Clays and Clay Minerals, Vol. 44, No. 5,672-676, 1996.

A M O R P H O U S A L U M I N U M H Y D R O X I D E F O R M E D AT THE EARLIEST WEATHERING STAGES OF K-FELDSPAR

MOTOHARU KAWANO 1 AND KATSUTOSHI TOMITA 2

i Department of Environmental Sciences and Technology, Faculty of Agriculture, Kagoshima University, 1-21-24 Korimoto, Kagoshima 890, Japan

2 Institute of Earth Sciences, Faculty of Science, Kagoshima University, 1-21-35 Korimoto, Kagoshima 890, Japan

Abstract--Weathering products formed on the surface of K-feldspar in Yakushima Island, Japan were investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission elec- tron microscopy (TEM), energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS). XRD confirmed that the weathering products were composed mainly of gibbsite and halloysite. SEM, TEM and EDX clearly showed formation of amorphous aluminum hydroxide exhibiting 2 distinct habits: 1) curled fibrous or circular forms less than 0.02 p,m in diameter; and 2) a spherical habit less than 1.0 p~m in diameter. The fibrous aluminum hydroxide exhibited curled fibrous forms or circular forms less than 0.02 p.m in diameter and gave a diffuse electron diffraction halo. EDX indicated that the material consisted mainly of A1 and very small amounts of Si and Fe. The spherical aluminum hydroxide also gave similar EDX and electron diffraction characteristics to the fibrous material. These fibrous and spher- ical aluminum hydroxides must be formed as a metastable phase in the earliest weathering stages, and transformed into a stable phase of gibbsite and halloysite as the reaction proceeded.

Key Words--Aluminum hydroxide, Gibbsite, Halloysite, K-feldspar, Weathering product.

I N T R O D U C ~ O N

Interaction of feldspar with H20 is one of the im- portant geochemica l processes supplying cations re- quired for life support system in soil, and produces various clay minerals such as gibbsite, halloysite, ka- olinite, smecti te or illite as an end product of the re- actions (Lodding 1972; Al len and Hajek 1989; Hsu 1989; Kawano and Tomita 1994). The first stage of the reactions is dissolution, which has been extensive- ly studied experimental ly in laboratory. It is well- known that the dissolution proceeds nonstoichiometri- cally throughout the reaction depending on the pH of solutions (Casey et al. 1988, 1989; Hel lmann et al. 1990; Nesbit t et al. 1991). Conversely, formation of the clay minerals has been simply considered f rom a v iew point of thermodynamics. Dissolut ion of feldspar proceeds and precipitation of clay minerals takes place when the concentrat ion of cations in solutions reaches saturation (Helgeson 1971). However , these consider- ations do not provide any information about the for- mation of a metastable phase as the earliest stage prod- acts.

Numerous studies on the end weathering products of feldspars have been carried out by the X R D tech- nique, however there are a few reports on the early stage noncrystal l ine products. Eggleton and Buseck (1980) observed development of 10 ,& layer structure and 10 ~ layer silicates at the surface of weathered K-feldspar f rom Australia using high-resolution trans- mission electron microscopy (HRTEM). They con- cluded that the materials may be precursors o f crys-

talline US mixed layer materials. Tazaki and Fyfe (1987a, 1987b) studied the earliest weathering prod- ucts of K-feldspar f rom Brazil using analytical elec- tron microscopy, and reported that Fe- and Si-rich high defect fibrous materials appeared at the earliest stages of weathering and transformed into crystal l ine hal loy- site. These studies provided new information concem- ing the formation processes of clay minerals during interaction o f feldspar with water under weathering conditions in which the reaction proceeded very slow- ly. But, more extensive study is required to clarify the interaction and the formation processes of the end weathering products. The present study describes such early weathering products of K-feldspar formed before the crystal l ine phases appear.

M A T E R I A L S A N D M E T H O D S

The material used in this study was K-feldspar mega- crysts f rom granitic rock, dated 13 to 14 Ma (Shibata and Nozawa 1968), f rom Yakushima Island, Kagoshi- ma Prefecture, Japan. A large number of the mega- crysts were col lected f rom a cut of a path through a forest at the south of the Yakushima Land. The K-feld- spars contained many inclusions of bioti te and quartz, which are significantly concentrated at the surface of the megacrysts . In this study, a K-feldspar sample con- taining a lesser amount of inclusions was carefully hand-selected. The sample was slightly weathered, and exhibited hard and roughly smooth surfaces on which weathering products were hardly recognized by the na- ked eye.

