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United States Department of Agriculture

Agricultural Research Service

Title: An Aem-Tem Study of Nanometer-Scale Mineral Associations in An Aquifer Sand: Implications for Colloid Mobilization

Authors
item Swartz, Christopher - MIT
item Ulery, April - U. C. RIVERSIDE
item Gschwend, Philip - MIT

Submitted to: Geochimica et Cosmochimica Acta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 3, 1996
Publication Date: N/A

Interpretive Summary: Particle to particle interactions and relationships were studied using high resolution transmission and analytical electron microscopy. This allows us to determine the arrangement of individual minerals in a Southeastern Coastal Plain aquifer sand from Georgetown, SC. The aquifer is mostly quartz sand with about 12% silt- and clay-sized materials, including kaolinite, goethite, gibbsite, and vermiculite. Goethite is an iron oxide mineral that occurs as 95% of the coatings on the larger sand grains and was also present as discrete aggregates among the clays. There was also some amorphous, or noncrystalline, iron oxide and biologically derived opal coating the sand grains and associated with the clay particles. These amorphous materials tend to act like an electrically charged cement or web, holding the individual clay particles together. Changes in chemistry, especially pH and ionic strength, may alter the cohesive nature of these coatings and cause the release of colloids, or extremely fine particles. Colloid mobilization or release is an important area of study in environmental sciences because radioactive, heavy metal, or organic contaminants are often closely associated with the colloids.

Technical Abstract: Analytical and transmission electron microscopy (AEM-TEM) techniques were used to identify the mineral juxtapositions at the nanometer-scale in the interstitial matrix of a shallow, Southeastern Coastal plain aquifer sand (Georgetown, SC). 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 silt- to clay-size 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 of the surface iron revealed that goethite constituted 95% percent by weight of the surface iron in the >63 um size fraction, but TEM/HRTEM indicated that the goethite occurred only in discrete aggregates among the clays. Conversely, the remaining 5% of the surface iron 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 oxide phase could act as the effective electrostatic intermediary among the clay-clay associations. In addition, HRTEM indicated the presence of another amorphous phase which appeared to hold the clay particles in a cementitious "web". AEM suggested that this amorphous phase was silicon enriched, probably biogenic opal.

Last Modified: 12/21/2014
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