Accurate crystal chemistry of ferric smectites from

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ABSTRACT: Lateritic weathering profiles developed on serpentinized peridotites of Murrin ... KEYWORDS: weathering, ultrabasic rock, smectite, crystal chemistry, nickel, XRD, Mцssbauer, ICP, SEM,. TEM-EDX. ...... Mineralogical Magazine,.
Clay Minerals (2004) 39, 301–315

Accurate crystal chemistry of ferric smectites from the lateritic nickel ore of Murrin Murrin (Western Australia). I. XRD and multi-scale chemical approaches A. GAUDIN1,3,*, O. GRAUBY2, Y. NOACK3, A. DECARREAU4

AND

S. PETIT4

1

CNRS-UMR 6112, Laboratoire de Plane´tologie et Ge´odynamique, Faculte´ des Sciences et Techniques, Universite´ 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, Universite´ Aix-Marseille III, Europoˆle Me´diterrane´en de l’Arbois, BP 80, 13545 Aix-en-Provence Cedex 04, France, and 4 CNRS-UMR 6532, Laboratoire HydrASA, Faculte´ des Sciences, 86022 Poitiers Cedex, France

(Received 27 August 2003; revised 5 May 2004)

AB ST R ACT : 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 Mo¨ssbauer 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 two, 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, Mo¨ssbauer, ICP, SEM, TEM-EDX.

The thick (>30 m) lateritic profiles of Murrin Murrin, Western Australia (~121º50’00’’E and 28º40’00’’S), developed on serpentinized peridotites, have been worked as a nickel and cobalt ore since 1999 (Fig. 1). After Camuti & Gifford (1997), a typical Murrin Murrin lateritic profile comprises, at the bottom, a saprolite zone (dominated by serpentine with smaller amounts of smectite, chlorite and maghemite), overlain by a smectite zone of 10 15 m * E-mail: [email protected] DOI: 10.1180/0009855043930136

thick (smectite and traces of maghemite and chlorite), and capped by a ferruginous zone (predominantly goethite, kaolinite and small amounts of smectite). From the bottom of the profile to the top, the increased weathering is accompanied by a progressive disappearance of the initial mesh texture of the parent rock. However, relict textures subsist even at the top of the smectite zone. Weathering profiles are disturbed by numerous fissures and cracks leading locally to greater weathering of the surrounding rocks. In the saprolite, Camuti & Gifford (1997) described Mg-rich smectites (Ni: 0.5 5%) repla-

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A. Gaudin et al.

FIG. 1. Geological map of the Murrin Murrin site (after Hallberg, 1985). Location of deposits (Monti & Fazakerley, 1996) and of the cbdd5 crop. MM1 9 areas correspond to the worked zones.

Crystal chemistry of ferric smectites

cing the serpentinized olivine grains; the smectite zone being wholly dominated by nontronites (Ni: 1 4%) with locally, rare beidellites. The ferruginous zone is dominated by kaolinite and goethite. This mineralogical and chemical zonation appears common when compared to other weathering profiles of ultrabasic rocks reported in the literature (Delvigne et al., 1979; Fontanaud, 1982; Nahon & Colin, 1982; Nahon et al., 1982a,b; Paquet et al., 1983; Colin et al., 1985, 1990) and is consistent with geochemical modellings (Noack et al., 1993). A smectite zone between the saprolite and the ferruginous zone is often reported in weathering profiles in numerous countries: Cuba (Linchenat & Shirokova, 1964), Indonesia (Ku¨hnel et al., 1978), Brazil (Esson & Santos, 1978; Colin et al., 1985, 1990; De Oliveira & Trescases, 1992), Ivory Coast (Nahon & Colin, 12982; Nahon et al., 1982a), Australia near Murrin Murrin (Elias et al., 1981) or in the Queensland region (Zeissink, 1969; Golightly, 1981). The development of such a smectite zone is connected with limited cation leaching, due to either a tropical wet-dry climate and/or impeded drainage (Golightly, 1979, 1981). On the basis of XRD data and bulk chemical analyses, the main clay mineral of the smectite zone is described as nontronite (Linchenat & Shirokova, 1964; Elias et al., 1981; Golightly, 1981; Paquet et al., 1981, 1983; Nahon et al., 1982b). The present study focuses on the detailed mineralogy and crystal chemistry at various scales (from rock sample to the clay particle scale) of smectites from the smectite zone from the Murrin Murrin weathering profiles. Another aim of this study was to specify the actual location of nickel in the smectite structure. The Murrin Murrin weathering profiles were chosen because, according to the previous studies (Camuti & Gifford, 1997), smectites are in great abundance and show apparently large chemical differences.

