Structural Characteristics of Hematite and Goethite and Their Relationships with Kaolinite in a Laterite from Cameroon

Bull. Minéral. (1988). 111, 149-166

Structural characteristics of hematite and goethite and their relationships with kaolinite in a laterite from Cameroon. A TEM study.

by MICHELINEBOUDEULLE? and JEAN-PIERREMULLER** * Laboratoire de Minéraiogie-Cristallographie, UA C.N.R.S. 805. Université Claude Bemard - Lyon 1 43, Boulevard du II Novembre 1918,69622 Villeurbanne Cedex, France: ** ORSTOM, UR Pétrologie de la Surface and Laboratoire de Minéralogie-Cristallographie.UA C.N.R.S. 09,

Universités Paris VI et VII. 4, place Jussieu, 75252 Paris Cedex 05, France. ,

Abstract. - TEM investigations on goethite and hematite associated with kaolinite in lateritic weathering profiles (Cameroon) have shown : (I) the variability of goethite habit, in contrast to hematite, according to sampling locality, (2) the occurrence of intermediate phases, primary or resulting from a topotactic transformation of hematite into goethite, (3) intergrowths of hematite and goethite, and (4) epitaxy of goethite upon kaolinite. These data are discussed in terms of growth and genetic relationships between minerals (mineral development and relative stability). Petrological implications are considered. Key-words : hematite, goethite. kaolinite. TEM, growth Caracrérisation structurule des liémurires er goerliiies dune kurérite (Cumeroun) et érude de leurs relurions uvec lu kaolinire. (Microscopie e: dijjiiucrion élecrroniques). Résumé. - Une étude en microscopie et diffraction électroniques a été réalisée sur des échantillons de goethite et d'hématite, associées aux kaolinites, prélevés le long de profils d'altération latéritique (Cameroun). La morpholo- gie de la goethite varie suivant le site de développement, au contraire de celle de l'hématite. Des phases intermédiaires entre ces deux minéraux, soit natives, soit résultant d'une transformation topotactique hématite- goethite ont été identifiées. Des surcroissances de goethite sur hématite et I'épitaxie de la goethite sur la kaolinite ont été observées. Ces résultats sont discutés en termes de mode de développement. de stabilité et de relations génétiques entre les minéraux. Des implications pétrologiques en sont tirées. Mots-clés : hématite, goethite, kaolinite, microscopie et diffraction électroniques, croissance

I. INTRODUCTION However, at the same time, independent data on the crystallinity (and/or particle sizes) Iron oxy-hydroxides, hematite and goethite, (Kühne1 er al., 1975) and Al/Fe substitutions in are with kaolinite the main mineral components natural samples. obtained by X-Ray diffracto- of laterite. A better understanding of lateritisa- metry (Schulze and Schwertmann, 1984 ; tion processes implies a reconstitution of secon- Schwertmann and Latham, 1986), I.R. (Cam- dary mineral nucleation and growth conditions bier, 1986), ESR (Pinnavaia, 1981) and Möss- and the determination of their stability fields. bauer (Coey. 1980) spectroscopies, emphasized the variability of their structural characters. Various morphologies, texture and spatial ar- rangements of hematitic and goethitic materials These characters reflect the initial weathering have been described in numerous field and conditions, but depend also on the subsequent and continuing processes of dissolution and petrographic studies of metric profiles (see refe- crystallization which have affected the materials , rentes in Bocquier er al., 1984). Thermodyna- (Muller, 1987~). mic and kinetic modelling,-- together with labora- tory experiments, have attempted to afford Deciphering the part of each phenomenon im- ORSTOM Fonds Documentaire global valuable interpretations of the facts plies combined studies at different scales : (Schwertmann and Taylor, 1977 ; Tardy and Transmission electron microscropy (CTEM and No .: Nahon, 1985). HRTM) and diffraction, coupled with in siru f7 Cote : 8 @ Sofibé française de Miniralogie et de Cristallographie, Pans. 1988 Y

