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Clays and Clay Minerals, Vol. 49, No. 1.44-59, 2001.
THE RELATIONSHIPS BETWEEN KAOLINITE CRYSTAL PROPERTIES A N D THE ORIGIN OF MATERIALS FOR A BRAZILIAN KAOLIN DEPOSIT
ANGI~LICA E DRUMMOND C. VARAJ,~O, I ROBERT J. GILKES, 2 AND ROBERT D. HART 2
DEGEO/EM/UFOR Campus Morro do Cruzeiro, 35400-000 Ouro Preto, MG, Brazil 2 Soil Science and Plant Nutrition, Faculty of Agriculture, The University of Western Australia, Nedlands,
Western Australia 6907, Australia
Abst rac t The clay particles in a kaolin deposit from Brazil were investigated by X-ray diffraction (XRD), differential thermal analysis (DTA), analytical transmission electron microscopy (ATEM), and electron paramagnetic resonance (EPR) to examine the relationships between morphological and chemical properties of the crystals and to relate these properties to formation conditions. The XRD patterns show the dominant presence of kaolinite with minor amounts of gibbsite, illite, quartz, goethite, hematite, and anatase. ATEM observations show two discontinuities in the deposit as indicated by changes in mor- phology and size of the kaolinite crystals. At the base of the deposit, hexagonal platy and lath-shaped particles (mean area of 001 face = 0.26 p,m 2) maintain the original fabric of the parent rock which characterizes an in situ evolution. In the middle of the deposit a bimodal population of large (mean area of 001 face > 0.05 ixm:) and small (mean area of 001 face < 0.05 p.m 2) sub-hexagonal platy kaolinite crystals occurs. This zone defines the boundary between the saprolitic kaolinite and the pedogenic kao- linite. Near the top of the profile, laths and irregular plates of kaolinite, together with sub-hexagonal particles, define two different depositional sources in the history of formation of the deposit. Crystal thickness as derived from the width of basal reflections and the Hinckley index are compatible with the morphological results, but show only one discontinuity. At the base of the deposit, kaolinite has a low- defect density whereas in the middle and at the top of the profile, kaolinite has a high-defect density. Likewise, EPR spectroscopy shows typical spectra of low-defect kaolinite for the bottom of the deposit and typical spectra of high-defect kaolinite for the other portions of the deposit. Despite the morphological changes observed through the profile, the elemental composition of individual kaolinite crystals did not show systematic variations. These results are consistent with the deposit consisting of a transported pedogenic kaolinite over saprolite consisting of in situ kaolinized phyllite.
Key Words--ATEM, Defect, EPR, Formation Condition, Kaolinite, Morphology, Saprolite, Soil, XRD.
I N T R O D U C T I O N
Kaol in i te is an impor t an t wea the r ing product at low lat i tudes. It is a c o m m o n cons t i tuen t of saprol i tes and the mos t a b u n d a n t clay minera l in the over ly ing soils (Dixon, 1989). Kaol in i te can be fo rmed by in s i tu chemica l evo lu t ion (weather ing) of the under ly ing rocks and f rom col luvia l mater ia ls der ived f rom highly wea the red lateri t ic profi les (Gilkes et al., 1973; Mul- cahy, 1973). Accord ing to the A I P E A Nomenc la tu re C o m m i t t e e Repor t ( G u g g e n h e i m et al., 1997), the t e rm kaol in m a y be e i ther a minera l group n a m e or a rock name. In the present work, kaol in i te is des igna ted as a mine ra l n a m e and kaol in is used only as a rock nanle .
Crys ta l s of kaol in i te in saprol i te and in kaol in de- posi ts are typica l ly large ( 1 - 2 Ixm) hexagona l plates (Dixon, 1989), a l though var ious shapes (Keller, 1978), such as laths and pseudo-hexagona l plates, m ay occur. In m a r k e d contrast , kaol in i te in topsoi l hor izons com- m o n l y cons is t s of smal l anhedra l platy to tubu la r par- t icles (S ingh and Gilkes , 1992a). Such var iabi l i ty in kaol in i te m o r p h o l o g y is c o m m o n and widespread in na tura l deposi ts . Similar ly, the defect densi ty and p robab ly chemica l compos i t i on of kaol in i te m ay differ a m o n g occur rences and this m a y inf luence the surface
chemica l and phys ica l proper t ies of clay mater ia l s (Robson and Gilkes, 1981; M c C r e a et al., 1990).
