical factors, and the question IZVESTIYA, PHYSICS OF THE SOLID EARTH
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whether in these cases it is possible to speak about the Rehbinder effect, has a terminological nature and it hardly deserves serious consideration. Another aspect is more important: long-term studies showed that the spectrum of phenomena, considered as the Rehbinder effect or the processes close to it, is characterized by the prerequisites of the manifestation and by the specific features of behavior, common for systems of very different natures (metals, ionic crystals, glasses, polymers, and rocks). At the same time, the selectivity of the action of specific media on the bodies of one or another type, which follows from the aforesaid, is manifested very clearly. This combination of universality and specificity makes it possible to use the model approach extensively, i.e., to carry out experiments on some systems and to make conclusions about the behavior of other ones, with the fulfillment of all requirements of similarity theory. From the very beginning, minerals and rocks related to a number of the most important objects of physicochemical mechanics (this name was given to the field of science, created by Rehbinder, which studies the role of physicochemical factors both in the processes of deformation and destruction, and in the course of the formation of dispersed structures, solid bodies, and materials). Moreover, similar objects (or synthetic materials close to them) were actually the only type of solid bodies, on which the Rehbinder effect was investigated for a period of the first decade by Rehbinder himself and by his coworkers. The end of the 1930s was marked by the transition of these works on a qualitatively new stage: the study and intense practical use of hardness reducers of rocks for borehole drilling, tunnel construction, and in other fields of mining were begun [Rehbinder, 1979c]. In this case, the information obtained, in combination with the results of laboratory experiments and with the continuously incoming new data of the Earth sciences, led Rehbinder in the last years of his life to a very important conclusion: the facilitation of the deformation and disintegration of rocks under the action of surfaceactive (in a broad sense) liquids on them can occur not only with anthropogenic interference, but also in the course of natural processes. In other words, the appearance of tectonic faults, especially under conditions of lithostatic compression at elevated temperatures, is impossible in “dry” rocks, taking into account the small (by comparison with the strength of dry rocks) levels of the differential stresses, which act in the Earth’s interiors. Since the ideas about the role of fluids in the formation of tectonic faults are sufficiently widespread, it is worthwhile to keep in mind, which aspects of the influence of fluids are not necessary for the manifestation of the Rehbinder effect (but, certainly, they can be combined with it): (1) a large quantity of fluid; (2) its chemical activity; and (3) a high pore pressure. In the present work, some new experimental results are presented, which, in my opinion, can find application in tectonophysics, and their connection with the previously published data is discussed. No. 11
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ADSORPTION REDUCTION OF STRENGTH Adsorption reduction of strength is observed in those cases when a solid body is subjected to mechanical loads in the presence of solutions, which contain a component, capable of being adsorbed at the interface, reducing its surface energy γ. In such systems it is possible, by smoothly changing concentration c, to evaluate adsorption Γ and a reduction in γ. With the observance of a number of conditions (the reversible nature of adsorption, the monomolecular coating of the surface, the absence of adsorbate–adsorbate interactions) the interrelated equations of Gibbs (2), Langmuir (3), and Shishkovskii (4) are valid: dγ RTΓ = – ---------------- , (2) d ( ln c ) ac Γ = Γ max --------------, (3) 1+ ac γ = γ 0 – b ln ( 1 + ac ), (4) where a is the adsorption activity and b = ΓmaxRT. The physicochemical parameters of the surface of the interface determined in such a way can be then compared with the directly measured strength êÒ. In a number of cases of brittle fracture it proves to be possible to describe this relation quantitatively, if the known Griffiths relationship is fulfilled: P c ∼ Eγ . 2
(5)
The first convincing proof of the validity of this approach was given in the work [Shchukin, 1966]. The authors measured the strength of the porous samples of synthetic brucite Mg(OH)2 after holding them above the solutions of sulfuric acid of different concentrations with the well-known pressure of water vapor p; according to the NMR data, the water was concentrated on the surface of the samples in the form of an adsorptive layer, but not as the liquid phase, so dissolution was excluded. Determining the value of Γ(p) from the increase in weight, it was possible to estimate a reduction in γ: p
∫
– Δγ = γ 0 – γ = RT Γ d( ln p ) .
