Clay Minerals (1977) 12~ 281.

CLAY MINERAL FORMATION UNDER LATERITIC WEATHERING CONDITIONS HERMANN

HARDER

Sedimentpetrographisches lnstitut der Universit6t G6ttingen, Goldschmidtstrasse 1, D-3400 GOttingen, Germany

(Received 12 February 1977)

A B S T R A C T : Amorphous hydroxides of AI, Fe, Mn, Mg, Zn, Co, Ni, etc., are capable of coprecipitating SiO2 even from very dilute (weathering) solutions. Silica minerals form only in those precipitates from solutions undersaturated with respect to amorphous silica (100 ppm SiOz at 20~ With higher SiOz concentrations the precipitates remain amorphous. The size of the cations should allow 6-fold coordination and giving a brucite-like layer. Most suitably sized octahedral ions are Mg, Zn, Ni, Co and Fe z + (size 0.78-0.82/k), and chemically pure three- and two-layer clay minerals with these ions are easily synthesized. With a relatively high content of silica in solution (60 ppm SiO2 with 1-2"5 ppm metal) several smectite minerals could be synthesized. With a low content of silica in solution (5 20 ppm SiOz and ca. 2 ppm metal) the serpentine minerals, could be synthesized. It is possible to crystallize the difficult to form Al-clay minerals in a solid solution with these more easily synthesized clay minerals. Clay minerals with the heavy metal ions of Ni, Co, Zn, and with Cu, Cr, etc., can be found in the weathering zone of Gossan and in the lateritic weathering crust of ultrabasic rocks. INTRODUCTION Clay mineral formation under lateritic weathering conditions is an important geological process. However, the reactions leading to clay mineral formation under surface conditions are not fully understood. The synthesis of clay minerals at elevated temperatures has been studied in m a n y papers. R o y & Osborn (1954) and m a n y others have synthesized clay minerals from mixtures of oxides and hydroxides at elevated temperatures and pressures. Smectite-, illite- and kaolinite-minerals were obtained as the reaction products at temperatures between 250 and 500~ But investigations have shown that clay mineral formation takes place mainly during chemical weathering at surface conditions. Only a few investigations have been made on the synthesis of the clay minerals at lower temperatures and pressures. Stresse & H o f m a n n (1941), Caill~re & Henin (1948), Siffert & Wey (1962), Wollast et al. (1968), lglesia & Martin-Vivaldi (1975) and Harder (1965) reported the synthesis of sepiolite and smectite minerals from magnesium silicate gels under alkaline p H conditions at low temperatures. Clay mineral formation takes place in very dilute solutions. The silica content o f solutions in which clay minerals are known to form can be lower than would be predicted f r o m the stability field o f the clay minerals. The question is, how do silicate minerals form from these apparently undersaturated Si-solutions ? The experiments reported here show that clay mineral formation under weathering conditions is in part a question o f how silica precipitates from natural solutions. 281

H. Harder

282 SILICA CONTENT

IN WEATHERING

SOLUTIONS

Amorphous hydroxides are capable of coprecipitating silica by chemisorption even from solutions as dilute as weathering solutions. Hydroxides were precipitated by increasing the pH from solutions containing Si and one or more of the cations A1, Fe, Mn, Zn, Co, Ni, Cu, etc. After 1 day the solution was filtered. The Si-content in the solutions and in the precipitates were then analysed. As Table 1 indicates, all hydroxides contain approximately the same distribution proportion of silica relative to the solution in hydroxide precipitates. The SiO2 distribution depends on several factors. The most important ones are: (1) the ratio of silica to hydroxide in the initial solution; (2) the temperature: more silica is found on hydroxide precipitates at lower temperatures, (3) sorption of silica is much less on crystallized substances. TABLE 1. SiO2-sorption in hydroxide precipitates. Concentration in solution before precipitation: 3 mg metal oxide/l, 3 ppm SiO2, pH mostly 7, 22~ Filtration after 1 day after precipitation. SiO2-content after precipitation Substance

In solution: mg SiO2/1 (ppm)

In precipitate: weight (~)

Co Ni Be La Cu Zn Y Cr V Pb Th Sn Zr Fe 3+ Mn4+

1.7 2.3 1.6 2.2 1.6 1.8 1.6 2"0 2.6 2-0 2"1 2.0 1.6 1.6 1.6

39 30 40 32 40 38 40 35 25 35 33 35 40 35 35

Under natural conditions climatic factors will have a very strong influence on the silica content of waters. In the rainy season the silica contents of ground waters are very low. In the dry season the silica content of ground waters increase. In desert areas closed drainage systems can lead to very high concentrations of silica in the ground water which are too high for a silica mineral formation as shown later. Under lateritic weathering conditions the most important factor leading to clay mineral formation is silica absorption on hydroxides. It has been shown that even from very undersaturated solutions (concentrations down to 0.3 ppm SiO2) silica can be adsorbed by hydroxides of aluminum, iron, manganese, nickel and other elements. The SiO2 concentration of such solutions is comparable to those of natural waters. By this