c * I 150 M. BOUDEULLE. J.-P. MULLER HEM.4TITE AND GOETHITE STRUCTURAL CHARACTERISTICS IN LATERITE 151 chemical analyses (EDS), allows morphological III. METHODS and crystal chemical characterizations of particles and aggregates at the micrometre scale The studied samples have been drilled from (Greenland er al., 1968 ; Felipe-Morales and optically selected materials. Pans were powde- Russel, 1972 : Jones er al.. 1982 Fordham er red by gentle grinding in agate mortar, in order al.. 1984 ; Amouric er al.. 1986). to record XRD patterns (Figure I B) using a Phi- lips PW 1730 vertical goniometer with a back The present paper reports results of investiga- monochromator and CuKa radiation. Drops of tions carried out on petrographically well defi- powder suspensions in methanol were air dried ned samples from Cameroon (Muller. 1987~).in on carbon coated grids. Other parts have been order to get more informations about hematite embedded in resin and thin-cut. to preserve the and goethite geneses and their interrelationships original spatial relations. with kaolinite. A standard Philips EM 300 electron microscope ( 100 kV) was used for low magnification. A JEOL. 1200 EX equipped with a TRACOR II. GENERAL LITHOLOGY EDS system. for high magnification and chemi- AND MATERIALS cal analyses. Results of HRTEM observations, made on a JEOL 200 CX, will be used briefly in The studied samples come from a soil topose- this paper. and published in details in a next quence in the Central East Cameroon (Muller, paper. 19876). In the upper zone of the toposequence. the vertical profiles exhibit. from the bottom to the top, three main zones (Figure IA) : IV. RESULTS (I) A lower weathering zone or saprolire. The weathering products preserve the original 1. Goethite texture and structure of the rock (gneiss). The mineralogical heterogeneity of the rock is evi- Goethite undergoes some destabilization un- denced on the centimetre scale through the alter- der extended exposition to the electron beam. nating whitish layers, with dominant kaolinites which likely modifies the crystal aspect. That and quartz, and iron-coloured red layers. This has to be taken into account when studying the horizon has a loose consistency. crystal surfaces. Goethitic materials present a global variability according to the sampling (2) An inrermediare nodular zone, in which place in lateritic profiles and local evolution in a two great types of indurated nodules are distin- very place. in terms of morphology, particle guished : (i) large and red lithorelictual ferrugi- size. crystal association and mineral interrela- nous nodules with a more or less preserved rock tionships. However. these characters lead to the structure and texture, and (¡i) small and red ar- distinction of three types of materials : gillomorphous nodules, with a soil texture, be- coming more abundant from the bottom to the a. From .velloti. arid white zones of rhe saprolire top of the zone. The internodular matrices have GoHe GoHe He Go the same general characters as the ferruginous When associated with fresh neofonned kaoli- matrices of the upper zone. nites and halloysites, goethite forms multido- Materials wilh inherited Materials with soil texture mainic almond or netting-needle like cystals gneiss texture (3) An upper loose clayey zone with a soil with irregular and pitted edges, non symmetrical small nodules texture, showing upwards a gradual transition terminations. and length up to the micrometre saprolite rEd clayey matrices from red (mainly hematitic) to yellow (goethitic) (Figure 2A). Single goethite particles (Figure 2C) 0 matrices, the red one often appearing as non- developed parallel to the (100) plane, and are large nodulis 0 yellow clayey matrices indurated nodules within the yellow one. elongated along c ([OOI]). as shown by selected area electron diffraction (SAD) patterns (Figures FIG. I. - [A] Sxnrheric derch map of 11 lareriric profile (Muller. 1987a). showing rhe sampling zones. [BI Sche- Seven great types of representative materials 2D and ZE). Normal SAD patterns. representing marir simplified represertrariort qf X-Ra? diacgrarns for hemarire and goerhirefrom reference samples. have been sampled along weathering profiles the (loo).* reciprocal lattice plane of goethite. Schéma sxnrhérique d'iin profil laréririque (Muller, 1987a). monrranr les zorles de prélèvemen!. (Figure IA) and characterized. often present anomalous intense 001 reflexions. Représenrarion schémaiique simplifiée dun diagramme de diffracrion RX de I'hémarire er de la goerhire dans les échanrillons de r@érence. I M. BOUDEULLE. J.-P. MULLER i. HEMATITE AND GOETHITE STRUCTURAL CHARACTERISTICS IN LATERITE I53 I j The reflexions, forbidden on account of goethite (001) plane of kaolinite. Figure 6A displays a I space group (Pbnm), could appear through mul- more complex organisation with three different tiple diffraction. However, after careful observa- directions ( 120) of goethite development, indu- tions, it seems more likely to invoke some struc- cing a twin-like diffraction pattem, with again tural disorder. The same effect, for example, is some misorientation, superimposed on a kaoli- noticeable on pattems of Al-substituted synthetic nite single crystal pattem (Figure 6B). goethites (Mann et al.. 1985). This oriented growth is interpreted in terms of Complex tridimensional star-shaped aggrega- epitaxy of goethite crystals upon kaolinite (Fi- tes are frequent (Figure 2B). Six armed stars gure 6), although the hypothesis of endotaxy with pseudo-hexagonal symmetry result from could not be ruled out definitively. multiple twinning with rotation about [loo], as The epitaxial relationships (Boudeulle and previously described on natural and synthetic Muller, 1986) are given by : goethites (Comell er al.. 1983 ; Cornell and Giovanoli, 1986). No particular relationships Goethite Kaolinite between kaolinite and goethite have been obser- (100) (001) ved in these samples. bG,=9.95 8, [OlO] [200] 2%= 10.32 Å 3~~,=9.068,. [O031 [OIO] b~=8.938, b. From red materials (red saprolite. ferruginous nodules. red clayey matrices) : It should be noted that the theoretical discre- pancies between the corresponding lattice pe- Different morphologies are evidenced in sam- nods (Figure 7) are not clearly expressed by ples from red materials, where goethite is inti- diffraction patterns (Figure 6B). That could be mately associated or tied to residual micas, kao- indicative of some lattice accommodations at the linite and hematite. interface, suggesting a semi-coherent or cohe- t ! ( 1) Large multidomains platelets (Figure rent contact between the two phases instead of i 3A), showing pronounced intragranular poro- an incoherent one. sity, with irregular outlines, developed parallel The kaolinite sheet presents a ternary symme- 1 to the (100) plane in form of leaf. Twin diffrac- ! try, while goethite structure parallel to (100) just tion pattems (Figure 3B) are frequent, with displays a binary symmetry. As a matter of I (02 1) as twin plane (Figure 3C). Pattems do not consequence, three equivalent directions of posi- show the spot splitting which would be expec- tioning for goethite upon kaolinite are possible. ted, according to lattice parameters (Sampson, That corresponds to the setting in figure 6A 1969). That indicates a quite perfect lattice and would favour goethite twin formation (Fi- match accross the boundaries, which has been gure 5A). Similar SEM images have been pu- confirmed by HRTEM as shown on synthetic blished by Triar (1983). crystals by Comell et al. (1983).