(6)
0
As one would expect, taking into account the Griffiths relationship, the linear dependence of the relative reduction in the square of the strength on the reduction of the free surface energy was confirmed (Fig. 1a); for γ0 a reasonably acceptable value ~300 mJ/m2 was found. Similar results were obtained in a number of other cases, for example, for the chloride potassium–n-propyl alcohol–n-heptane system. The strength of polycrystals of KCl was measured in the C3H7OH– C7H16 of different concentrations and was compared with the reduction in the surface energy of KCl with the adsorption of alcohol, determined from the interfacial angles [Traskin, 2004]. This method also confirmed 2 the proportionality of P c ~ γ (Fig. 1b); for γ0 of potassium chloride, a value of 110 mJ/m2 was obtained, which is close to the literature data. Generally, alkaline halides (NaCl,
KCl, KBr, etc.) are exceptionally convenient for studying the role of the affinity of a medium and a solid body (in this case the degree of affinity is connected with the polarity of the liquid). One of the most spectacular examples is described in the textbooks [Shchukin, 2007]: the strength of polycrystalline potassium chloride drops in the continuous series of solutions n-heptane–dioxane–water (Fig. 2). Combining equations (2) and (5), one can obtain the expression, which directly connects the adsorption of an active component with the strength: dP c γ0 1 -------P ---------------. Γ = – 2 -----------2 RT c d ( ln c ) ( Pc )0
(7)
Knowing the maximum adsorption Émax, it is possible to estimate the area s, occupied by an active molecule on the nascent surface of the destroyed crystal: s = 1/NAΓmax (NA is the Avagadro number). This calculation gives a correct order of magnitude of s (several Å2). Even more striking evidence of the adsorptive nature of the action of water, dissolved in a less active medium, on the strength of hydrophilic solid bodies is observed with a variation of the solvent. The results of the measurement of the strength of the polycrystals of potassium iodide in the solutions of water in alcohols are given in Fig. 3 [Skvortsova, 2005]. If one takes its value in dry alcohol (depending on its nature) as the original value of KI’s strength, the curves in Fig. 3 merge into one curve and, correspondingly, the presented value of water adsorption, calculated from these data, is also described by a single curve (Fig. 4). The methodology of the studies of adsorption reduction of strength, worked out on ionic crystals, was used for the elucidation of the mechanisms of the well-known weakening action of water on rocks. In particular, the work of M.Z. Abdrakhimova, executed at the Kola ultradeep bore hole SG-3 [Abdrakhimov, 1988; Traskin, 1997; 1998] was concerned with this item. In this work, vast experimental material, obtained in the course of laboratory experiments on the bore specimen and on the analogous rocks, free of the anthropogenic changes, is contained. In particular, the compression tests of the samples of amphibolite (95% of green hornblende, 5% of iron sulfides, biotite, and accessory minerals) were carried out in a thermobaric chamber at temperatures up to 250°ë in the solutions of water in acetone; the results indicated the adsorption reduction of strength as one of the most probable mechanisms of the action of water. Further data processing with the use of equations (2)–(5) makes it possible to speak about this with greater confidence (Fig. 5). The characteristic shape of the adsorption curve, constructed according to the results of strength measurements of amphibolite, the value of maximum adsorption and the corresponding surface site for water molecule (several Å2), the linearity of the temperature dependence of the rock–solution interfacial energy (Fig. 6), and a reasonable value of the slope of this straight line, i.e., the specific surface entropy (~0.7 mJ/(m2 degree)), can be considered as the final confirmation of the fact that in all these regular patterns, at least in similar situations, the well-
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2
1 – ( Pc )A ⁄ ( Pc )0 0.9
0.9
(‡)
(b)
0.6 08 0.3
0
50
100
150
07 200 250 –Δγ, mJ/m2
15
20
25
30
Fig. 1. Dependence of (a) the strength of the porous samples of magnesium hydroxide and (b) polycrystals of KCl on the reduction in the surface energy of the solid phase. Indices 0 and A relate to inert (a) air, (b) heptane and surface-active (a) water vapor, (b) propanol, respectively.
expressed strength reduction of silicate rock that occurs is caused by water adsorption. STRENGTH REDUCTION UNDER CONTACT WITH THE LIQUID PHASE If one now traces how the strength of solid bodies is reduced with the increase of the content of a very soluble surface-active component in the solution (in this case, the interface free energy begins to depend practically linearly on lnc), it is possible to see that a sharp transition from the adsorption reduction of strength to the effects, caused by contact with the phase layer of liquid, does not exist. With such contact, the abrupt changes in the mechanical characteristics of solid bodies of any nature are observed. These effects occur with the physicochemical interaction of the
solid and liquid phases, related by chemical nature and structure; moreover, the criteria of affinity, which are entirely obvious at the intuitive level, cannot always be formulated quantitatively. For materials of contrasting polarities, the dielectric permeability of different liquids can serve as a reference point. Thus, the KI strength in welldried alcohol is lowered in comparison with the strength in air (Fig. 3); moreover, the effect is reduced with the decrease of the polarity of alcohol, i.e., with an increase in the length of the hydrocarbon radical [Skvortsova, 2005]; at the same time, for nonpolar materials (naphthalene), an inverse dependence is observed (Fig. 7) [Rehbinder, 1979b]. For metals and some covalent crystals, it is possible to take the phase diagrams as the basis for the calculatation of the interface free energy from them and to compare it with the measured strength. However, as usual, the (Pc)A/(Pc)air 1.0
Pc, mPa 30
0.9
C8H17OH
20
C6H13OH 0.8
C4H9OH C3H7OH
10 0.7
0 Heptane
50
0 100 wt % Dioxane
50
100
0
1
Water
Fig. 2. The strength of the polycrystals of KCl in media with different compositions. IZVESTIYA, PHYSICS OF THE SOLID EARTH
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5 6 c(H2O), mol/l
Fig. 3. The relative strength of the polycrystals of potassium iodide in the presence of the solutions of water in alcohols of a saturated series. No. 11
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nent composition should give a possibility of the “selection” of the optimally interacting components, which provide the maximum reduction in the free energy of the interface [Pertsov, 1981]. The quantitative assessments prove to be frequently difficult, since more or less exact values of free surface energy γ are known only for separate minerals (rock salt, calcite).