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adsorption process silica is enriched in the precipitates compared with the corresponding solutions. Harder (1965) suggested that during diagenesis clay minerals and quartz could be formed from such precipitates. The experiments on the synthesis of clay minerals were carried out in the following way. Hydroxides were precipitated from solutions containing silica and some other ions and a range of different cations. These precipitates which contained Si were aged at low temperatures. After ageing for from 2 weeks to 3 months the precipitates were studied by X-ray diffraction to see if a clay mineral has formed. CONDITIONS

FOR

FORMATION MINERALS

IN SYNTHETIC

CLAY

Clay minerals formed from some (but not all) hydroxide-silica precipitates, after a few days ageing at low temperatures. Certain conditions seem to be very favourable (see Table 2) for clay mineral formation, others not. The most important factor for silica mineral formation is the silica concentration in the solution which should not be too high or too low. Experimental conditions such as the concentration, the p H and temperature can be changed to some extent. But the initial silica concentration in the solution should be > 5 p p m SIO2, and the Si/metal ratio in the initial solution should be kept constant. The adsorption of silica on to the hydroxides leads to silica concentration TABLE2. Favourable conditions for clay mineral formation at surface temperatures (1) Low silica content in solution: Monomeric Si-acid, lower than 100 ppm, higher than 5 ppm. Polymerization inhibits formation of clay minerals. (2) Slow precipitation: (a) by changing the pH, (b) by decomposing complex-ions ; (c) by oxidation-processes. (3) Composition of precipitates: Should be similar to the composition of clay minerals. (4) Size of cations: Should be suitable for installation in 6-fold coordination and build up a brucite-like layer (see Table 3).

within the pore solutions of the precipitates which are necessary for clay mineral formation. The silica concentrations in the pore waters of the amorphous hydroxide precipitates are more important for the mineral formation than the silica concentration in the whole solution. Unfortunately it is not possible to give values of the concentration in these micro environments in the pore waters of the hydroxides. 10-60 p p m SiO2 seems to be the optimum concentration range for clay mineral formation. Lower silica content in solutions makes clay mineral formation impossible but allows relatively rapid formation of quartz or feldspar at low temperatures (Harder & Fleming, 1970). Much higher SiO2-concentrations in solution inhibit silica mineral formation. Silica mineral formation takes place in those precipitates from solutions undersaturated with respect to amorphous silica (100 p p m SiO2 at 20~ If the SiO2-concentrations in the solutions are

284

H. Harder

higher, the precipitates stay amorphous eved after long ageing times. It seems that polymerization of the silicic acid inhibits the process of silica mineral formation. This experimental result agrees with observations in nature in Australia. Many recent SiO 2rich and X-ray amorphous products are (or probably were) in contact with SiO2-rich ground waters. For instance some ground waters in the Andamooka Opal Fields, South Australia, have 80 ppm. Allophane, opal containing chert, silificed wood, chrysoprase and different kinds of opal are the signs that during the formation of these minerals the SiO 2 content was too high for quartz or clay mineral formation. A further important condition for the synthesis of clay minerals at room temperatures is a sufficiently slow rate of precipitation. The precipitation can be the result of pH changing, decomposition of complex-ions or of oxidation processes. The composition of the hydroxide-silica-precipitates should be similar to the composition of the analysed minerals. TABLE 3. Influence o f ionic radii on the clay mineral formation at surface temperatures from chemically pure solutions

Effective radii of I o n in 6-fold co-ordination

Mg 2 + Ni 2 + Co 2 + Zn 2 + Fe 2 + Fe 3+ Cu 2 + V 3+ Cr 3 + M n 2+ M n 3+ A13+

(A)

Synthesis of clay minerals

0"78 0.78 0-82 0"83 0-82 0"67 0.70 0'65 0'64 0.91 0-70 0"57

Possible Possible Possible Possible Possible N o t possible under oxidation condition Difficult in chemically pure solutions Difficult in chemically pure solutions Difficult in chemically pure solutions Probably too great for clay minerals Difficult N o t possible in chemically pure solutions at low temperatures