(2) Smaller, anhedral dumpy cnstuls. still c. From loose ?ellow materials with a soil developed parallel to the (100) plane, with irre- te.vture (Figure IA, zone I) : gular and festooned outlines like oak-leaves (Fi- gure 14). They generally form confuse aggrega- X-Ray diffraction identifies goethite as the tes or coatings, often associated with hematite. dominant iron phase (Figure IB, sample 1). Goethite appears as rod-shaped particles or mi- (3) Larhs or acicular custals. filling voids nute rounded crystallites, less than 100 .& in and cracks (Figure 4), with irregular outlines diameter, forming lenticular to framboiidal ag- and terminations. FIG. 2. - [A] Goethite (Go) crystals upon a large kaolinite (KI flake from a yellow parch in saprolite. gregates (Figure 8). Similar observations have [BI Complex tridimensional aggregate of goethite crystals. [CI Individual from the same sample. In some cases, kaolinite coating is observed been frequently reported on materials from soils D] Electron dirracrion pattern (SALI) of a goethite cvstal parallel to (100). [E] Corresponding representation (Figures 5A and 6A). On figure 5A, a parallel (Jones et al., 1982). The type or degree of inte- of (100)*, reciprocal lattice plane. array of goethite crystals, with some starshaped raction within goethite aggregates and between [A] Cristaux aè goethite reposant sur une plaquette de kaolinite (K)provenant dune zone jaune de In twins, gives a characteristic texture pattern the two phases (Go-K) cannot be assessed : saprolite. [BI Agrégar tridimensionnel complexe de cristaux de goethite. [CI Cristal isolé provenant du même (Figure 5B), with arced Okl reflexions from goethite is strongly tied to kaolinite platelets, as échantillon. [Dl Diagramme de microd$%action électronique dun cristal de goethite développé parallèlement goethite, indicative of some relative desorienta- their separation can hardly be achieved even af- d (100). [E] Repr6senta:ion du plan correspondant du réseau réciproque de la goethite tion. Texture axis c ([OOI]) is parallel to the ter long ultrasonic shaking of suspensions with a L-

154 M. BOUDEULLE. J.-P. MULLER HEMATITE AND GOETHITE STRUCTURAL CHARACTERISTICS IN LATERITE 155

granular aspect, with some moire-like cons- tram, and serrated or stair-like edges (Figure 9A and 9B). The SAD pattern (Figure 9C) is a typical (O00 I)" reciprocal net of hematite single crystals (Figure 9D). Weathering, posterior to growth, seems to be responsible for their aspect, as illusmted by figure IO, which shows the effects of partial dissolution on a characteristic hexagonal crystal. However, as dissolution processes affect prima- rily the zones of weakness of crystals, either in terms of lattice defects or growth features, weathering could emphasize an initial subgrai- ned texture. Crystals of hematite are readily synthesized by. ageing precipitates from aqueous Fe(II1) so- I lutions (Feitknecht and Michaelis, 1962 ; Atkin- son er al., 1968 ; Schwertmann, 1969). TEM examinations of the reaction products (Fischer and Schwertmann, 1975 ; Johnston and Lewis, 1983) show that the original ferrihydrite particles coalesce to form hexagonal hematite platelets, which increase to 300-400 A with ageing. The same process, with a layer-by-layer tridimensional development, would readily explain the observed microstructures of the natural crystals. FIG.4. - Goethite laths coating and crackfilling. C Rzzowremenr et remplirsnge de feme par des lanes Typical rounded residual hematite crystals are @e@*@ de goethite. imaged in soil materials (Figure g), generaily without interactions with kaolinites. Apart from o o o disperser or even with a chemical treatment the only observed case of oriented growth of (Muller and Calas, 1987), precluding good high hematite upon a kaolinite platelet (Figure 14), resolution examination. direct interrelationships between the two minerals have not been found. In contrast to hematite, kaolinites of the zones where hematite is 2. Hematite present do not display any sign of dissolution or destabilization effects. Whatever the sampling place (red and loose saprolite, indurated nodules or even red clayey matrices in the top zone), hematitic materials do not display major or significant differences. 3. Intermediate phases

Large hematite crystals are not easily charac- Some granuleous particles present features terized, owing to the sample preparation like pock-marks or craters, up to a fifty methods : they are either too thick for TEM Angstroms in diameter, quite evenly distributed observation or have been pulled of because of (Figure 1 I), which give them a different look, their hardness during cuting. When imaged FIG.3. - [A] L.eaL.es of multiabmainic goethite crystals from upper red ;one af saprolite with kaolinite (K). when compared with hematite and goethite crys- [BI Electron dijfraction pattern (SAD) of (OZI) twin association. [Cl Corresponding representation of (loo)*, along [OOOI] they show sharp edges, with tals. No evolution of this aspect during beam reciprocal lottice plane rvirli 2 individuals. I and II. subhexagonal outlines. exposure is noticed, indicating that these featu- [A] Cristau multidomaines en feliilles de goethite provenant de la partie supérieure de la saprolite (zone Smaller crystallites, in the micrometre size res are not radiation-induced damages, like in rouge). [BI Diagramme de microdifiaction produit par un crinaì maclé (plan de macle, (021)). [Cl Représen- range or less, have a typical and reproducible kaolinites, but intrinsic weathering effects. tation correspondante des plans (KU)*des réseaux réciproques des deux individus. appearance : the anhedral platelets present a These particles are found in samples from red P---A -