Copyright �9 1996, The Clay Minerals Society 672

Vol. 44, No. 5, 1996 The earliest weathering products of K-feldspar 673

Figure 1. SEMs of the surface of weathered K-feldspar be- fore A) and after B) ultrasonic cleaning. A) = aggregates of gibbsite, halloysite and amorphous aluminum hydroxide. B) = spherical particles of amorphous aluminum hydroxide.

The selected K-feldspar sample was crushed into several pieces. One of the pieces was used for scan- ning electron microscopy (SEM) and energy disper- sive X-ray analysis (EDX) to investigate weathered surface before cleaning, the other pieces were cleaned ultrasonically and then used for SEM, EDX and X-ray photoelectron spectroscopy (XPS). The <2 ~m size fractions in the suspension obtained by ultrasonic treatment were collected by sedimentation and centri- fugation, and were used for X-ray powder diffraction (XRD) and transmission electron microscopy (TEM).

The SEM was carried out with a HITACHI S-4100 scanning electron microscope equipped with EDX fa- cilities operated at an accelerating voltage of 15 kV.

XPS analysis was determined with a SHIMADZU ESCA-1000 instrument using samples mounted on a carbon holder. The Mg-Kcx radiation (8 kV, 20 mA) was used as an X-ray source. The TEM was performed with a JEOL 200FX transmission electron microscope equipped with EDX facilities (ultra thin window) op- erated at an accelerating voltage of 200 kV using a carbon coated sample deposited on a Cu grid covered with a collodion film. The XRD profile of the fractions was obtained using a sample deposited on glass slides with a RIGAKU RU-200 diffractometer (Cu-Kct ra- diation, 30 kV, 100 mA) equipped with a graphite monochrometer.

RESULTS AND DISCUSSION

SEM of the K-feldspar surface before ultrasonic cleaning showed some aggregates of weathering prod- ucts composed of micrometer to sub-micrometer size materials exhibiting irregular to spherical forms (Fig- ure 1A). EDX indicated that the aggregates consisted mainly of A1 and Si, and small amounts of K and Fe (Figure 2A). Gibbsite, halloysite, biotite and K-feld- spar were identified by XRD in the weathering prod- ucts collected by ultrasonic cleaning and centrifuga- tion (Figure 3). Small amounts of K and Fe detected by the EDX are probably due to the K-feldspar and biotite. The gibbsite and halloysite must be weathering products of K-feldspar, whereas the biotite came from inclusions of K-feldspar megacryst. After ultrasonic cleaning, spherical products less than 1.0 Ixm in di- ameter appeared on the K-feldspar surface (Figure 1B). EDX indicated that the products consisted mainly of A1, and a very small amount of Si (Figure 2B).

The TEM of the weathering products collected by ultrasonic cleaning showed large amounts of lath- shaped gibbsite (Figure 4A) and tubular halloysite (not shown), both are common weathering products from feldspars (Lodding 1972; Tazaki 1976, 1979; Wilke et al. 1978; Anand et al. 1985). However, significant amounts of spherical particles (Figure 4B) and fiber ag- gregates (Figure 5) were clearly observed within the products. These spherical products gave similar EDX spectrum to the spherical products that appeared on the K-feldspar surface (Figure 2B), and also gave a diffuse electron diffraction halo. These results indicate that the spherical particles are noncrystalline aluminum hydrox- ide formed by the K-feldspar weathering processes. Conversely, the fibrous material (Figure 5) exhibited roughly mottled texture with partly circular forms less than 0.02 p~m in diameter and gave a diffuse electron diffraction halo. The EDX of the fibers displayed a strong A1 peak and very small Si and Fe peaks (Figure 6). These results also indicate that the fibers must be amorphous aluminum hydroxide containing small amounts of Si and Fe. Generally, amorphous aluminum hydroxide prepared by neutralization of acid solutions containing A1 commonly exhibits fibrous habit, which

674 Kawano and Tomita Clays and Clay Minerals

A

Si

K Fe

o 2 ,i g Energy (KeV)

Ai B

Si L

- l " 7 ~ I I 1 I I

0 ~' 4 6 8 10 Energy (KEY)

Figure 2. Energy dispersive X-ray spectra of weathering products on the surface of K-feldspar before A) and after B) ultrasonic cleaning. The spectra were obtained from weath- ering products which are shown in Figures 1A and B, re- spectively.

is similar to the present fibrous material (Watson et al. 1957; Violante and Huang 1993). Conversely, spherical particles of amorphous aluminum hydroxide are known to be produced, within a few hours, from a solution containing SO42- by the urea method (Brace and Ma- tijevi6 1973; Sift et al. 1989; Matijevi6 1985). Shi et al. (1989) explained that the strong attraction between A13+ and SO42- would prevent A13+ and OH- from first nu- cleation, but the weaker attraction between A13+ and NO3- or C1 would allow rapid nucleation. Similarly, the present spherical material can simply be considered

