SREL Reprint #2220
GOETHITE MORPHOLOGIES INVESTIGATED VIA X-RAY DIFFRACTION OF ORIENTED SAMPLES
ROBERT G. FORD, PAUL M. BERTSCH AND JOHN C. SEAMAN
University of Georgia, Advanced Analytical Center for Environmental Sciences, Savannah River Ecology Laboratory, Aiken, South Carolina 29802
INTRODUCTION
Synthetic and natural goethites display a variety of particle morphologies that are dependent on solution conditions during formation (Schwertmann 1990). Factors influencing goethite morphology include pH and the presence or absence of specifically adsorbing ions in solution. Particle morphology can affect goethite dissolution behavior and will govern the distribution of coordination sites available on the particle surface (Comell et a]. 1974; Colombo et al. 1994). Morphological information can be obtained via electron microscopic imaging (Schulze and Schwertmann 1984) or profile analysis of X-ray diffraction (XRD) patterns (Koch et al. 1986). These methods are essential for determination of absolute particle dimensions, but they may require a significant time conunitment or advanced level of analysis. In order to facilitate rapid preliminary comparison of synthetic goethite samples, we have investigated the use of powder XRD to derive similar qualitative information via the influence of preferred orientation during sample preparation.
Samples for XRD analysis were prepared according to the procedure outlined by Drever (1973) with some modifications. This and similar methods are commonly used to prepare clay powder samples displaying enhanced relative intensities in basal reflections (Brown and Brindley 1980). Filtering clay minerals from suspension leads to the formation of films of particles preferentially oriented with their basal planes parallel to the plane of the support. This preparation results in a sample with a biased distribution of crystallographic planes (in this case 001) coincident with the diffracting plane of the XRD instrument. This is manifested inthe diffraction pattern as enhanced and suppressed intensities for basal and nonbasal reflections, respectively. In principle, the tendency towards preferred orientation could be used for preliminary comparison of any suite of minerals displaying systematic morphological differences due to the conditions of formation.
Several authors have noted the effects of preferred orientation on XRD intensities in patterns of goethites synthesized under various conditions even after attempts to achieve a random distribution (Landa and Gast 1973; Cornell et al. 1974; Brown 1980; Schulze and Schwertmann 1984; Schwertmann et al. 1985). The effect most often observed is a change in the relative intensity of the (I 10) and (I I 1) Bragg reflections compared to that predicted for a random sample (Schulze and Schwertmann 1984; Schwertmann et al. 1985). In addition, preferred orientation in some natural goethites is expected based on published electron micrographic images (Rozenson et al. 1982; Schwertmann 1990). In an attempt to derive the maximum amount of information from limited sample quantities, we have employed a method for powder XRD analysis which takes advantage of the preferred orientation phenomenon.
Key Words- Goethite, Oriented Film, Particle Morphology, Powder X-ray Diffraction, XRD.
SREL Reprint #2220
Ford, R. G., P. M. Bertsch, and J. C. Seaman. 1997. Goethite Morphologies Investigated via X-ray Diffraction of Oriented Samples. Clays and Clay Minerals 45:769-772