156 M. BOUDEULLE, J.-P. MULLER HEMATITE AND GOETHITE STRUCTURAL CHARACTERISTICS IN LATERITE 157

FIG.5. - [A] Oriented growth of goethite upon ho- FIG. 6. - [A] Epitaxial growth of goethite upon linite :purullel te.nure with nvins. Texture axis c~~ kaolinire : ternary network of goethite crystals ([O0 I]) [BI Corresponding electron diflacrion (Go) : K1 = original kaolinite flake, K2 = parallel te-nure punern. (Inner spots are O20 reflexions from growth. [BI Electron diffraction pattern (SAL)) twins). showing rhe nvin-like pattern [( 100)*Goreciprocal lanice plane : inner spots : 110 and 020 type spots [A] Croissunce orientée de lu goethite sur lu &oli- from kaolinite]. nire. Texture parallèle avec macles. [BI Diagramme de diffruction ussocié. [A] Croissance épitacrique de la goethite sur la kaolinire : réseau temaire de cristaux de goethite (Go). KI, plaquette de kaolinite support : K2, croissance parallèle de kaolinite. [BI Diagramme de (OOOl)* reciprocal plane (Figure 12B, a), a se- di3action associé. montrant la répartition des ré- cond weak hexagonal net with a 30 rotation flexions Okl de la goethite analogue à celle d’une macle (021). Les spots proches du centre sont des with respect to the other and dhkl 2.9 for = ,& réflexions de type 110 et 020 de la kaolinite. central spots (Figure 12B, b, : they fall midway between two equivalent (I 120) type hematite re- topotactic and pseudomorphic transformation of flexions (dhkl = 2.5 A). A similar pattern with hematite iato goethite can be considered. Never- 2.9-3 b; additional reflexions has been described theless, this process implies a complete recons- by Amouric er al. (1986), for hematite from truction and expansion of the lattice, and it layers of saprolite and red intemodular matrices iron-crust pisolites, and interpretated as topotac- As a matter of fact, the observed pattems re- seems doubtful that such a transfomation would of the nodular zone. tic maghemite upon hematite. However, a dis- preserve quite perfectly the lattice orientations. tinct arcing of-the spots, and a radial net present a logical series f” (OO01)* reciprocal Different SAD pattems, of single-crystal type, contraction, with respect to the hematite net, net of hematite single crystal to (loo)* recipro- have been recodecl that kind of An other interpretation is proposed in were noted, which are not obviously in the cal net of (021) twinned goethite as shown by (Figure 12), which contain Only iron and a few seen parallel : Atkinson er al. (1968, studying the present case. A more complex diagra (Figure figure 3C. aluminum, as shown by X-Ray EDS. effect of pH on crystal nucleation in Fe(II1) so- 12B, c) shows additional spots with hl= 5 ,&, As the diagrams are different from those des- lutions, noted that “Intermediate types occur The simplest pattern (Figure 12A) exhibits, m which can be compared to doro (4.98 1) for cribing the dehydration process of goethite into and were difficult to distinguish, since the dif- addition to the strong reflexions of hematite goethite. hematite published by Watari et ol., (1983). a fraction pattems for goethite twin crystals and ~--. t L

158 M. BOUDEULLE. J.-P. MULLER HEMATITE AND GOETHITE STRUCTURAL CHARACTERISTICS IN LATERITE 159

FIG. I. - Epitaxial lanice relationships between kaolinite and goethite. The hexagonal cell refers to kaolinite octahedral layer. Orientation relative des réseaux de la kaolinite et de la goethite épitaxiée. La maille hexagonale ha- churée décrit le feuillet octaédrique de la kaolinite. FIG.8. - Typical association of goethite (Go), hema- hematite crystal, on [lo01 and [OOOI] zone axes tite (He) and kaolinite (K)crystallites forming the respectively, are very similar". The patterns can upper level with soil tenure of the lateritic se- quence. be indexed as a superstructure of hematite, with a doubling of basal cell parameters (Figure 12B). Association caracréristique de cristaux de goethite It should be noted that the same samples, stu- (Co),hématite (He) et kaolinite (K)apparaissant duns le niveau supérieur à texture de sol de la died by means of XRD (Figure lB), give ano- séquence latéritique. malous intensities for hematite diagrams, interpreted as resulting from structural disorder by Perinet and Lafont (1972). On symmetry grounds (as for kaolinite), three equivalent directions of goethite development FIG.9. - Hematite crystals. [A] Thin cut : note the moiré effects. [BI Suspension : note the typical edges. Goethite and hematite smctures are based from a hematitic nucleus are possible, and ac- [Cl Single crystal electron diffraction pattem. [DI Corresponding reciprocal lattice plane (o001 )*H~: upon hexagonal close packed layers of oxygen tually occur, inducing twin formation (Figure Monocristaux d'hématite. [A] Coupe ultra-mince : noter les effets de moiré. [BI Suspension : remarquer les or/and hydroxyl groups, giving a common sub- 13). Such particles are probably less stable than bords caractéristiques. [Cl Diagramme de microdiffraction assmié à [BI. [Dl Plan riciproque (OOOI)* corres- lattice, within which the octahedral vacancy fil- hematite ones and would undergo alteration pondant de I'hématite. ling scheme differs according to the phase more drastically (Cornel er al., 1974). (Goldsztaub, 1931 ; Bemal er al., 1959 ; Fran- oriented and random intergrowth of goethite. requires that indicators or references of the dif- combe and Rooksby, 1959). Yet, the anion net Composite particles, presenting both an interme- ferent stages, identified as relicts, can be found. retains similar parameters in two directions. diate structure and goethite, have been imaged Although crystal growth conditions in natural 4. Intergrowths of oxy-hydroxides (Figure 15A and 15B). medium differ on many points from sxperimen- Hematite Goethite tal ones, some correlations can be drawn. As As noted above, intergrowths of hematite and iron oxy-hydroxides development from aqueous (ooo1) (100) goethite have been found. The most significant v* DISCUSS1oN CoNCL'"S1oN solutions has been extensively studied in labora- 8.12 8, [21.0] [O031 3~,,=9.068, picture, in terms of crystal growth, is presented 2h,= 10.071 [02.0] [O101 &,,=9.95 8, tories in the very purpose to get correlations on figure 14 which shows at low magnification, 1. Mineral development with the natural phenomena, hematite and [OO. 1JHe : 3-fOld symmetry axis upon a large kaolinite platelet, oriented dendri- goethite morphological studies afford valuable [ IOO]G, : 2-fold symmetry axis tes and a continuous layer of hematite with The reconstitution of lateritisation processes informations. = Id- Í 1 M. BOUDEULLE. J.-P. MULLER HEMATITE AND GOETHITE STRUCTURAL CHARACTERISTICS IN LATERITE 161