É/Émax 1.00 C8H17OH C6H13OH C4H9OH C3H7OH
0.75 0.50 0.25
0
2
4
6 8 c(H2O), mol/l
Fig. 4. Adsorption of water from alcohols of a saturated series on the surface of potassium iodide, which is formed during destruction.
comparison of different systems gives less numerous and fewer clear dependences, than a smooth change in the parameters of the same system. For this particular reason Rehbinder in 1928 did not try, following the spirit of the time, to determine exact values of γ of solid bodies (now, they are also absent in reference books), but he traced the relation of the change in adsorption of γ with the accompanying changes in its hardness, that made it possible for him to make such a fundamental discovery. The thermodynamic condition of the manifestation of the Rehbinder effect, apparently, is fulfilled for the majority of the rock–fluid pairs, that are in contact in a natural situation. Despite all the diversity of these systems, they are always formed by polar substances, which are frequently close in composition and, furthermore, their multicompoÉ × 105, mol/m2 8
Pc, MPa 180
6
160 140
For quartz and silicates the usual methods of γ determination give overestimated values. There is still less data on the interfacial energy of minerals at the interface with the liquid media, whose most important component is, as a rule, water. In those few cases, when such data are available, it is possible to be convinced of the validity of the Griffiths relationship. Besides the examples given above, it is possible to mention the calcite–water system: the values of the surface energy of calcite are given in [Gilman, 1960], and the interfacial angles and the strength are presented in [Vakar, 1986]. In the case of destruction at the interface between the different phases, characteristic for polymineral rocks, half of the work of adhesion instead of γ should appear in the Griffiths formula [Skvortsova, 1992, Traskine, 2000]. The direct use of the interaction potentials for the solution of the problem on the weakening of interatomic bonds in a solid in the presence of impurity molecules is difficult at the present time. With the decomposition of quartz and framework silicates, silicone bonds necessarily break; in the chain and banded silicates, the slip and break along the specific planes, formed only by Me–O bonds are also possible; in nesosilicates the Si–O–Si bonds are absent. The enumerated bonds differ by the geometric parameters (the length, the bond angles), by the distribution of electron energy and binding energy. The scatter of these values for the separate classes of silicates has the narrower limits [Dikov, 1979]. γ(solid–liquid), mJ/m2 1.00
0.96
4 0.92
120
2 0.88
100 0
10
20 30 40 c(H2O), mol/l
50
0 60
Fig. 5. The strength of amphibolite in the water-acetonic mixture (circles) and water adsorption on the surface of destruction (triangles). Temperature is 150°C, hydrostatic pressure is 90 MPa.
50
100
150
200
250 t, °C
Fig. 6. Linear decrease of the amphibolite–solution interfacial energy with a temperature increase, calculated according to the strength data (for the initial surface energy of amphibolite the value ~1 J/m2 is accepted).
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It is important that within the entire range of the change in the polarity of Si–O bonds they remain substantially covalent, in spite of the large difference of the electronegativities of silicon and oxygen. This is explained by the shift of the unshared electron pair of oxygen to one of the free dπ coupling) 3d-orbital of silicon (by the pπ [Voronkov, 1976]. Therefore, the hydrolytic decomposition of silica-oxygen bonds is hindered in comparison with the hydrolysis of the more polar cation-oxygen bonds [Mitsyuk, 1980]. This leads to an important conclusion: the more metaloxygen bonds are broken with the rock’s disintegration, the larger the effect that water should exert. The existence of such a regular pattern is confirmed by systematic tests under identical conditions of rocks with the predominance of minerals of specific structural types. For example, in a series of rocks, which contain predominantly feldspar– pyroxene—olivine, the sensitivity of strength increases both in relation to nonaqueous active media (Fig. 8) [Sal’nikov, 1985] and to water, while for the dry rocks, on the contrary, the general tendency is considered to be an increase in the strength with an increase in basicity [Krasilova, 1985]. However, the existence of a similar regular pattern under natural conditions is confirmed by the specific features of geomorphology of sea shores at high latitudes: it is well-known that the intrusions of the basic rock are destroyed much faster than the enclosing acid rocks, and in their place narrow bays (fiords) are formed. Apparently, the freeze-thaw cycles play a role in their formation. M.Z. Abdrakhimov investigated the samples of basic and acid rocks before and after thirty such cycles. An increase in the fracturing amounted to 70% for amphibolite and 5– 10% for granite [Abdrakhimov, 1988]. THE ROLE OF KINETIC FACTORS The degree of strength reduction of solid bodies under the action of surface-active media, besides the chemical nature of components, is connected with the conditions, under which the processes of deformation and destruction proceed, usually having the kinetic sense. The mobility of a medium, which can limit the rate of the development of a rupture crack, is the most important kinetic parameter. The account of this factor made it possible to connect the laboratory results, obtained on the polymeric model material, whose strength and plasticity were sharply lowered by organic liquid, and the data on the rate of emplacement of magmatic intrusions in the Hawaiian islands [Pertsov, 1977]. The rate of the growth of the crack v and the dynamic viscosity of liquid η and pressure , which differ individually in the model and in nature by two–four orders of magnitude, form in both cases the invariant combination vη/p ≈ 2 × 10–9 m. Such cracks grow relatively slowly (the rate of their development is determined by the viscous drag of the intruding liquid phase and it does not depend on the applied stresses); with the achievement of the break point stress of a “dry” rock, the crack is separated from the liquid phase and grows at a rapid rate. The latter picture is frequently observed under laboratory conditions, when, in IZVESTIYA, PHYSICS OF THE SOLID EARTH
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(Pc)A/(Pc)0, % 100 Glycerine Propylene glycol
80
C4–C8 Water
C3
60
C2
40 0
20
40
60
80 ε
Fig. 7. Dependence of the relative strength of potassium iodide (circles) and naphthalene (triangles) on the dielectric constant of the testing medium; C2–C8 designate alcohols of a saturated series from C2H5OH to C8H17OH.