The size of the cations (see Table 3) is a further factor which is very important for clay mineral formation at low temperatures. The size of the cations should be suitable for 6-fold coordination and build up of a brucite-like layer. There was some difficulty with the Al-clay mineral synthesis under surface conditions. Probably the small size of the Al-ion (0.57 A) makes installation of A1 in the octahedral positions difficult. More suitably sized for installation in the octahedral positions are the ions of Mg, Zn, Ni, Co and Fe 2+ (between 0.78 and 0.82 A). The synthesis of chemically pure three- and twolayer clay minerals with these ions is relatively easy (Table 4), with a relatively high content of silica in solution (60 ppm SiO2 with 1-2-5 ppm metal; or with a similar Si/metal ratio). Several smectite minerals could be synthesized: Ni- (pimelite) and Co-smectite, Fe III smectite, sauconite (Zn-smectite). With a lower concentration of silica in solution (5-20 ppm SiO2 and c. 2 ppm metal; or with a lower Si/metal ratio) the serpentine minerals Co- and Ni-serpentine (garnierite), chamosite, etc., could be synthesized.

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285

TABLE4. Conditions of low temperature clay mineral synthesis (20~ Smectite Element Co Ni Zn Fe Mg

mostly pH 8)

Serpentine

SiO2 (ppm)

Metal (ppm)

SiO2 (ppm)

Metal (ppm)

Remarks

60 60 30 20 20

1 1 2"5 7.5 20

15 15 5 20

2.5 2"5 20 20

+0-1~ dithionite

pH 10

It is possible to produce crystallization of Al-clay minerals (see Tables 5 and 6), which are difficult to f o r m by direct precipitation o f Al-hydroxide, in a solid solution with these more easily synthesized clay minerals. F o r instance the synthesis o f Al-rich three-layer silicate minerals is possible with the help of magnesium. The presence of hectorite (Mg-smectite) or nontronite in the Al-rich three-layer minerals makes a rapid low temperature synthesis possible. The p H is an important factor in the precipitation and co-precipitation of magnesium. The synthesis o f hectorite is only possible when the precipitates contain at least 6~o MgO. The precipitates stay a m o r p h o u s if the magnesium content is lower. U n d e r alkaline conditions (pH 10) 10 p p m Mg but at p H 8, t250 p p m Mg (sea water concentration) are necessary in the solution to obtain the required 6% MgO in the precipitate. Other not easily formed clay minerals with Mn, Cu, Cr and Fe 3 § can be formed in a similar way with the help of the ions o f the more easily synthesized clay minerals in a solid solution with the m o r e difficultly formed clay minerals. The smectite mineral TABLE5. Solid solution in synthetic smectite-minerals at low temperatures (20~ (a) Dioctahedral Montmorillonite Montmorillonite Nontronite (hisingerite) Wolchonskoite

A1-Mg A1-Fe2§ Fea +-AI-Fe 2+ Cr-A1-Fe3§

(b) Trioctahedral Saponite Minnesotaite Stevansite Hectorite Sauconite Sauconite Nickel smectite (pimelite) Cobalt smectite Ferri smectite Medmontite

Mg-Fe 2+ Fe z +-Mg-(Fe 3+-A1-Si) Mg-M n Mg-Li Zn-Mg Zn-AI Ni-Mg Co-Mg Mg-Fe 3+ Cu-Mg-A1

H. Harder

286

TABLE6. Solid solution in synthetic serpentine minerals at low temperatures (20~ (a) Dioctahedral Ferri-chamosite

Fe a+-Fez+-Mg AI-Fea+ (Fez+ in solution) Not possible in pure inorganic solution?

Kaolinite (b) Trioctahedral Serpentine Greenalite Grovesite Chamosite Cronstedtite Mangan serpentine Nickelserpentine (garnierite) Maufite Cobalt serpentine Chromium amesite

Mg-Fe2+ Fe 2+-Fe3§ Fe z+-Mn-AI Fe3+-Fe2+-Mgz§ Fe2+-Fe3+-(Fe3+-Si) Mn-Mg Ni-Mg Mg-Fez+-Ni-A1 Co-Mg Cr-Mg

medmontite (Cu-Mg-A1) and wolchonskoite (Cr-A1-Fe3+-Mg) and the serpentine minerals grovesite (Fe2+,Mn-A1), Mn-serpentine, Cr-amesite (Cr-Mg) have been synthesized. The catalytic formation of Al-rich clay minerals can be carried out with the help of inorganic or organic compounds as has been done in the case of kaolinite syntheses. CLAY MINERAL