FIG. 1 I. - Particles with inrermediate structure (see test) showing dissolution indices. [A] Thin cut, and [BI suspension. Panicules ayant une srrucrure intermédiaire (voir rese) présentant des indices de dissolution. [A] Coupe uha-mince. [BI Suspension. Hematite morphological characters support very high scale, precludes realistic correlations the assumption of formation of hematite after with the complex experimental data. Neverthe- nucleation from amorphous Fe@) hydroxides less, some comments can be made.

tion and reprecipitation growth mechanism or The diversity of goethite morphologies accor- “reconstructive transformation”, as frst prop- ding to sampling place reveals unambiguously sed by MacKay (1960), inducing more equant differences in the environmental growth condi- habits or twins : the precursors are femhydrite tions. ‘Unfortunately, the local variability, at a germs or hematite nuclei.

RG. 10. - [A] Hematite (He). goethite (Go). inrermediate phase (IP)and kaolinire in a thin section. [BI Hematite cvstal. viewed along [OOOI]. with the corresponding qpical hexagonal outline, affected by partial dissolution. [CI Detail of B (rectangle) showing the preserved hematite lattice (lattice or wedge fringes) in the dissolution area. [A] Hémarite (He). goethite (Go), phase intermédiaire (IP)et kaolinite (coupe ultra-mince). [BI Le crisral d’hématite. développé parallèlement à (0001). est affecté par une dissolurion partielle. [CI Ce détail de [E] montre le réseau préservé de l’hématite fianges de coin ou de réseau) dans la zone de dissolution. ---- Ii 162 M. BOUDEULLE. J.-P. MULLER / HEMATITE AND GOETHITE STRUCTURAL CHARACTERISTICS IN LATERITE 163 !

-0 i IB e *

.J dUJU I

I I I FIG. 12. - [A] cvpicul dfiuction purrern for inrermediure structure. [BI 1nde.rarion us hemarire superstructure. with doubling of :he unit cell busal puramerers (Compare with Fig. 9D (a) hematite normal reflexions ; (b) first kind of additional reflexions (d -. 2.9 A) ; (c) second kind (d = 5 ,&i [A] Diagramme cpique dune phase inrermédiuire. [BI Indexution comme surstrucrure de I’hématire avec des puramèrres de réseau doubles (voir Figure 9D) : a) Réflexions normales de l‘hématite : b) Premier type de reflexions additionnelles (d = 2.9 1); c) Deuxième type (d = 5 Ål. FIG. 14. - Inrergrowrhs of hemutite (He) und goerhire (Go) upon u kaolinite plareler (K): oriented dendrites FIG. 13. - Structural relationships benceen hemurire (left) and large crystals of hematite and leaf-like association of goethite crystallites. und goerhire (Bi-dimensional representation). Note Surcroissances dhémarire (He) er de goerhire (Ga) sur une pluquerre de kaolinire : the anion common sub-structure and the three equi- (K) dendrites orientées et larges cristaux d’hématite ; association cle cristallites de goethite en forme de feuilles de chêne. valent settings for goethite (I. II. III). Rttw for goethite unit cells correspond to domains in (021) twin position. terrelationships (in fact true bonding) between undergo a later weathering and constitute a se= Relurions srrucriiraies de la goerhire et de I’héma- the minerals. condary source of iron. Goethite stability cannot rite :noter la sous-smcture anionique commune et be apprehended directly, no distinctive signs of les trois orientations équivalentes de la goethite (1. Iron hydroxides precipitation in the presence destabilization being detected. II, III). Les zones hachurées des mailles de goethite of well crystallized reference kaolmitc:s has been représentent des domaines en position de macle studied (Greenland and Oades, 1968 ; Saleh and (021). Jones, 1984 ; Jones and Saleh, 1986). Coatings Besides, because of the close associations of of different types of oxyhydroxides showed only hematite and goethite either in a same and one weak association with basal clay surfxes, gene- particle (intermediate phase) or as intergrowths, rally explained in terms of electrostatic interac- the problem arises of the stability field jointness gamets, ...) playing the rôle of seeds in the first or overlapping for the two minerals. Neverthe- steps of weathering and goethite development. tions. Epitaxial growth depends on the physico- chemical parameters of the medium, and among less, it should be recalled that goethite, when Goethite epitaxial growth upon kaolinite them specially the surface state of the supports. associated to hematite, seems often to postdate would, at frst sight, represent another mode of It is believable that this state, for fresh, even it and tends to develop in more open sites. Mo- development, with “heterogeneous nucleation”. still growing kaolinite crystals, in a natural me- reover, goethite, in these cases, is associated However, the extent of Fe- substitution within dium, would be drastically different from the with hematite or hematite-like particles which / / / / 1 kaolinite octahedral layer (and AI in goethite), experimental one (Schwemann, 1979 ; Robert present dissolution marks and are potential pre- on one hand, the close structural relations et al., 1987). cursors for goethite growth, in a hematite- between this layer and hematite layer, on the goethite transformation involving a dissolution- other hand, lead to consider that the scheme pro- crystallization mechanism (Bedarida er al., The observed morphologies and the existence posed for describing intermediate phase forma- 2. Mineral stability 1973). Such kinetic processes would explain the of intermediate forms suggest that the second tion (Figures 7 and 13) can be readily transpo- failure of classical thermodynamic approaches to process has been the more efficient, with proba- sed : iron substituted domains within kaolinite In contrast to kaolinite which remains preser- describe iron oxyhydroxide behaviour in lateriti- bly primary iron compound remnants (micas, layer could act like nuclei, inducing definite in- ved in the same zones, hematile crystals zation . ?

