Allophane and imogolite neogenesis in soils may occur in the presence of organic matter. To understand this process under conditions relevant to soils, the influence of dissolved organic carbon (DOC) as humic acid (HA), on aluminosilicate formation was studied at , pH 6, and low-DOC concentrations. For solutions with initial Al/Si ratios of 1–2.1, and 0–6？mg/L DOC, precipitates recovered after 20？h had Al/Si ratios of 2.2–2.7. The formation of allophane, imogolite-like material, and aluminosilicate gel was confirmed by XRD, FTIR, and NMR. The effect of DOC was to produce a small, but systematic increase in imogolite-like Si in the precipitate, and a decrease in the formation of aluminosilicate gel. Results suggest that the presence of DOC as HA slows the otherwise rapid polymerization of Al and Si at low temperature, and may also promote the formation of imogolite. The high C content of these precipitates indicates that this process may facilitate the sequestration of organic matter, slowing C cycling in soils. 1. Introduction Complexation of Al with natural organic matter is important for the development of soils, influencing both the weathering and neogenesis of Al-bearing minerals [1] including aluminosilicate clays. The metastable clay minerals allophane and imogolite, when present, may control the solubility of both Al and Si relative to more stable phases such as gibbsite and kaolinite [2]. Allophane and imogolite are poorly crystalline hydroxyaluminosilicates with Al/Si ratios ranging from 0.5–4. They are composed of Si and Al polyhedral layers shaped into hollow tubes and spheres [3]. Imogolite, (OH)3Al2O3SiOH [4], which has long-range order, is tubular in shape and has an Al/Si ratio of 2. Allophane has short-range order and forms hollow spherules with a range of Al/Si ratios. Al-rich allophane is also called protoimogolite or imogolite-like allophane and has an Al/Si ratio of ~2 [4]. Imogolite and allophane occur frequently as, but are not limited to, what are termed allophanic horizons in volcanic soils such as andisols and spodosols [5]. Their occurrence is contingent upon conditions such as soil pH, the availability of sufficient Si, and the presence of organic material, which can either inhibit the formation of [5], or conversely, stabilize these minerals [6]. The large surface area and high reactivity of these minerals promote complexation with organic matter [6], the essential soil nutrients P and N, and toxic contaminants such as As, Cd, Cu, and Pb [7, 8]. Laboratory synthesis of aluminosilicate phases in the presence of organic
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