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GEOCHEMISTRY OF THE .EARTH'S SURFACE AND OF ìv-IINERAL FORbiATION 133 2nd INTERNATIONAL SYMPOSIUM, July, 2-8, 1990, Aix en Provence, .

I DISTRIBUTION OF HYDRATED AND DEHYDRATED MINERALS IN LATERITIC PROFILES AND LANDSCAPES

l TARDY Y. *, TROLARD F. **, ROQUIN C *** and NOVIKOFF A. * * Institut Français de Recherche Scientifiquepour le Développement en Coopération, (ORSTOM), B.P. 2528 Bamako, Mali. ** Université Catholique de Louvain (UCL) - Laboratoires de Géologie générale,Unité de Géochimie, 3, place Louis Pasteur, B 1348 Louvain-la-Neuve, Belgique. ** "Centre de Géochimie de la Surface (CNRS), 1, rue Blessig, 67084 Strasbourg, France.

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Lateritic profiles and particularly , 1976; Tardy and Nahon, 1985), the optimal ferricretes and kaolinitic mantles, show successions induration and accumulation of correspond to of horizons more or less indurated and sequences of the zone of nodule formation, and to the maximum minerals which are in different hydration states. In value of the ratio /goethite. The secondary t the unsaturated zone located above the ground water goethite which appears at the top of the profiles table, the less hydrated phases are interstratified corresponds to a rehydration process which is between two horizons containing more hydrated thought to be responsible for the destruction of the minerals. This general distribution is explained by origiñal nodules and for femcrete dismantling seasonal fluctuations of the thermodynamic activity (Nahon, 1976). of water within the profiles. Such fluctuations also According to hydration reactions, goethite, explain how duricrusts such as fenicretes for and gibbsite are hydrated minerals while example, are formed, conserved or destroyed (Tardy hematite, , boehmite and kaolinite are and Novikoff, 1988, Tardy et al., 1988). dehydrated minerals. Hydrated minerals appear at In lateritic profiles several sequences of the bottoms of profiles, close to the ground water hydrated-dehydrated-hydrated minerals have been table, or downslope where the unsaturated layer is observed : gibbsite - kaolinite - gibbsite, in oxisols; reduced. They also appear again close to the surface goethite - hematite - goethite, in femcretes; gibbsite - of thick profiles, at the edges of large channels, boehmite - gibbsite in lateritic bauxites or even : temporarily hydrated during the wet season. kaolinite - (boehmite) - gibbsite in karst Dehydrated minerals appear preferentially in the bauxites. These mineral sequences are not always intermediate part of the unsaturated zone (Tardy complete, however the maxima of boehmite, et al., 1988). They form , almost always diaspore and sometimes corundum, always accompany induration, and contribute to duricrust- correspond to the maxima of hematite and to the formation. optimal development of ooids and pisoids (Bardossy, 1982). A similar process occurs in Chemical composition and climatic manganese duricrusts in which the sequence distribution of goethite and hematite manganite pyrolusite - cryptomelane is observed (Weber et al., 1979; Nahon et al., 1984). In Schwertmann (1988) analyzed occurrence and femcretes in which the mechanism of hydration- formation of iron oxides in various

I dehydration-rehydration was demonstrated (Nahon, pedoenvironments. In profiles, the ratio . % I--

134 GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MIh'ERAL FORMATION 2nd INTERNATIONAL SYMPOSIUM, July, 2-8, 1990, Aix en Provence, France.

