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Accurate crystal chemistry of ferric smectites from the lateritic nickel ore of Murrin Murrin (Western Australia). I. XRD and multi-scale chemical approaches

¹ CNRS-UMR 6112, Laboratoire de Planétologie et Géodynamique, Faculté des Sciences et Techniques, Université de Nantes, BP 92208, 44322 Nantes Cedex 03, ² CRMC2, CNRS-UPR 7251, Campus de Luminy, Case 913, F-13288 Marseille Cedex 09, ³ CEREGE, CNRS-UMR 6635, Université Aix-Marseille III, Europôle Méditerranéen de l’Arbois, BP 80, 13545 Aix-en-Provence Cedex 04, France, and ⁴ CNRS-UMR 6532, Laboratoire HydrASA, Faculté des Sciences, 86022 Poitiers Cedex, France

^* E-mail: anne.gaudin{at}chimie.univ-nantes.fr

(Received 27 August 2003; revised 5 May 2004)

ABSTRACT

Lateritic weathering profiles developed on serpentinized peridotites of Murrin Murrin (Western Australia) exhibit thick smectite zones (10–15 m). The smectites from plasma and fissures were characterized by XRD, chemical analyses (ICP-AES, SEM-EDX and TEM-EDX) and Mössbauer spectroscopy. These Fe-rich smectites, previously described as nontronites, are in fact more complex. Their layer charges originate from both the tetrahedral and octahedral sheets. Plasma and notably fissure smectites exhibit, from the bulk sample scale to the particle scale, large and continuous Al for (Fe+Cr) substitutions, covering a chemical gap previously described for dioctahedral smectites ranging between nontronite and beidellite end-members. Lastly, they exhibit an octahedral occupancy slightly above 2, due to a low (Mg+Ni) trioctahedral contribution. Thus, the smectites occurring in weathering profiles of ultrabasic rocks can have actual chemistries intermediate between four dioctahedral end-members (beidellite, nontronite, montmorillonite and previously rarely described ferric-montmorillonite) and a trioctahedral one ((Mg+Ni)-saponite).

KEYWORDS: weathering, ultrabasic rock, smectite, crystal chemistry, nickel, XRD, Mössbauer, ICP, SEM, TEM-EDX

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