164 M. BOUDEULLE. J.-P. MULLER HEMATlTE AND GOETHITE STRUCTURAL CHARACTERISTICS IN LATERITE I65

Clay Minerals, 34, 45-52. (1984). - Occurrence of microcrystalline goethite ATKINSON,R.J., POSNER,A.M. and QUIRK,J.P. in an unusual fibrous form. Geodem, 34, 135- (1968). - Crystal nucleation in Fe(II1) solutions 148. and hydroxide gels. J. Inorg. Nucl. Chem., 30, FRANCOMBE,M.H. and ROOKSBY, H.P. (1959). - 2371-2381. Structure transformations effected by the dehydra- BEDARIDA,F., FLAMINI,F., GRUEESSI,O. and PEDE- tion of diaspore, goethite and delta ferric oxide. MONTE, G.M. (1973). - Hematite to goethite sur- CluyMiner. Bull.. 4, 1-14. face weathering. Scanning electron microscopy. GOLDSZTAUB,S. (1931). - D@hydratation des hy- Amer. Min., 58,794-795. drates femques naturels. C.R. Acad. Sci. Paris, BERNAL,J.D., DASGUPTA,D.R. and MACKAY,A.L. 193. 533-535. (1959). - The oxides and hydroxides of iron and GREENLAND,D.J. and OADES,J.M. (1968). - Iron their swctural relationship. Cloy Miner. Bull., 2 1, hydroxides and clay surfaces. In Proc. 9th Intem. 15-30. Congr. Sol Sci., Adelaïde, Australia, 657-668. BOCQUIER,G., MULLER,J.P. and BOULANGÉ,B. GREENLAND,D.J., OADES,J.M. and SHERWIN,T.W. (1984). - Les latérites. Connaissances et perspec- (1968). - Electron-microscope observations of tives actuelles sur les mécanismes de leur différen- iron oxides in some red soils. J. Soil Sci., 19, 123- ciation. in Livre Jubilaire Cinquantenaire AFES, 126. . Assoc. fr. Etude du Sol, éd., Paris, 123-138. JOHNSTON,J.H. and LEWIS,D.G. (1983). -A detai- BOUDEULLE,M. and MULLER,J.P. (1986). - Elec- led study of the msformation of femhydrite to tron microscopy (AEM-HRTEM) of goethite from a hematite in an aqueous medium at 92 “C. Grdchim. latente. Twinning and epitaxy on kaolinite. in Proc. Cosmochim. Acta, 47. 1823-1831. IMA 1986, Stanford, USA, 60. JONES,R.C., HUDNALL,W.H. and SAKAI,W.S. CAMBER,P. (1986). - Infrared study of goethites of (1982). - Some highly weathered soils of Puerto varying crystallinity and particle size I. Interpreta- Rico 2. Mineralogy. Geodem, 27, 75-137. tion of OH and lattice vibration frequencies II. JONES,A.A. and SALEH,A.M. (1986). - Electron Crystallographic and morphological changes in se- diffraction and the study of ferrihydcte coatings on ries of synthetic goethites. Clay Miner., 21, 191- kaolinite. Cloy Miner., 21, 85-92. 210. KÜHNEL,R.A., ROORDA,H.J. and STEENSMA,J.J. COEY, J.M.D. (1980). - Clay minerals and their (1975). - The crystallinity of minerals. A new variable in pedogenetic processes. A study of FIG. 15. - [A] Association of intermediute phase und goethite (Go). [BI C0mple.r electron difracrion pattern transformation studied with nuclear techniques. The contribution of Mössbauer suectroscouy. Atomic goethite and associated silicates in laterites. Clays showing the parullel orientanon of the m’O lattices (the indexations correspond to figures ZE and 12B). .- Energy Rev., 18,73-124. and Clay Minerals, 23,349-354. [A] Association d’une phase intermédiaire er de goethite (Go). [BI Le diagramme de dzruction associé montre CORNELL,R.M. and GIOVANOLI,R. (1985). - Effect MACKAY,A.L. (1960). - Some aspects of the topo- l‘orientation parallèle des deu réseaur. of solution conditions on the proportion and mor- chemistry of the iron oxides and hydroxides. in. phology of goethite formed from femhydrite. Clays Proc. 4th Intem. Symp. Reactivity of Solids, Ams- 3. Petrological implications . ACKNOWLEDGEMENTS and Cloy Minerals, 33, 424-432. terdam, 571-583. CORNELL,R.M. and GIOVANOLI,R. (1986). - Fac- ~NN,S., CORNELL,R.M. and SCHWER~ANN,U. At the scale of lateritic profiles, the above This work was supported by a research grant tors that govem the formation of multidomainic (1985). High resolution transmission electron- data would imply : goethites. Clays and Clay Minerals, 34,557-564. microscopic (HRTEM) study of aluminous goethi- provided by C.N.R.S. and ORSTOM (ATP tes. Clay Miner., 20, 255-262. “Latérites”). The authors are greatly indebted CORNELL,R.M., W, S. and SKARNULIS,A.J. (1) A fist generation of goethitic material, (1983). - A high-resolution electron microscopy MULLER,J.P. (1987~).-Analyse pétrologique d’une resulting from in situ direct weathering of iron- to D. Tessier and G. Ehret for electron micros- examination of domain boundaries in crystals of formation latéritique meuble du Cameroun. Essai bearing rock-forming minerals. cope preparations. They would like to thank synthetic goethite. J. Chem. Soc., Fura& Trans., de traçage d’une différenciation supergène par les Rhône Poulenc Society (C.I.D.) for providing I. 79. 2679-2684. paragenèses minérales secondaires. Thèse Dr. Sci., (2) Iron transfers via solutions as soon as the microscope facilities. Fruitful discussions with CORNELL.R.M., POSNER,A.M. and QUIRK,J.P. Palis. intergranular porosity of the weathered rock Profs. U. Schwert”-n and P. Michel and sug- (1974). - Crystal morphology and the dissolution MULLER,J.P. (1987b). - Analyse de la différencia- would allow it, inducing nucleation and growth, gestions from the referees have been much ap- of goethite. J. Inorg. Nucl. Chem., 36. 1937-1946. tion d’une couverture latéritique de l’Est du Came- as achieved experimentally, of hematite, FEITKNECHT,W. and ~~ZICHAELIS,W. (1962). - Über roun. in Regional Seminar on Laterite and Lateritic preciated. The graphical assistance of J. Dyon is Soils, Douala, Cameroon, 1986, ORSTOM, hematite-goethite and goethite particles. acknowledged. die Hydrolyse von Eisen (III)-Perchlorat-Liisungen. ed., Helv. Chim. Acta, 212-224. (in press). generations of goethite, resulting (3) Next Reçu le 22 octobre 1987 FELIPE-MORALES,C. and RUSSEL,A. (1972). - Ob- MULLER,J.P. and CALAS,G. (1987). - Kaolinite from dissolution-crystallization processes affec- Accepté le 9 décembre 1987 servations sur le fer libre des fractions argileuses paragenesis in a laterite (Cameroon). Tracing kaoli- ting hematite and intermediate particles. des sols à l’aide de la microscopie élemnique. nites through their defect centers (submitted to Pédologie, 13. 119-157. Clays and Cloy Minerals). FISCHER,W.R. and SCHWERTMA”,U. (1975). - PINNAVAIA,T.J. (1981). - Electron spin resonance REFERENCES The formation of hematite from amorphous iron studies of clay minerals. in Advanced Techniques (III) hydroxide. Clays and Cloy Minerals, 23, 33- for Clay Mineral Analysis, J.J. Fripiat, ed., Else- AMOURIC, M., BARONNFT,A., NAHON,D. and DI- tions of iron oxyhydroxides and accompanying 37. vier, Amsterdam, 139-161. DER, P. (1986). - Electron microscopic investiga- phases in latentic iron-crust pisolites. Clays and FORDHAM,A.W., MERRY, R.H. and NORRISH,K. PERINET,G. and LAFONT,R. (1972). - Sw les para- Bull. Mineral. 166 M. BOUDEULLE. J.-P. MULLER (1988). 111, 167-171