spite of the presence of the thermodynamic prerequisites of strength reduction, it is not manifested because of the excessively high viscosity of the active medium. Thus, the silicate melts, close by their composition to the rock samples, do not influence their strength, if the melt viscosity is not lowered by heating or the introduction of the corresponding additives (for example, PbO) [Sal’nikov, 1985]. The role of such additives under natural conditions is played by the volatile components of magmatic melts, primarily, water, which considerably decreases the viscosity of magma. In a number of experiments, in which the abrupt drop in the strength of granites, aplites, and other rocks with the appearance of melting at the grain boundary, which facilitate the brittle fracture of a solid skeleton even at premelting temperatures, was studied, water was always present, which facilitated intergranular melting and gave mobility to the meltings [Arzi, 1978; Paquet, 1980; Van der Molen, 1979]. As far as the role of water as an adsorptiveactive component of such systems is concerned, it is unlikely to be significant in the background of the activity of the silicate components of melt. In those cases, when water is the main strength reducer of silicate minerals, the processes, which limit the rate of destruction, are more frequently not connected with the viscosity and are not localized in the liquid phase bulk, but near the front of the growing crack and include the thermal fluctuation events of the hydrolytic decomposition of interatomic bonds, which is activated by the applied stresses. Under these conditions, the destruction was studied both directly on the rock samples (see review in [Atkinson, 1984]) and on the other materials (glasses, ceramics), on which for many years a large number of results have been obtained, which can be applied to geological systems [Charles, 1958; Wiederhorn, 1974; Bershtein, 1987]. As far as the activation barriers are concerned, which control the transport processes in water, containing electrolytes, for their estimation the ideas of the school of No. 11
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Pc, MPa 180
Ar, 1000°C Ar, 1100°C (Fe, Pb, Cu(S)), 900°C
150
PbO + SiO2, 900°C 120 PbO + SiO2, 1000°C Na2B4O7, 900°C
90
é olivinite 60 B bronzitite 30
é
B
D
D dolerite
Fig. 8. The strength in the different media of rock samples, which consist of (from left to right) predominantly island (olivine), chain (pyroxene) and skeleton (plagioclase) silicates.
O.Ya. Samoilov [Samoilov, 1957] can be successfully applied, based on which it is possible to explain the differences in the action of aqueous solutions on the strength of sylvine (potassium chlorite), rock salt, and others [Skvortsova, 2005]. However, the role of the mobility of water is most clearly manifested in the tests of model materials: hydrophilic polymers (cellophane, cellulose and the others). The quantitative criterion of the mobility of water is served by the value ΔU, a change under the action of ions of the potential barrier, which separates the equilibrium states of its molecules in the solution. An increase in the activation barrier of self-diffusion implies a change in the macroscopic properties of the liquid. In the presence of ions, named by Samoilov “positively hydrated” (Na+, Li+, Mg2+, Ca2+), the coefficient of self-diffusion of water and its thermodynamic activity are reduced. In the presence of “negatively hydrated” ions (Cs+, K+) the opposite effects are observed. On the basis of the Samoilov concept it was possible to show that the degree of influence of the aqueous solutions, which contain ions of the first type, depends on the deformation rate: at low rates the samples are destroyed as in pure water; with an increase in the rate of extension, their strength reaches the strength of dry samples (Fig. 9) [Ankudinova, 2004]. In the presence of solutions, which contain negatively hydrated ions (for example, cesium chloride), the strength within the entire range of rates investigated is as small as the strength in the presence of water.
The results of measuring the stress-rupture strength (the long-term strength) of the material in water can prove to be most promising from the point of view of tectonophysics: it sharply grows with an increase in the concentration of ions, which bind water. Moreover, the effect correlates with the data on the decrease of the coefficient of self-diffusion of water. The data obtained make it possible to use the theory, developed by S.N. Zhurkov and based on the ideas about the kinetic nature of strength, for the analysis of the results obtained (both with the measurements of stress-rupture strength and under conditions of active loading). With the tests of polymers in the static and dynamic behaviors, it was shown [Skvortsov, 2005] that in the presence of “positively hydrated” ions, the energy of the activation of destruction grows in value, not strongly differing from the values given in the literature, ΔU [Samoilov, 1957]. The results of numerous works on static fatigue and on the kinetics of crack growth are sometimes discussed in terms of “stress corrosion.” If one considers corrosion as dissolution with the transition of the atoms of the solid phase in the volume of solution, then such a process sometimes makes a significant contribution to the general picture. However, most frequently, the fate of the atoms, which formed the bounds, is of no consequence after their hydrolytic decomposition. In a number of cases, it is possible to assert that they remain on the spot, since the active medium does not form a liquid phase, but is present in the form of an adsorption layer [Shchukin, 1966]. However, even if they are transferred into solution (perhaps with the redeposition elsewhere, if the solution is saturated), then, nevertheless, the measure for the action of the medium can be served by the work of adsorption, chemisorption or a topochemical reaction, i.e., the thermodynamics of surface interactions. INTERGRANULAR WETTING AND PERCOLATION One additional group of factors, which influence the reduction in the adsorption of the strength of solid bodies, includes the specific features of their real structure, i.e., the type and distribution of the defects. The roughest defects, such as cracks, open pores, and voids, facilitate the transport of liquid media and create stress concentrators, which strengthen their action. However, the most universal defects, which are always present even in continuous rocks, are the intergranular boundaries. It is well-known that the transition from the transcrystalline fracture to the intercrystalline fracture accompanies the action of the surface-active media on the polycrystals of any chemical nature, for example, water on rock salt and on sylvinite, gallium on zinc, etc. The intercrystalline fracture is connected with a continuous series of intermediate situations with the crack opening along the intergranular boundaries, which contain only small residual stresses and are even entirely free from them, if only the liquid phase is sufficiently active on the surface, in order to penetrate such a boundary [Traskine, 1996].
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The condition for the formation and stability of the liquid interlayers, which separate solid surfaces, for the first time was expressed by Faraday, it was strictly formulated by Gibbs [Gibbs, 1982], and was used in physical metallurgy by Smith [Smith, 1948]. This condition, thermodynamically obvious, requires the decrease of free energy during the replacement of the contact surface of grains T1 and T2 by the surface of their contact with the liquid: γ(í1–í2) > γ (í1–liquid) + γ (í2–liquid). (8) In accordance with this condition, at the usual temperature, the intergranular penetration of water and aqueous solutions is limited by rocks of the type of potassium and sodium salts. The temperature increase sharply changes the situation. A steeper abrupt drop of γ(solid–liquid) in comparison with γ(solid–solid) serves as a prerequisite of the extension of the number of systems, in which water (or supercritical fluids) can penetrate along the intergranular boundaries. Indeed, temperature thresholds were observed on metals, on ceramics with melts and on many rocks. In the latter case, the anisotropy of the thermal expansion of separate grains is also added, which intensifies the deconsolidation of rocks [Zaraiskii, 1978]. The water-containing magma is also liable to penetration along the triple junctions of boundaries and on the boundaries themselves (many authors write about this, including [Sisson, 1993]). In those cases, when the free energy of the grain boundary is insufficient for the formation of a liquid interlayer on it, it can be increased due to the externally applied shear or tensile stresses, and the boundary becomes permeable. The joint action of stresses and elevated temperatures, apparently, is the reason for the developed intergranular fracturing, observed in the core samples from the ultradeep bore holes. It is significant that the species from the areas, which for a long time were in contact with the drilling mud in the dwelling periods of boring, prove to be more strongly developed along the boundaries than the fresh core, extracted at the end of drilling [Abdrakhimov, 1988]. The liquid interlayers along the grain boundaries totally change the elastic, rheological, and strength properties of rocks. Apparently, precisely these interlayers are those inclusions, which make a large contribution to the attenuation of elastic waves [Abdrakhimov, 1988], serve as ways of mass transfer with recrystallization creep, and gradually prepare the catastrophic rock fracture in the depths of the Earth during the earthquakes. The grain boundaries in the polycrystalline bodies are distributed in a specific manner over the energies. The parameters of this distribution can be found, for example, from the distribution of angles in the triple intergranular junctions or from the distribution of disorientations (one of the examples of the relation of the boundary’s permeability with the disorientation of grains, which reflects the energy criterion of permeability, is shown in Fig. 10) [Traskin, 1986]. Furthermore, knowing the value of solid–liquid interfacial energy, it is possible to estimate an important value: the fraction of the hydrophilic boundaries, i.e., the boundaries, for which the IZVESTIYA, PHYSICS OF THE SOLID EARTH
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(Pc)/(Pc)air 100 1 2 3 4 5 6
50
0
3
4
5
6
air water MgCl2 K2CO3 CaCl2 LiCl
7 8 ln(v, mm/min)
Fig. 9. The strength of cellophane in the aqueous solutions of electrolytes under tension at different rates.
Gibbs–Smith condition is fulfilled. This value makes it possible to pass to the solution of the question regarding the degree of connectivity of the liquid phase, which is located on the boundary. An effective apparatus for this is served by the percolation theory, which enables one to answer the most important question: whether the rock with liquid interlayers is beyond the percolation threshold pc or within this threshold? In other words, whether liquid inclusions form an infinitely extended connected network, i.e., an infinite cluster? A number of works, published in recent years [Volovich, 2002; Volovitch, 2002; Traskine, 2000; 2001; Frary, 2004] are concerned with the application of percolation theory for the description of the intergranular wetting. Special attention needs to be paid to the question about the role of the scale factors, whose knowledge enables one to compare the results of laboratory experiments with the events, which affect the objects, more extended by many orders of magnitude. The works, carried out on model objects in recent years showed that the percolation threshold on a sample with a length H and a width L exceeds the threshold for the infinite space by the value p c ( H,L ) – p c ( ∞ ) = L
1 – --v
1 ---
H v C ⎛ ln ----⎞ , ⎝ L⎠
(9)
where ë is a constant and v is a universal critical index of correlation radius, independent of the coordination number and equal to 4/3 and ~0.88 for 2D and 3D space, respectively [Traskin, 2005; 2005]. The knowledge of such relationships makes it possible to carry out a quantitative interpretation of the results of computer and physical model experiments on intergranular wetting. No. 11
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TRASKIN n, % 100
Halite
Sylvinite
NaCl–NaCl
NaCl–NaCl
KCl–NaCl
50
0
30
60 30 60 Grain-boundary angle, degrees
30
60
Fig. 10. Water permeability of the cohesive and adhesive grain boundaries in halite and sylvinite depending on the grain-boundary angle of the adjacent grains.
FACILITATION OF PLASTIC FLOW Several mechanisms for increasing the plasticity of a solid body as a result of its physicochemical interaction with an adsorption-active medium are known. The most efficient mechanism, which is apparently very widespread in nature, is the recrystallization creep, or the dissolution–reprecipitation (pressure solution in the English-language literature). The generally accepted three-stage scheme of the process is the local dissolution of a solid body in the stressed sections with an increased chemical potential, diffusion towards smaller concentrations, and the reprecipitation at the stress-free places. In this case, the role of the surface activity of a medium consists of the provision of a developed surface of interface contact and in the reduction in the energy of heterogeneous nucleation at the third stage of the process (with good wetting, practically up to zero). Recrystallization creep is intensively studied by many research teams from different countries both in the laboratory experiments and, also, in the attempts of quantitative interpretation of different natural micro(stylolites) or macro- (salt and other diapirs) structures [Skvortsova, 2004; Zoubtsov, 2004]. This range of problems is also one of the high-priority aspects of studies, conducted in the laboratory of physicochemical mechanics of the Department of Chemistry, Moscow State University. From the results obtained in recent years, the following are worth mentioning:
(1) A methodology has been developed, which makes it possible to distinguish the conditions (the stress state, the deformation rate, the composition and the geometry of a system, etc.), under which the limiting stage of recrystallization creep is either diffusion or the processes at the interface (the kinetic regime) [Skvortsova, 2005]. (2) The correlation between the well-known action of additives, promoting or inhibiting the process of crystal dissolution, or changing their crystal habit (such as urea on NaCl) in the absence of stresses, and the deceleration or acceleration of crystal creep in contact with these additives is revealed (Fig. 11) [Skvortsova, 2008a; 2008b; Traskine, 2008a]. (3) The spectacular effect of the sharp acceleration of recrystallization creep with cyclic loading has been discovered (Fig. 12) [Traskine, 2008b]. The effect appears only under the diffusion conditions of creep and, most probably, is explained by the facilitation of mass transfer through the extended channels at the stage of unloading. Further study of the frequency band, in which the effect appears, must answer the question, whether seismic activity can lead to a noticeable intensification of the rock strain by the mechanism of recrystallization creep. Thus, testing of different materials within a wide range of media and conditions leads to the conclusion that the role of the Rehbinder effect in the appearance of tectonic faults is not only possible, but in a number of cases it is also undoubted. To separate these cases
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ΔH, μm 500
(dε/dt) × 1011, s–1 8 7
400
6 300 5 4
200
3
100
NH4NO3
Cyclic loading NaCl
2 –7
–6 –5 –4 –3 –2 c(oxyethylidenediphosphonic acid), mol/l
Fig. 11. Deceleration of the recrystallization compaction of the calcite powder with an increase in the concentration of the aqueous solution of oxyethylidenediphosphonic acid. The mean grain size is 30 μm, the nominal stress on the plunger is 0.3 MPa, pH = 8.3, the temperature is 20°C.