M a n y studies re la t ing the defect densi t ies in kaol in- ite to chemica l compos i t ion have been pe r fo rmed since the early work of Rober t son et al. (1954), and several t echniques us ing spect roscopic me thods were p roposed to inves t iga te defect concen t ra t ions (Jones et al., 1974; Ange l et al., 1974; M e a d s and Malden , 1975; Herb i l lon et aL, 1976; M e s t d a g h e t aL, 1980; Br ind ley et al., 1986). These studies s h o w e d that bo th the con ten t and the locat ion of s t ructural i ron m a y in- vo lve re la t ionships wi th the defect densi t ies in kaol in- ites, a l though the m e c h a n i s m s respons ib le for these re- la t ionships are unknown . There is some unders t and ing of the geochemica l process tha t controls the compo- s i t ion of kaol in i te fo rmed dur ing wea ther ing (Mul le r e t al., 1995), bu t the great var iabi l i ty in i ron con ten t and m o r p h o l o g y of kaol in i te in soils (S ingh and Gil- kes, 1995; Har t et al., unpubl , data) raises ques t ions about the role of the i ron conten t of kaol in i te in af- fec t ing crys ta l propert ies . For example , does i ron in solut ion or as crys ta l l ine phases dur ing the fo rmat ion of kaol ini te affect crys ta l g rowth or is i ron acciden- tal ly incorpora ted into the kaol ini te so that the extent of incorpora t ion is af fected by crys ta l m o r p h o l o g y and not the reverse (Stone and Torres-Sanchez , 1988).
Copyright 9 2001, The Clay Minerals Society 44
Vol. 49, No. 1, 2001 Kaolinite crystal properties and the origin of materials 45
The present study focuses on the morphological and chemical features of kaolinite crystals from Brazil and the relationship to formation conditions in a low-tem- perature environment. The deposit, sampled from a drill core, was formed by the superposition of a col- luvial unit upon an in s i tu saprolitic unit (Varaj~o et al., 1989, 1990, 2000). This study allows us to distin- guish the processes of formation of the kaolinite and is potentially useful for mineral exploration in highly weathered terrains and for interpreting geological his- tory.
Size and morphology of the kaolinite crystals were examined by high-resolution transmission electron mi- croscopy (HRTEM), and the elemental composition of single crystals was obtained by energy dispersive spectroscopy (EDS). Bulk chemical analyses, X-ray diffraction (XRD) patterns, and electron paramagnetic resonance spectroscopy (EPR) were used to determine the defect content and crystal chemistry of kaolinite.
LOCATION AND GEOLOGICAL SETTING
The study is based on samples from a kaolin deposit located 15.5 km from Belo Horizonte, capital of the Minas Gerais State, southeastern Brazil (Figure 1). The kaolin deposit is situated in the Moeda Syncline trough on the western border of the Quadril~tero Fer- rifero, a mountainous Precambrian region with alti- tudes varying from 650 to 2000 m and with an area of --7000 km 2. The main geomorphologic features of this syncline are related to both lithologic and struc- tural control. The syncline is characterized by crest lines consisting of itabirite and quartzite rocks border- ing depressions originating from differential weather- ing of the substratum consisting of sericitic schists, phyllitic dolomite, and dolomitic itabirite. Many eco- nomically important kaolin deposits occur in these de- pressions and one is characterized here.
Owing to their economic interest and because these deposits belong to a well-known structural unit of the Quadril~tero Ferrffero, their origin has been studied by field observations and relatively unsophisticated min- eralogical analysis. Thus, according to Pomerene (1964) and Souza (1983), these deposits result from an in s i tu evolution from alluvial sediments within karstic depressions (Dorr, 1969; Fleisher and Oliveira, 1969; Barbosa, 1980). Souza (1983) suggested that sediments were derived from carbonatic phyllite and Fleisher and Oliveira (1969) suggested that they orig- inated from basic rocks. In both hypothesis, these sed- iments were subjected to climatic and hydrological variations. More recent studies (Varaj~o et al., 1989, 1990, 2000) based on petrological analysis revealed a more complex genesis for these occurrences involving the superposition of two units. The lower unit shows an in s i tu evolution from the underlying phyllitic rocks. The upper unit is allochtonous and was derived from lateritic (Miocene age) colluvial material origi-
nating from the weathering of phyllitic rocks that com- prised the adjoining slopes.
MATERIALS AND METHODS
Six samples, from depths of 76, 58, 54, 24, 13, and 10 m, representing the general evolution of the profile, were selected from a core drilled through the entire clayey sequence and the saprolite (Figure 1). Extract- able Fe, AI, and Si were dissolved using, successively, three extractants: sodium pyrophosphate (Blakernore et al., 1981), acid ammonium oxalate (Tamm, 1922; McKeague and Day, 1966), and citrate-dithionite (Holmgren, 1967). Content was determined with a Per- kin-Elmer Analyst 300 atomic absorption spectropho- tometer. The XRD patterns of whole samples and the clay fraction, as both oriented and random samples, were obtained before and after