mètres cristallographiques des hématites alumineu- of aluminum. Soil Sci., 128, 195-200. ses. C.R. Acud. Sci. Paris. 275. 1021-1024. SCHWERTMANN,U. and LATHAM. M. (1886). - Pro- Description de la ludjibaïte, un polymorphe de la pseudomalachite, ROBERT, M., VENEAU,G. et ABREU,M.M. (1987). perties of iron oxides in some new Caledonian ox¡- - - Etudes microscopiques d’associations sols. Geoderma, 39, 105-123. CU,(PO,), (OW4 aluminium-argiles ou feragiles. in Proceed. VIIth SCHWERTMANN,U. and TAYLOR,K.M. (1977). - Intem. Working Meeting on Soil Micromorpho- Iron oxides. in Minerals in Soil environments, par PAIJLPIRETI et MICHELDELIENS** logy, 1985, Paris, Fedoroff, N., Bresson, L.M. et Dixon J.B. and Weed S.B. ed., Madison, Wiscon-. Courty, M.A., eds, A.F.E.S., Paris. 467-474. * Laboratoire de Chimie physique et de Cristallographiede l’Université, Bâtimgnt Lavoisier, sin, USA, Soil Sci. Soc. Amer., 145-180. Place Louis Pasteur I, B 1348 Louvain-la-Neuve, Belgique. - SALEH,A.M. and JONES,A.A. (1984). - The crys- TARDY.Y. and NAHON. D. (1985). - Stability of tallinity and surface characteristics of synthetic fer- AI-goethite, Al-hematite and Fe”+-kaolinite in bau- ** Section de Minéralogie et de Petrographie, Institut royal des Sciences Naturelles de Belgique, rihydrite and its relationship to kaolinite surfaces. xites, ferricretes and laterites. An approach of the tue Vautier 29, B 1040 Btuxelles, Belgique. Cluy Miner.. 19. 745-755. mechanism of the concretion formation. Amer. J. SAMPSON,C.F. (1969). - The lattice parameters of Sci., 285, 865-903. natural single crystal and synthetically produced TRIAT,J.M. (1983). - Ochrification of the mid- goethite. Acru Ctysr.. E25. 1683-1685. Résumé. - La ludjibaïte a été muvée clans un horizon de schiste micacé rouge à Ludjiba, Shaba, Zaïre, en cretaceous glauconitic greensands, in Provence, association avec la pseudornalad~t;ll~eet la libithénite. Eile forme des agrégats crêtés de petites lamelles bleu-vert SCHULZE,D.G. and SCHWERTMANN,U. (1984). - France. Mineralogical sequences and facies succes- The influence of aluminium on iron oxides. X, The (0.3 mm maximum) à la surface de cristaux vert foncé de pseudomalachite. Système triclinique, groupe spatial sion. Sci. Géol.Mém., 72, 161-167. PÏ, avec a = 4,446(3), b = 5.871(4), c = 8,680(7) A, a = 103,9(2), ß=90,3(1), y = 93,2(2)”, V = 219,5(3) properties of Al-substituted goethites. Chy Miner., WATARI,F., DELAVIGNETTE,P., VAN LANDUYT.J. 19. 521-529. A3, Z = J. Densité calculée :_4,36. Raies principales iu diagramme de poudre [d(A) (I) hkl] : 4,46(10)100, and AMELINCKX,S. (1983). - Electron microsco- 2,462(5)120, 2,353(5)103 et 113,3,02(2)102et 2,408(2)103. Biaxe positif ou négatif, 2V grand, n’,, = 1,786(2) SCHWERTMANN,U. (1969). - Die Bildung von Eise- pic study of dehydration transformations III. High noxidmineralen. Forrschr. Miner., 46, 274-285. et n’ = 1,840(5). Composition chimique : Cu0 69,1, P205 24,6, H206.3 %. Le composé artificiel correspon- resolution observafi‘on of the reaction process dant: la ludjibaïte avait été synthétisé et sa structure déterminée par Shoemaker er al. (1981). SCHWERTMANN,U. (1979). - The influence of alu- FeOOH-Fe203. J. Solid State Chem. 48,4964. minium on iron oxides V. Clay minerals as sources Mots-clb : ludjibaïte - nouveau minéral, phosphate, pseudomalachite, polymorphisme, Zaïre. Descriprion of ludjibaire. a polymorph of pseudomalachite Cu5(P04)Z(OH).j. Abstract. - Ludjibaïte occurs on red micaceous shale at Ludjiba, Zaire, in association with pseudomalachite and libethenite. It forms crest-like aggregates of small bluesreen blades (up to 0.3 mm) on the surface of dee green crystals of pseudomalachite. Triclinic, space group PI, a = 4.446(3), b = 5.871(4), c = 8.680(7) f,a = 103.9(2), ß = 90.3(2), y = 93.2(2)”, V = 219.33) A3, Z = I. Calculated density : 4.36 g.cmL3. Strongest lines of X-ray-powder pattem id(.&) (I) hkl] : 4.46(10)100, 2.462(5)120, 2.353(5)103 and 113, 3.02(2)102 and 2.408(2)103. Biaxial positive or negative, 2V large, a’ = 1.786(2) and y’ = 1.840(5). Chemical composition : Cu0 69.1, Pz05 24.6, H20 6.3 %. The corresponding artificial Compound had been prepared by Shoemaker er al. (1981) who had also determined the crystal structure. Key-words :ludjibaite - new mineral, phosphate, pseudomalachite, polymorphism, Zaire.