from the others, where alternative mechanisms are not excluded, and to reconstruct as accurately as possible, a quantitative scenario of the events that are taking place or have occurred, is a task of future studies. CONCLUSIONS (1) Numerous experiments show that the fluids, which reduce the free surface energy of a solid body, can produce an appreciable change in their mechanical properties (the Rehbinder effect): facilitation of plastic flow or, vice versa, a sharp drop in the strength and the appearance of brittleness, and also a spontaneous fragmentation along the grain boundaries. (2) The possibility of the manifestation of the Rehbinder effect is determined by the intensity of interfacial interactions at the microscopic level, by spatial and temporary accessibility of the broken or rearranged interatomic bonds for the molecules of an active medium and by the specific features of the mesostructure (the energy spectrum of the grain boundaries, fracturing, etc.). (3) The Rehbinder effect is characterized by the prerequisites of the manifestation and by the specific features of behavior, common to the systems of most diverse nature (metals, ionic crystals, glasses, polymers, rocks), with the clearly expressed selectivity of the action of specific media on the bodies of any particular type. This combination of universality and specificity offers the possibility of extensively using the model approach, i.e., to carry out the experiments for some particular systems and to draw conclusions about IZVESTIYA, PHYSICS OF THE SOLID EARTH
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16 Time, h
Fig. 12. Recrystallization compaction of the powders of NaCl and NH4NO3 under the static and cyclic (frequency of 0.03 Hz) conditions. The nominal stress on the plunger is 0.3 MPa. In the first case, the process of dissolution is limited by diffusion and, in the second case, by the events at the interface.
the behavior of other systems, fulfilling all requirements of similarity theory. (4) The direct laboratory experiments, carried out on the rocks, make it possible with sufficient validity to determine the ranges of the P-T-X-conditions, which prevent the Rehbinder effect or an effect, favorable for one or another form of its manifestation. The estimation of the scale factor with the help of the percolation theory helps to extrapolate the results on objects, which exceed the laboratory samples in terms if their size by several orders of magnitude. (5) There is a high probability that the Rehbinder effect is very widespread in nature and in many cases it plays the determining role in the reduction of macrostrength and in the increase in the deformability of rock massifs. REFERENCES 1. M. Z. Abdrakhimov, “The Role of Physicochemical Processes in the Development of Intergranular Destruction in the Silicate Rocks,” Candidate’s Dissertation in Geology and Mineralogy (GEOKHI, Moscow, 1988) pp. 1–218. 2. M. Z. Abdrakhimov, V. Yu. Traskin, N. V. Pertsov, et al., “The Study of Deconsolidation of the Crystalline Rocks of Ultradeep Bore Holes by the Methods of Physicochemical Mechanics,” in Deep Structure and Geodynamics of the Crystalline Shields of the European Part of the USSR Ed. by F. P. Mitrofanov and V. I. Bolotov (Apatity, 1992) pp. 128–136. 3. A. M. Ankudinova, Z. N. Skvortsova, V. Yu. Traskin, and A. V. Pertsov, “The Influence of Aqueous Solutions of Electrolytes on the Mechanical Properties of Cellophane,” Dokl. Akad. Nauk, 397 (5), 633–637 (2004). No. 11
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4. A. A. Arzi, “Critical Phenomena in the Rheology of Partially Melted Rocks,” Tectonophysics, 44, 173–184 (1978). 5. B. K. Atkinson, “Subcritical Crack Growth in Geological Materials,” J. Geophys. Res., 89 (B6), 4077–4114 (1984). 6. V. A. Bershtein, The Mechano-Hydrolytic Processes and the Strength of Solids (Nauka, Leningrad, 1987) pp. 1−315. 7. R. J. Charles, “Static Fatigue of Glass,” J. Appl. Phys., 29 (11), 1549–1560 (1958). 8. Yu. P. Dikov, I. A. Brytov, Yu. N. Romashenko, and S. P. Dolin, Specific Features of the Electronic Structure of Silicates (Nauka, Moscow, 1979) pp. 1–126. 9. M. Frary and C. A. Schuh, “Percolation and Statistical Properties of Low- and High-Angle Interface Networks in Polycrystalline Ensembles,” Phys. Rev. B., 69 (134115), l–12 (2004). 10. J. V. Gibbs, Thermodynamics. Statistical Mechanics (Nauka, Moscow, 1982) pp. 1–584. 11. J. J. Gilman, “Direct Measurements of the Surface Energies of Crystals,” J. Appl. Phys., 31 (12), 2208–2218 (1960). 12. N. S. Krasilova and L. L. Panasyan, “The Strength Properties of Rock Grounds,” in Physicochemical Mechanics of the Natural Dispersed Systems Ed. by E. D. Shchukin (Publishing House of MGU, Moscow, 1985) pp. 90–107. 13. B. M. Mitsyuk and L. I. Gorogotskaya, Physicochemical Transformations of Silica under the Conditions of Metamorphism (Naukova Dumka, Kiev, 1980) pp. 1–236. 14. J. Paquet and P. Francois, “Experimental Deformation of Partially Melted Granitic Rocks at 600–900°C and 250 MPa Confining Pressure,” Tectonophysics, 68, 131– 146 (1980). 15. N. V. Pertsov and B. S. Kogan, “Physicochemical Geomechanics,” in Physicochemical Mechanics and Lyophilic Property of Dispersed Systems Ed. by F. D. Ovcharenko (Naukova Dumka, Kiev, 1981) vol. 13, pp. 53–65. 16. N. V. Pertsov, B. S. Kogan, and V. N. Balashov, “The Model of the Interstitial Intrusions of Magma under the Conditions of Manifestation of Adsorptive Decrease in the Rock Strength,” Dokl. Akad. Nauk SSSR, 235 (6), 1375–1378 (1977). 17. N. V. Pertsov and V. Yu. Traskin, “The Rehbinder Effect in Nature,” in Advances in Colloid Chemistry and Physicochemical Mechanics Ed. by E. D. Shchukin (Nauka, Moscow, 1992) pp. 155–165. 18. P. A. Rehbinder, “On the Influence of Changes in the Surface Energy on the Cleavage, Hardness and Other Properties of Crystals,” in P. A. Rehbinder. Selected Transactions. Vol. 2. Physicochemical Mechanics (Nauka, Moscow, 1979) pp. 1–142. 19. P. A. Rehbinder and E. D. Shchukin, “Surface Phenomena in Solids in the Processes of Their Deformation and Destruction,” in P. A. Rehbinder. Selected Transactions. Vol. 2. Physicochemical Mechanics (Nauka, Moscow, 1979) pp. 203–269. 20. P. A. Rehbinder, L. A. Shreiner, and K. F. Zhigach, “The Hardness Reducers in Drilling,” in P. A. Rehbinder. Selected Transactions. Vol. 2. Physicochemical Mechanics (Nauka, Moscow, 1979) pp. 270-320.