of hematite to hematite plus goethite (Hm/(Hm + with gibbsite in bauxites. Hematite generally has Go)) is controlled by temperature, humidity of the less substitution than goethite and both are less atmosphere, pH, organic matter content, altitude aluminous when kaolinite and quartz are present and latitude. Some of these factors such as altitude than when gibbsite forms. (elevation) or latitude include the effect of Obviously, the aluminum contents of goethites and temperature and moisture. Furthermore, at a regional are controlled by the activity of aluminum scale, some parameters such as soil pH and organic in solution which in turn is determined by the matter content are closely related to rainfall. of aluminous minerals, that is gibbsite and Therefore, it is difficult to separate their kaolinite. It turns out that the relative stability fields specific influence. Soil temperature is regulated by of goethite and hematite may depend on their degree climatic factors such as latitude, altitude and of Al-substitution which are dependent on the continentality. The relative humidity of the soil stability of the other aluminous minerals associated atmosphere, that is, the thermodynamic activity of with them. The conditions of formation of goethite water, is controlled by rainfall, evaporation, and hematite may be ultimately determined by the permeability, pore size, and depth of ground water activity of silica in solution and by the activity of table. water in pores in which these minerals form. If pH and organic matter are typically kinetic parameters, temperature and activity of water are Thermodynamic stability field of both thermodynamic factors which have to be taken Al-goethite and Al-hematite into account as well. In fact, the mass action law applied to the equilibrium hematite-goethite, yields : Stability fields of Al-goethite and Al-hematite, gibbsite, boehmite and kaolinite can be considered FeO1.5 (hematite) + 0.5 H2O (water) = FeOOH to be a function of particle size, temperature and the (goethite) thermodynamic activity of water (Trolard, 1988; Trolard and Tardy, 1987). K(T) = [FeOOH] / [Feol.~][H20]o.5 For a given temperature, T, and according to the equilibrium conditions expressed earlier, stability The constant of the mass action law (K(T)) is a diagrams are constructed as a function of the function of temperature and the equilibrium aluminum content and water activity. When conditions are chiefly dependent on the activity of temperature decreases, several modifications are water (aw = [HzO]). However hematite-goethite observed. The stability field of the association of equilibria are also regulated by the activity of the two Al-hematite and boehmite is reduced. The stability solid phases : [FeOOK] for goethite and [FeC$5] for field of the association of the Al-goethite and hematite. Solid phase activities are determined by gibbsite becomes larger. For a given composition three conditions which have to be taken into and a given water activity, the Al-substitution in consideration separately : purity, chemical compo- Al-goethite associated with gibbsite decreases. sition (aluminum content) and grain size. When temperature increases the contrary is observed. The stability field of Al-goethite is Distribution of Al-hematite and Al-goethite reduced, gibbsite disappears near 40°C and is in replaced by boehmite; the stability field of Al-hematite is extended; at about 75OC,Al-goethite The aluminum contents of goethite and disappears. hematite should be regarded as a function of the For a constant water activity ([HzO] = 1, for nature of the other associated minerals. In goethites example) and for a given composition (Al2O3/ of red , the substitution rates diminish when the (Fe203 + Al2O3)) the degree of substitution of climate becomes less humid or more arid and diaspore in Al-goethite, when Al-goethite is percolating solutions are richer in silica (Tardy, associated with gibbsite, increases with temperature. 1971). This is probably due to the presence of The degree of substitution of corundum in kaolinite which controls the chemical composition of Al-hematite also increases with temperature. ;the solution (Trolard and Tardy, 1989). An increase in temperature and a decrease in water Compositions range from 2 to 20 mole percent activity, have qualitatively similar effects on the of AlO(0H) in goethites of femcretes and other soils stability fields. This is because the entropy and the in which kaolinite is present. Compositions range heat capacity of water in gibbsite (A1203.3 HzO), from 18 to 27 mole percent in goethites associated goethite (FqO3.H2O) and boehmite (A1203.H20) GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORhIATION 135 2nd IKTERNATIONAL SYbIPOSIUM, July, 2-8, 1990, Aix en Provence, France. - as are smaller th.an those characterizing liquid water. their aluminum content depends neither on