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outcrop, 9 km apart, are now worked and designated Murrin Murrin North and South (Fig. 1). Material was collected from the smectite zones of the cbdd5 core (121º53’41’’E, 28º44’51’’S) and of the MM2 pit, in the Murrin Murrin North area (Figs 1 and 2). The unweathered parent rock was not reached. In the saprolite and in the smectite zone the initial mesh texture of the serpentinized peridotites is always observed. Smectites can occur either replacing the original polygonal serpentinized olivine grains (plasma smectite) or in the fissures (fissure smectite). The 0.18 and 0.18) randomly mixed with H.K. non-swelling layers with low tetrahedral charges (between 0 and 0.18). The proportion of H.K. swelling layers was determined from the experimental position of the H.K. (002)/(001) reflection (Table 2) and a calculated curve of d002/001 values vs. amount of swelling layers using NEWMOD simulations. The amounts of H.K.

TABLE 2. Layer charges and % of octahedral charge of the Murrin Murrin smectites calculated from IR data (CEC: cation-exchange capacity); XRD data: positions of the (001) and (002)/(001) reflections of. Murrin Murrin glycolated and H.K. treated samples obtained from DECOMPXR decompositions and, % of H.K. swelling layers (tetrahedral charge > 0.18) (XRD data) calculated from the d002/001 values. Plasma smectites Samples

5-25

Fissure smectites

5-19

5-23

5-36

IR data CEC (mEq/100 g) Layer charge Octa. charge (%)

89 0.37 42

89 92 98 0.37 0.39 0.42 43 55 59

XRD data after H.K. ˚ d001 A ˚ d002/001 A % H.K. swelling layers

17.03 9.06 52

18.05 17.82 17.91 9.38 9.27 9.17 36 41 46

5-19F 5-36F 5-37F 5-44F 5-46F 2-17F 2-21F 2-22F

100 93 101 89 99 102 102 91 0.41 0.39 0.40 0.37 0.40 0.42 0.42 0.38 66 54 61 61 56 61 50 62

17.87 17.4 18.00 9.08 8.95 9.12 51 61 49

.

.

.

17.61 9.09 51

17.81 18.02 17.81 9.19 9.26 9.13 45 41 48

Crystal chemistry of ferric smectites

FIG. 5. d values of the fitted (001) and (002)/(001) reflections of the 2-17 sample after H.K. treatment and glycolation, using the DECOMPXR program (background substracted).

swelling layers obtained for Murrin Murrin smectites were between 36 and 61% of the total (Table 2). The proportion of H.K. swelling layers for plasma and fissure smectites are similar. As both tetrahedral charges and the amount of H.K. swelling layers vary little for these samples, no correlation was found between these two parameters.

Morphology After TEM observations, plasma and fissure smectite particles appear to be composed of irregularly flat and fluffy layers with diffuse boundaries (Fig. 6). Their size varies between 0.1 and 2 mm. This flake morphology differs from the lath morphology often described for the Fe-rich smectites (Gu¨ven, 1988; Badaut et al., 1992; Petit et al., 2002). However, this morphology was observed previously for nontronites from other Australian weathering profiles and was considered as indicative of extensive weathering (Keeling et al., 2000). Microdiffraction patterns were pseudoannular, sometimes slightly punctuated. Such patterns are indicative of turbostratic disorder. Given the very fine particle size and poor crystallinity of the smectite particles, beam damage can affect some of the TEM-EDX analyses (Ma et al., 1998). No other phase, particularly no Ni-enriched phase, was detected.

Multi-scale chemical characterization Chemical analyses by ICP of Ca-saturated s m ec t it e s a m p le s ar e g iv e n in T a b l e 3 .

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FIG. 6. TEM images of a smectite particle of sample 5-36.

Compositions of the plasma smectites are relatively homogeneous and Fe-rich (the high Mg content of sample 5-19 is probably due to the occurrence of a small amount of chlorite). The fissure smectites are Fe-rich also but exhibit greater Al contents than the plasma smectites. Finally, significant Ni contents are detected in the two types of Ca-saturated smectites showing that most Ni is located in the layers of the smectites. Taking into account previous data on Ni-rich clay minerals, Ni is probably located within the octahedral sheet of smectites (Manceau & Calas, 1985, 1986; Decarreau et al., 1987). The nature of the interlayer cations was studied for some samples (Table 3). The dominant interlayer cation is always Mg2+ but significant amounts of Ca2+, K+ and Na+ occur. The amounts of interlayer Ni are always