21. E. H. Rutter, “Pressure Solution in Nature. Theory and Experiment,” J. Geol. Soc., 140, 725–740 (1983). 22. D. I. Sal’nikov, “The Influence of Liquid Phases on the Structural and Mechanical Properties of Rocks,” Candidate’s Dissertation in Chemistry (MGU, Moscow, 1985) pp. 1–190. 23. O. Ya. Samoilov, The Structure of Aqueous Solutions of Electrolytes and the Hydration of Ions (Publishing House of the Academy of Sciences of the USSR, Moscow, 1957) pp. 1–182. 24. E. D. Shchukin, M. V. Dukarevich, S. I. Kontorovich, and P. A. Rehbinder, “On the Adsorptive Reduction in the Strength of Highly Dispersed Porous Structures,” Dokl. Akad. Nauk SSSR, 167, 1109–1112 (1966). 25. E. D. Shchukin, A. V. Pertsov, and E. A. Amelina, Colloid chemistry (Higher School, Moscow, 2007) pp. 1–444. 26. T. W. Sisson and T. L. Grove, “Experimental Investigations of the Role of H2O in Calc-Alkaline Differentiation and Subduction Zone Magmatism,” Contrib. Mineral. Petrol., 113, 143–146 (1993). 27. Z. N. Skvortsova, “Deformation by the Dissolution– Reprecipitation Mechanism as a Form of the Adsorptive Plasticizing of Natural Salts,” Kolloid. Zh., 66 (1), 5–15 (2004). 28. Z. N. Skvortsova, “The Regular Patterns and the Mechanisms of Influence of Liquids on the Strength and Plasticity of Ionic Crystals,” Doctoral Dissertation in Chemistry (MGU, Moscow, 2005). pp. 1–254. 29. Z. N. Skvortsova, I. V. Kas’yanova, A. E. Muralev, E. V. orodenko, and V. Yu. Traskin, “Recrystallization Creep of Sodium Chloride in the Solutions of Different Composition. 2. Influence of the Additives of Urea,” Kolloid. Zh., 70 (5), 674–677 (2008). 30. Z. N. Skvortsova, I. V. Kas’yanova, E. V. Porodenko, and V. Yu. Traskin, “Recrystallization Creep of Sodium Chloride in the Solutions of Different Composition. 1. Effect of the Additions of Inorganic Salts,” Kolloid. Zh., 70 (5), 669–673 (2008). 31. C. S. Smith, “Grain, Phases and Interfaces: an Interpretation of Micro Structure,” Trans. Metall. Soc. AIME, 175 (1), 15–51 (1948). 32. V. Yu. Traskin, “The Interlayers of Liquid on the Grain Boundaries of Rocks and Model Materials,” in Physicochemical Mechanics and Lyophilic Property of Dispersed Systems Ed. by F. D. Ovcharenko (Naukova Dumka, Kiev, 1981) vol. 13, pp. 81–91. 33. V. Yu. Traskin, M. Z. Abdrakhimov, and Z. N. Skvortsova, “Manifestation of the Rehbinder Effect under to the Conditions for Ultradeep Drilling,” Kolloid. Zh., 32 (1), 23–25 (1998). 34. V. Traskine and J. Barralis, “Liquid Metal Induced Degradation of Alloy 7010,” Proceedings of V International Congress on Aluminum Alloys (London–Paris–New York, 1996) pp. 237–240. 35. V. Yu. Traskin, A. S. Bedarev, Z. N. Skvortsova, L. G. Arutyunyan, L. S. Bryukhanova, and N. V. Pertsov, “The Intercrystalline Destruction of the Polycrystals of Alkaline Halides with the Liquid Intergranular Layers,” Dokl. Akad. Nauk UkSSR (Ukrainian SSR), Ser. B, No. 11, 48–51 (1986). 36. V. Traskine, P. Protsenko, Z. Skvortsova, and P. Volovitch, “Grain Boundary Wetting in Polycrystals: Wettabil-
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