INTRODUCTION un échantillon provenant du Zaïre, ce qui nous permet de compléter les données déjà foumies Les stnctures cristallines de trois polymor- par Shoemaker er al. (1981) et de décrue le phes de CU,(PO,)~(OH), ont été determinées minéral sous le nom de ludjibaïte. jusqu’à présent (Tableau I). La pseudomalachite, composé naturel monoclinique, a été étu- diée par Ghose (1963) et par Shoemaker er al. LOCALISATION DU GISEMENT (1977). Un composé synthétique montdinique a ité décrit sous le sigle PPM par Anderson er al. Le site de Ludjiba se trouve au voisinage du ( 1977), tandis qu’un composé synthétique uicli- confluent de la rivière du même nom avec la nique l’a été sous le sigle QPM par Shoemaker Mura, a une douzaine de km au sud-ouest de et al. (1981). Un mélange de ces trois polymor- Kambove (Figure 1). Cette zone se trouve sur le phes a été trouvé à l’état naturel sur un échantil- flanc sud de l’anticlinal Pempere-Kitinda. Le Ion de la collection minéralogique de ]’Univer- long de son axe, on observe deux accumulations sité de Harvard (Shoemaker et Kostiner, 1981) de Roan (Katangien inférieur du Précambrien mais n’a pas fait l’objet d’une description. De- moyen) : à l’est, la roche minéralisée a éte puis lors, le composé PPM a été trouvé à l’état exploitée dans le gisement de Shamitumba ; naturel en RFA et décrit, sous le nom de rei- l’ouest par contre, le Roan affleure sous forme chenbachite, par Sieber er al. (1987). De notre de petits synclinaux stériles, à l’exception de côté, nous avons découvert le composti QPM sur celui de Ludjiba (direction N45”E, longueur :

@ Societe française de Mineralogie el de Crisrallogqhie. Paris. 1988