FORMATION

IN NATURE

The experimental conditions reported here are reflected very well by the actual conditions under which clay minerals are formed in nature. Clay minerals with the heavy metal ions of Ni, Co, Zn, and with Cu, Cr, etc., can be found in the weathering zone of Gossan and in the lateritic weathering crust of ultrabasic rocks which have relatively high contents of heavy metals in the parent rock. Such clay minerals are quite common in some weathered Australian ore deposits. For instance: Ni-deposits in New Caledonia, in the environment of Rockhampton, in Kalgoorlie, New Caledonia, etc. Facies conditions control the new formation of the different clay minerals. The experiments have shown that the synthesis of clay minerals is dependent on the SiO2 and metal content and the pH of the solution. This is in agreement with observations in nature. Smectite has been formed by the weathering of basic or acidic igneous rocks and also in sedimentary and metamorphic rocks. From basic rocks with a high content of magnesium, smectite mineral formation is possible under alkaline or neutral pH conditions. From acidic magmatic rocks with a low content of magnesium, smectite mineral formation is only possible under alkaline pH conditions which are produced by the K or Na-content of freshly weathered feldspar. Iron can favour the new formation of Al-rich three layer minerals only under reducing conditions. In lateritic weathering profiles these reducing conditions are sometimes present. During the run of a year in a tropical climate the conditions in the potentially clay mineral forming environments may change drastically. During the wet seasons the silica contents are mostly very low, and clay minerals cannot be formed. During the dry season the clay mineral formation is

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287

possible below water table level. It seems that three-layer clay mineral formation in nature occurs in inorganic basic solutions, and kaolinite formation in weakly acidic solutions aided by the presence of organic compounds. ACKNOWLEDGMENTS

For help in the laboratory I would like to thank Mrs Cierny. I would like to thank Deutsche Forschungsgemeinschaft, Schwerpunkt Tonmineralogie, for financial support for this study. REFERENCES CAILLERE S. & HENIN M.S. (1948) C.r. Acad. Sci Paris. 226, 580. DE CHETELAT E. (1947) Bull. Soc. Gdol. France (5), 17, No. 1-3, p. 105. CHtJKROV F.V. (1975) Fortschr. Miner. 52, Spec. Issue: IMA-Papers 9th Meeting Berlin, Regensburg 1974, p. 345. GARRELS R.M. & CHRIST C.L. (1965) Solutions, Minerals, Equilibria. Harper & Row, New York. GINSBURGI.I. & RUKAVISHNIKOVAI.A. (1951) Mineralien dee alten Verwitterungsrinde des Urals. Moskau. GROSS V. (1970) Diplomarbeit G6ttingen. HARDER H. (1965) Geochim. cosmochim. Acta 29, 429. HARDER H. (1972) Chem. GeoL 10, 31. HARDER H. & FLEMING W. (1970) Geochim cosmochim. Acta 34, 295. Lg IGLESIAA. ~7, MARTIN-VIVALDIJ.L. (1975) Clay Miner. 10, 399. JUBELT R. (1962) Geophys. Geol. 4, 3. RoY D.M. & MUMPTON F.A. (1956) Econ. Geol. 51,432. RoY D.M. & RoY R. (1954) Am. Miner. 39, 957. Roy R. & OSBORN E.F. (1954) Am. Miner. 39, 853. SIFEERT B. & WEY, R. (1962) Proe. Int. Clay Conf. I, 159. SMOL'YANINOVAN.N., MOLEVAV.A. & ORGANOVAN.I. (1960) Acad. Sci. U.R.S.S., Comm. for Study of Clays, Repts. to Meeting of Internatl. Comm. for Study of Clays, p. 45 [in Russian]. SPANGENBERGK. t~r MULLER M. (1949) Heidel. Beitr. 1, 560. STRESSE H. • HOFMANN U. (1941) Z. anorg, allg. Chem. 247, 65. TILLER K.G. (1968) 9th Intern. Congr. Soil Sci. Trans. 2, 567. TILLER K.G. & PICKERING J.G. (1974) Clays Clay Miner. 22, 409. DE VLETTER R.D. (1955) Engng Mining J. 156, 84. WERNER C.W. (1965) Ber. geol. Ges. D D R Band 10, Heft 5, p. 567. WEY R., SIFFERT B. & WOLFF A. (1968) Bull. Grpefr. Argiles 20, 79. WOLLASTR., MACKENZIE F.T. & BRICKER O.P. (1968) Am. Miner. 539 1645. R 1~S U M 1~: Les hydroxides amorphes de A1, Fe, Mn, Mg, Zn, Co, Ni, etc., sont capables de copr6cipiter du SiO2 mSme ~t partis de solutions alt6r6es tr6s dilu6es. Les min6raux siliceux se forment seulement dans les pr6cipit6s de solutions sous-satur6es en ce qui concerne la silice amorphe (100 ppm SiOz/~ 20~ Avec de plus fortes concentrations de SiO2, les pr6cipit6s restent amorphes. Les dimensions des cations devraient permettre une coordination sextuple dormant une couche semblable gt la brucite. Les dimensions des ions octa6driques de Mg, Zn, Ni, Co et Fe 2§ (dimensions 0.78-0"82 A) conviennent le mieux et les min6raux chimiquement puts/l trois et b. deux couches d'argile avec ces ions sont ais6ment synth6tis6s. Avec une teneur relativement forte de silice en solution (60 ppm SiO2 avec avec 1-2.5 ppm de m6tal) plusieurs min6raux de smectite pourraient 0.tre synth6tis6s. Avec une faible teneur en silice en solution (5-20 ppm SiO2 et ca. 2 ppm de m6tal), les min6raux serpentins pourraient ~tre synth6tis6s. I1 est possible de cristalliser les difficiles pour former des min6raux d'argile AI dans une solution solide avec les min6raux d'argile plus facilement synth6tis6s. L'on peut trouver les min6raux d'argile ~. ions m6talliques lourds de Ni, Co, Zn, et avec Cu, Cr, etc., dans la zone alt6r6e de Gossan et dans la crofite alt6r6e lat6ritique de roches ultrabasiques.