water ss activity nor on temperature. The Al-goethite and :nt CONCLUSIONS Al-hematite compositions depend then only on the ratio of (A12@/(A1203 + Fe2O3)) in the system. id It is clear that the chemical potential of water (vi) The stability field of baolinite in the piesence of m plays a major role in the transformation of gibbsite gibbsite or boehmite is dependent both on activity of ie into boehmite, for the transformation of boehmite water and activity of silica. At 25OC and 100 kPa id into corundum at low temperature (Bardossy, 1982) total ressure, if the silica activity is lower than dS and for the transformation of goethite into hematite 1O-4.R, kaolinite is not stable whatever the water :e (Tardy and Nahon, 1985). activit is. In the dissolved silica activity interval le Equilibrium diagrams show the conditions of Y7 to 10-4.501 gibbsite, then kaolinite and :d formation of Al-goethites,,Al-hematites,gibbsite and finally boehmite can be stabilized, successively, :e boehmite in latentes, bauxites and ferricretes. The when the water activity decreases. In the interval ie stability fields of the various associations are [10-4.50 to 10-4.01 only kaolinite and boehmite are ,f presented in terms of the thermodynamic activity of stable. For a decreasing water activity, the stability water, temperature and particle size. The diagrams field of boehmite progressively overlaps the allow for substitutions of Al for Fe in goethite and kaolinite domain (Trolard and Tardy, 1989). hematite and we have regarded these solid solutions (vii) The presence of kaolinite instead of gibbsite as ideal. and boehmite in soils or weathering profiles should induce changes in the Al-content of aluminous '7 In summary, the stability diagrams based on activity goethite, as the equilibrium with kaolinite decreases d of water and temperature permit the following when water activity or silica activity increases. This e conclusions. may explain why, in general, goethites are poorer in i; (i) Dehydration transformations of the type : aluminum downslope, in lowlands and in (goethite --->hematite), (gibbsite ---> boehmite ---> hydromorphic soils compared to ultisols or oxisols e I corundum), (gibbsite --->kaolinite), can be induced located at the top of profiles or in high landscape l by a decrease in the thermodynamic activity of water situations where the aqueous pore solutions are (at constant T), by an increase in temperature (at depleted in silica. i constant aw>or by a, decreasing and T increasing (viii) The iron content in kaolinite is shown to be l simultaneously. dependent on the water activity and on dissolved i (ii) A decrease in water activity has a similar effect to silica activity. An increase of water activity at a fixed an increase in temperature. The successions of silica activity, or an increase of silica activity at a i mineral phases and parageneses are similar in both fixed water activity should induce an increase of iron cases : Al-goethite ---> Al-goethite + Al-hematite relative to aluminum in the ferruginous kaolinite. 1 ---> Al-hematite, for systems of low aluminum Clearly the chemical composition of goethite, i content and, Al-goethite + gibbsite ---> Al-goethite hematite and kaolinite are controlled by equilibrium i + boehmite ---> Al-hematite ---> (Fe-corundum), with other phases present in the system, by silica for systems of high aluminum content. activity regulated by the rate of percolating waters, (iii) When minerals are in association such as by temperature and by the activity of water which is !l (Al-goethite + gibbsite) or (Al-goethite f a function of climatic and regional drainage Al-hematite), the Al-goethite or Al-hematite conditions. compositions do not depend on the composition of the system but only on water activity or temperature. ACKNOWLEDGEMENT i The aluminum contents increase as water activity decreases or as temperature increases. The present study was carried out as part of the ATP i (iv) When Al-goethite and boehmite are stable "Latérites" (CNRS), of the Project of the Programme together, the Al-goethite composition does not PIRAT (INSU-ORSTOM) and in the general framework of l depend on the composition of the system nor on EUROLAT (European Network of Laterite Study). temperature and water activity. It is remarkable that, 1 in this case, the aluminum content of goethite is REFERENCES i almost fixed (for example, (Al0.25 Feo.755)OOH at 25OC). The same considerations are valid for the BARDOSSY G., 1982. Karst bauxites. deposits on association of hematite and corundum. carbonate rocks. Elsevier, Scientific Publishing CO., I (v) When only Al-goethite or Al-hematite are stable, Amsterdam-Oxford-New York, Developments in Economy 136 GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION 2nd INTERNATIONAL SYMPOSIUM, July, 2-8, 1990, Aix en Provence, France.

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