I I I

2 10 20 30 40

Degree 2 0 CuK a Radiat ion

Figure 3. XRD profile of weathering products on the surface of K-feldspar collected by ultrasonic cleaning. The weather- ing products identified bYo XRD are gibbsite (4.95 A) and halloysite (7.15 and 4.40 A). Very small fractions of K-feld- spar (6.67, 4.40 and 3.17 A) and biotite (10.01 ,A,) are also contained in this sample.

formed from a solution containing SO42-, which may be supplied from acid rain.

The surface compositions of the feldspars that reacted experimentally with acid solutions tend to show a de- crease in alkali cations and AI, and an increase in Si (Casey et al. 1988, 1989; Hochella et al. 1988; Muir et al. 1989, 1990; Petit et al. 1989; Nesbitt et al. 1991). On the contrary, Al-enriched layers at the surface of natu- rally weathered feldspars have been confirmed by EDX (Nixon 1979) and secondary ion mass spectroscopy (Nesbitt and Muir 1988). Similar leaching behaviors of hornblende in natural environments were also recognized by Auger electron spectroscopy (Mogk and Locke 1988). These differences of the leaching behaviors of feldspars have been considered a result of reaction conditions such as temperature, pH, compositions, concentrations of so- lutions and biological activity (Muir et al. 1990). Such changes in chemical composition of the feldspar surface were caused by preferential leaching of Si or A1, and formation of cation-depleted leached layers. At the sur- face of the present K-feldspar, formation of a leached layer was not confirmed by XPS, but precipitates of amorphous aluminum hydroxide showing fibrous and spherical forms, which may be a precursor of crystalline gibbsite, were clearly observed.

It is well known that amorphous aluminum hydrox- ide transforms into crystalline materials such as gibbs- ite, bayerite, nordstrandite or boehmite by prolonged aging, depending strongly on pH, temperature and concentration of organic and inorganic ligands present in solution (Barnhisel and Rich 1965; Hsu 1966, 1989; Taylor 1987; Violante and Huang 1985, 1992, 1993; Violante et al. 1993). Gibbsite, bayerite and nordstran- dite have a similar structure composed of basic sheets of close packed hydroxyls with A1 in octahedral co-

Vol. 44, No. 5, 1996 The earliest weathering products of K-feldspar 675

Figure 5. TEM and electron diffraction pattern of amorphous aluminum hydroxide exhibiting curled or circular fibrous forms produced by the earliest stages of K-feldspar weathering.

Figure 4. TE/Vls of weathering products of K-feldspar. A) = aggregate of lath-shaped gibbsite. B) = spherical particle of amorphous aluminum hydroxide.

ordination with different stacked sequences (Taylor 1987). However bayerite and nordstrandite crystallize rapidly under extreme alkaline conditions, but gibbsite has a tendency to form in acid solutions (Schoen and Roberson 1970; Barnhisel and Rich 1965; Hsu 1989). In addition, gibbsite has great stability relative to boehmite in the system of A1203-HzO at 25 ~ and 1 atm (Chesworth 1972). These facts suggest that the gibbsite that appeared in the present weathering prod- ucts must be transformed from the amorphous alumi- num hydroxide.

CONCLUSIONS

Weathering products adhering to the surface of K-feldspar collected from Yakushima Island, Japan

A I

Si Cu Fe

11 ~ i i 0 2 6 8 10

Energy (KeV) Figure 6. Energy dispersive X-ray spectrum of amorphous alu- minum hydroxide exhibiting fibrous habit as shown in Figure 5.

676 Kawano and Tomita Clays and Clay Minerals

were examined. X R D conf i rmed that the weather ing

products were com posed primarily of gibbsi te and hal- loysite. The SEM, TEM and E D X results clearly showed format ion o f amorphous AI(OH)3 exhibi t ing 2 dist inct habits: 1) spherical habit less than 1.0 Ixm in diameter, and 2) curled fibrous or circular forms less

than 0.02 Ixm in diameter. These amorphous AI(OH)3 are probably formed at the earliest weather ing stages o f K-fe ldspar as a metastable phase and t ransformed into a stable phase of gibbsi te as reactions proceeded.

A C K N O W L E D G M E N T S

We would like to thank T. Kakoi (Kagoshima University) for the TEM technical assistance. This study was partially supported by a grant from Research Center for the South Pacific, Kagoshima University.

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(Received 16 May 1995; accepted 3 January 1996; Ms. 2650)