288

H. Harder K U R Z R E F E R A T : Die Synthese von Tonmineralen ist auch bei niedrigen Temperaturen

in kurzer Zeit m6glich, wenn einige wichtige Bedingungen beachtet werden. Nur aus verdiinnten monomeren Kiesels~iurel6sungen ist ein Aufbau von Silikatmineralen m6glich. Hohe polymere Kieselsaurekonzentrationen verhindern einen Bildung von Tonmineralen. Die Gr6sse der Kationen begiinstigt den Aufbau einer Oktaederschicht oder erschwert die Synthese von Tonmineralen. Die Ionengr6sse des Mg, Zn, Ni, Co und Fe 2+ (zwischen 0.78 bis 0'82 A) begiinstigt die Synthese yon Zwei-bzw. Dreischichttonmineralen mit diesen Kationen in den Oktaederpositionen. Wahrscheinlich ist das Ai-lon (0'57 A) f'tir einen energetisch begiinstigten Aufbau der Oktaederschicht zu klein und erschwert so die Synthese reiner A1-Tonminerale bei niedrigen Temperaturen. Als Mischkristalle yon leicht zu synthetisierenden Tonmineralen mit Kationen von schwer zu synthetisierenden Tonmineralen ist jedoch auch der Aufbau von aluminiumreichen Tonmineralen bei niedrigen Temperaturen leicht m6glich. R E S U M E N : Los hidr6xidos amorfos de AI, Fe, Mn, Mg, Zn, Co, Ni, etc., son capaces de coprecipitar SiO2 hasta partiendo de soluciones de intemperizaci6n diluidas. $61o se forman minerales de silice en los precipitados de soluciones no saturadas respecto a la sflice amorfa (100 ppm SiO2 a 20~ Con concentraciones m~is altas los precipitados permaneeen amorfos. E1 tamafio de los cationes deberia permitir la coordinaci6n s6xtuple que produce una capa semejante a la brucita. Los de tamafio mb.s adecuado en los iones octa6dricos son Mg, Zn, Ni, Co y Fe 2 + (tamafio 0"78-0.82 A), y los minerales de arcilla quimicamente puros de tres y de dos capas que tienen estos iones se sintetizan f~icilmente. Con un contenido relativamente elevado de sIlice en soluci6n (60 ppm SiO2 con 1-2-5 ppm de metal) podrtan sintetizarse varios minerales esmectiticos. Con un bajo contenido de silice en soluci6n (5-20 ppm SiOz y 2 ppm de metal) podrian sintetizarse los minerales de serpentina. Con estos minerales de arcilla que se sintetizan m~.s f~icilmente es posible cristalizar en una soluci6n s61ida los minerales de Al-arcilla mils dificiles de formar. Pueden hallarse minerales de arcilla con los iones met~ilicos pesados de Ni, Co, Z n y con Cu, Cr, etc., en la zona de intemperizaci6n de Gossan y en la corteza lateritica de intemperizaci6n de las rocas ultrabgtsicas.