An AEM-TEM study of nanometer-scale mineral associations in an aquifer sand: Implications for colloid mobilization
Analytical and transmission electron microscopy (AEM-TEM) techniques were used to identify mineral juxtapositions at the nanometer-scale in the interstitial matrix of a shallow, Southeastern Coastal plain aquifer sand (Georgetown, South Carolina, USA). In doing so, we sought to infer particle-particle interaction mechanisms holding the matrix intact. The aquifer is a fine-to-medium quartz sand with approximately 12% by weight <63 μm size fraction composing the interstitial matrix. The clay-size fraction contains kaolinite, goethite, gibbsite, and vermiculite. The arrangement of the clay minerals is that of a framework of face-associated domains. Selective extraction revealed that goethite constituted 95% by weight of the free iron oxyhydroxides in the <63 μm size fraction, but AEM-TEM and high resolution TEM (HRTEM) indicated that the goethite occurred only in discrete aggregates among the clays. Conversely, the remaining 5% of the free iron oxyhydroxides comprised an amorphous iron phase which was found to be distributed throughout the matrix and directly associated with the clay particles. This evidence suggests that the amorphous iron oxyhydroxide phase could act as an effective binding intermediary among the clay-clay associations, possibly electrostatically or through bond linkages with the clay surfaces. In addition, HRTEM indicated the presence of another amorphous phase which appeared to hold the clay particle aggregates in a cementitious web. AEM suggested that this amorphous phase was silicon enriched, probably biogenic opal. This evidence suggests that instigating dissolution of the opaline silica phase may be necessary to induce substantial colloid mobilization in this aquifer sediment.