Structure and distribution of allophanes, imogolite and proto-imogolite in volcanic soils C. Levard a, b , E. Doelsch c, ⁎, I. Basile-Doelsch a , Z. Abidin d, e , H. Miche a, b , A. Masion a, b , J. Rose a, b , D. Borschneck a, b , J.-Y. Bottero a, b a CEREGE, CNRS, Aix-Marseille University, IRD, College de France, Europôle Méditerranéen de L'Arbois, BP 80, 13545 Aix en Provence, France b iCEINT, international Center for the Enviromental Implications of NanoTechnologies, Europôle de l'Arbois, 13545 Aix-en-Provence, France c CIRAD, UPR Recyclage et risque, F-34398 Montpellier, France d Laboratory of Applied Chemistry for Environmental Industry, Faculty of Agriculture, Ehime University. Tarumi 3-5-7, Matsuyama shi, Ehime, 790-8566, Japan e Laboratory of Inorganic Chemistry, Faculty of Mathematics and Science, Bogor Agriculture University, Kampus Darmaga IPB, Jl, Meranti Darmaga, Bogor, West of Java, 16680, Indonesia abstract article info Article history: Received 5 July 2011 Received in revised form 29 February 2012 Accepted 11 March 2012 Available online 8 May 2012 Keywords: Short-range aluminosilicate Andosol Allophane Imogolite Réunion Despite half a century of intensive research, the structure and presence of short-range ordered aluminosili- cates in volcanic soils is still the focus of debate. Imogolite is easily distinguishable because of its tubular structure, whereas allophane compounds—usually described as spheres—are harder to identify, especially be- cause of their variable structure and occurrence patterns. In addition, the local structure of allophanes can be very similar to that of proto-imogolite (imogolite precursor). Strangely, this similarity is seldom considered in most characterization studies. In this context, our study focuses on the structure of two Al-rich short range-ordered aluminosilicates of two different origins, from: (i) an Andosol B horizon (Andosol sample); and (ii) a weathered pumice grain (pumice sample). These natural samples were compared to a synthetic proto-imogolite. The three samples were analyzed using experimental tools that are commonly used for the identification of these nanophases (chemical composition, X-ray diffraction, nuclear magnetic resonance, Fourier transform infrared spectroscopy and transmission electron microscopy). The three samples exhibited the same local structure, but significant differences were observed at a larger scale. The pumice sample clear- ly showed ring-shaped particles, while the Andosol sample and the synthetic proto-imogolite were amor- phous. Our results suggest that poorly ordered proto-imogolite, rather than Al-rich allophones and imogolites, is present in Andosol horizons. We believe that the mineralogy of these phases needs to be reas- sessed along with their growth mechanisms. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Imogolites and allophanes are naturally occurring nanoparticles formed through the alteration of volcanic parent materials. These par- ticles are known to play a significant role in the physico-chemical properties of volcanic soils, in terms of organic matter (OM) storage (Basile-Doelsch et al., 2005; Basile-Doelsch et al., 2007; Parfitt et al., 1999), phosphorus (Parfitt, 1989), heavy metal (Doelsch et al., 2006; Levard et al., 2009a), water retention (Khan et al., 2006) and biogeo- chemical (Parfitt, 2009) processes.(Cradwick et al., 1972; Farmer et al., 1977b). Imogolites are aluminosilicate single-walled nanotubes (Al 2 SiO 7 H 4 ) of 2–3 nm diameter and a few hundred microns long (Cradwick et al., 1972; Farmer et al., 1977b)(Table 1). Imogolite is composed of a curved gibbsite layer on the outer surface and Si tetrahedra monomers (Q 0 environment (Barron et al., 1982) linked to six Al octahedra inside the vacancies. Hereafter, we call this specific local silicon configuration “imogolite local structure” (ILS) (Fig. 1A). The growth mechanisms of these nanotubes were investigated using several complementary tech- niques, including transmission electronic microscopy (TEM) and dy- namic light scattering (DLS) (Yang et al., 2008). Moreover, improved protocols and synthesis of aluminogermanate analogues (Al 2 GeO 7 H 4 ) with higher electronic density has facilitated characterization of mech- anisms involved in the formation of imogolite type structures (Levard et al., 2008; Levard et al., 2011; Maillet et al., 2010; Maillet et al., 2011; Mukherjee et al., 2005; Mukherjee et al., 2007; Wada and Wada, 1982). For the first time, models of Ge-imogolite precursors (commonly referred to as proto-imogolites) were thus proposed that consist of roof tile shaped nanoparticles, up to 5 nm in size, with Ge vacancies and varying curvatures (Levard et al., 2010). The local proto-imogolite order was proved to be very similar to that of ILS. Hereafter, all short- range ordered alluminosilicates with an ILS will be called imogolite- type materials (ITM). Geoderma 183–184 (2012) 100–108 ⁎ Corresponding author. Tel.: +33 4 42 97 75; fax: +33 4 42 97 15 59. E-mail address: doelsch@cirad.fr (E. Doelsch). 0016-7061/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2012.03.015 Contents lists available at SciVerse ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma