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1 Departamento de Química Inorgánica, Cristalografía y Mineralogía, Facultad de Ciencias, Campus de Teatinos, Universidad de Málaga, Spain
2 Instituto Andaluz de Ciencias de la Tierra, CSIC-Universidad de Granada, Facultad de Ciencias, 18071 Granada, Spain
* E-mail address of corresponding author: mdruiz{at}uma.es
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Abstract |
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 TOP Abstract INTRODUCTIONGEOLOGICAL SETTING AND MATERIALSANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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Key Words: Betic Cordillera • Dickite • Illite • Mixed-layer • Pyrophyllite • Spain • Sudoite
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INTRODUCTION |
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TOPAbstractINTRODUCTIONGEOLOGICAL SETTING AND MATERIALSANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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Although sudoite was initially recognized in association with ore deposits (e.g. Bailey and Tyler, 1960; Hayashi and Oinuma, 1964; Sudo and Sato, 1966), it has also been reported in diagenetic and low-grade metamorphic terrains (e.g. Fransolet and Bourguignon, 1978; Daniels and Altaner, 1990; Livi et al. 2002; Theye and Siedel, 1993). In some cases it is associated with low-temperature, high-pressure assemblages (Theye et al., 1992).
In the Betic Cordillera, sudoite has been identified in Triassic sequences from the transition Maláguide-Alpujárride Complexes (Abad et al. 2003; Lázaro et al. 2003; Ruiz Cruz et al., 2005). Sudoite is common in fine-grained rocks (red lutites – typical from the Maláguide complex, and blue phyllites – typical of the Alpujárride complex), and less frequent in red sandstones and conglomerates interbedded with red lutites in rocks lithologically similar to the Maláguide complex.
Nevertheless, two contrasting interpretations have been made about the origin of sudoite in these Triassic sequences. In fine-grained rocks from the eastern part of the Cordillera (Sierra Espuña), Abad et al. (2003) interpreted sudoite as a retrograde product of tri-octahedral chlorite, based on transmission electron microscopic (TEM) observations. In contrast, in the central part of the Cordillera (Sierra Arana), Ruiz Cruz et al. (2005) interpreted sudoite as formed from dickite during the prograde stage of metamorphism. This interpretation was based on chemical analyses of bulk rocks and on textural evidence observed by optical microscopy.
This work summarizes the transmission/analytical electron microscopic (TEM/AEM) study of sudoite from several mineral associations observed in the sequences of Sierra Arana (Ruiz Cruz et al., 2005), and reveals the presence of sudoites with Fe contents greater than those previously described. In addition, this work shows that microscopic and submicroscopic sudoites include different types of interstratifications, and this probably explains the chemical variability observed among microprobe analyses of sudoite from different sources.
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GEOLOGICAL SETTING AND MATERIALS |
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TOPAbstractINTRODUCTIONGEOLOGICAL SETTING AND MATERIALSANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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The Maláguide complex includes sediments from Paleozoic to Tertiary ages. The Triassic sequences, characterized by the presence of red conglomerates, red sandstones, red lutites and minor carbonates, show a transition from low diagenesis to low anchizone (Ruiz Cruz and Rodríguez Jiménez, 2002). The Paleozoic sequences mainly consist of blue phyllites, limestones, and a greywacke-shale alternation. Two metamorphic stages have been identified in the Maláguide Paleozoic: the Hercynian and the Alpine (Mäkel, 1985). The Alpine parageneses range from the chlorite to the biotite zones (Ruiz Cruz & Rodríguez Jiménez, 2002). In addition, ‘intermediate units’ between the Maláguide and the Alpujárride complexes have been described in different zones of the Betic Cordillera (Sanz de Galdeano et al., 2001).
In the central area of the Betic Cordillera (Sierra Arana), the intermediate units, previously characterized by Sanz de Galdeano et al. (1995a, 1995b, 1995c), have been sampled in two sectors: Diezma and El Molinillo (Figure 1
). In both cases, the intermediate units appear as several tectonic slices (from several tens to several hundreds of meters thick), showing a progressive increase in metamorphic grade from the top to the base of the pile. These tectonic slices exhibit notable horizontal continuity. The uppermost tectonic slices show lithological characteristics similar to the typical Maláguide complex whereas increase in depth is characterized by the presence of intermediate lithologies, and finally, by lithologies typical of the Alpujárride complex, in the deepest slices.
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From this study, three samples with different lithologies and mineral associations were selected for TEM/AEM investigation.
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ANALYTICAL METHODS |
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TOPAbstractINTRODUCTIONGEOLOGICAL SETTING AND MATERIALSANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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For the TEM/AEM study, slices were removed from petrographic thin-sections and thinned to electron transparency by argon ion-milling, using a GATAN DUAL Ion Mill-600. Specimens were coated with carbon and examined using a 200 kV Jeol 2000 FX microscope, coupled with a Kevex Quantum X-ray energy-dispersion spectroscopic system (University Complutense, Madrid), and a Philips CM-20 transmission electron microscope, equipped with an EDAX solid-state EDX detector (University of Granada). Scanning TEM mode was used for quantitative analyses (AEM) of particles using a 40 Å diameter beam and variable scanning area, based on the particle size. Muscovite, albite, spessartine, olivine and titanite were used as standards to calculate K factors by the thin-film method of Lorimer and Cliff (1976).
The XRD patterns were obtained using a Siemens D-5000 powder diffractometer at Málaga University. For XRD analysis of the fine fractions, oriented samples which were air dried, solvated with ethylene glycol (EG), and heated (550°C) were used. Semi-quantification of the phyllosilicates involved the intensity factors of Islam and Lotse (1986). Randomly oriented powders of 2–20 µm and <2 µm size fractions were used for determination of b parameters of the phyllosilicates. The 211 quartz reflection (1.541 Å) was used as an internal standard for the exact determination of the position of the 060 reflections of the phyllosilicates. Oriented samples were scanned from 2 to 30°2
, using CuK
radiation at 35 mA and 40 kV, and a graphite mono-chromator (step size = 0.02°2 and counting time of 2 s), as recommended by Kisch (1991). Illite crystallinity (KI) was measured using the method of Kübler (1968), as the width of the 10 Å peak at half maximum peak height expressed in °2. The illite polytype was determined from the intensity ratio of the 2.80 and 2.58 Å mica peaks in oriented samples, following Maxwell and Hower (1967). Our KI measurements (y) were transformed into CIS values (x) (Warr and Rice, 1994) according to the equation y = 1.23x – 0.07. Chlorite crystallinity was determined from the 7 Å peak, under the same experimental conditions as the illite.
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RESULTS |
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TOPAbstractINTRODUCTIONGEOLOGICAL SETTING AND MATERIALSANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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) include a red conglomerate (CL-64) from the El Molinillo section and a red lutite (CL-26) and a red sandstone (CL-34) from the Diezma section. The mineral assemblage is different in these samples. In addition to quartz, Fe oxides (hematite and magnetite), and occasional albite and carbonate, these samples show the following phyllosilicate associations (Figure 2a):
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Assemblages 1 and 2 are typical, but not exclusive of the El Molinillo section, and appear well developed in coarse-grained rocks (conglomerates and sandstones) and in lutites, respectively. Assemblage 3 is typical of the Diezma section, and has been identified in both fine- and coarse-grained lithotypes. In coarse-grained rocks sudoite is observed in pores and veins, frequently replacing dickite. Nevertheless, most grains of sudoite show grain sizes insufficient for study by electron microprobe. In fine-grained rocks, sudoite is not discernible by optical microscopy.
The KI values of illite in sudoite-bearing rocks range from 0.31 to 0.48 in the Diezma section to 0.32–0.74 in El Molinillo. Samples CL-64 (KI = 0.48) and CL-34 (KI = 0.42) can be ascribed to the late diagenetic zone, whereas sample CL-26 shows a KI (0.39) typical of the low anchizone, according to the nomenclature by Merriman and Peacor (1999). On the other hand, the XRD patterns obtained from unoriented samples (not shown) reveal that the illite polytype 1M is dominant in the El Molinillo section and the polytype 2M1 in the Diezma section. In addition to typical K-illite, sample CL-26 contains intermediate Na-K-illite, as deduced from the presence of reflections at 3.26 Å and 1.98 Å (Frey, 1970; Jiang and Peacor, 1993).
Sudoite was identified in the XRD patterns according to the criteria by Eggleton and Bailey (1967) and Bailey and Lister (1989). The XRD patterns of sudoite usually show a distinctly more intense 003 reflection than trioctahedral chlorites, and the 060 reflection at intermediate positions between those of dioctahedral (1.49–1.50 Å) and trioctahedral (1.53–1.54 Å) chlorites. Given the presence of some trioctahedral chlorite together with sudoite in samples CL-26 and CL-34, the relative intensities of the basal reflections do not exactly correspond to those of sudoite (Anceau, 1992). The differences observed in the position of the 060 reflection of sudoite (Figure 2b
) can be related to different Fe contents. The XRD pattern of unoriented sample CL-26 shows 20l (13l) diffraction intensity distribution, characteristic of a IIb ordered polytype with b = 9.055 Å. The lower sudoite content in the coarse-grained samples has prevented the determination of the sudoite polytype by XRD. The b value in these samples ranges from 9.078 to 9.101 Å.
Other structural differences among the described sudoites affect the widths of the 00l basal reflections (crystallinity). Figure 3 shows the XRD patterns of the <2 µm size fractions of samples CL-26 and CL-34, in the zone 11–13°2. In sample CL-26, the 7 Å sudoite peak is narrow and symmetrical, whereas in sample CL-34, the sudoite peak is broad and asymmetrical, and, in addition, small peaks with higher d values are observed in this zone, the best defined at 7.78 Å. This pattern suggests the presence of interstratifications of sudoite and another phase with greater basal spacing.
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), or isolated and included in quartz. In sample CL-64, sudoite appears associated with illite or dickite, whereas in samples CL-26 and CL-34, sudoite appears inter-grown with illite or illite and pyrophyllite, respectively. In most cases, sudoite appears in the external parts of the composite packets, also in contact with quartz, suggesting a genetic link between quartz and sudoite.
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), displaying variable composition from K-rich particles to Na-K intermediate particles. Large crystals of sudoite show lattice-fringe images with regular 14 Å or 28 Å periodicities and SAED patterns characteristic of either one-layer or two-layer ordered polytypes (Figure 5a). The presence of 10 Å fringes interstratified in the sudoite structure is only rarely observed (Figure 5b). Some representative AEM data for sudoite from this sample are shown in Table 2. The formulae calculated for O10(OH)8 and assuming all Fe as ferrous reveal that the Si content ranges from 2.96 to 3.20. Sudoite is also characterized by a low Fe/(Fe+Mg) ratio, which ranges between 0.09 and 0.14. These values are close to those described for conventional sudoites (Billault et al. 2002, and references therein). Small amounts of Na or K in some analyses reveal the presence of mica layers interstratified in sudoite.
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). The SAED patterns obtained from these packets (Figure 6a, inset), show a 14 Å periodicity along c* and poorly defined reflections in the 0kl or hhl reflection rows, reflecting abundant stacking disorder. These packets show a variable composition as shown in the AEM data in Table 3 (analyses 1–6). The formulae calculated for O10(OH)8 and assuming all Fe as ferrous reveal that Si content ranges from 3.26 to 3.46 a.p.f.u. Although the Fe+Mg content varies considerably, the Fe/(Fe+Mg) ratio is rather constant (0.27–0.32) and greater than that measured in the fine-grained sample. The total octahedral occupancy ranges from 4.79 to 5.20, within the compositional range of sudoite (Bailey, 1980).
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, analyses 7–9), suggesting the presence of dickite/sudoite intergrowths or interstratifications. The lattice-fringe images from these areas show a uniform 14 Å periodicity. Nevertheless, medium magnification images occasionally permit the observation of thin packets (~200–500 Å thick) showing fringes with 42 and 56 Å periodicities (Figure 6b) which suggest the presence of interstratifications of one sudoite + two dickite (or two sudoite + one dickite) layers and two sudoite + two dickite layers.
Lattice-fringe images of sudoite from sample CL-34 show small areas with a 14 Å periodicity which evolve laterally toward areas with more complex structures. The AEM data obtained from the 14 Å areas show, however, a wide compositional variability which mainly affects the Si content and the Fe/(Fe+Mg) ratio (Table 4, analyses 1–6). The Si content ranges from 3.17 to 3.56 a.p.f.u., whereas the Fe/(Fe+Mg) ratio ranges between 0.37 and 0.59. These latter values are notably larger than the values commonly determined for sudoite (Newman and Brown, 1987). The total octahedral occupancy ranges between 5.02 and 5.32 when the formulae are calculated assuming all Fe as ferrous. These values are slightly higher than the ideal value for sudoite, suggesting that some Fe can be Fe3+. Indeed, previous studies of sudoite indicate that in oxidizing conditions, the Fe3+content can be high (e.g. Billault et al. 2002). With the aim of estimating the Fe3+/Fe2+ ratio, the sudoite analyses from this sample were also tentatively normalized to 14 oxygens and 9 cations (Table 4). Following this method, the formulae obtained indicate the presence of both Fe2+ and Fe3+ in variable proportions.
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). These packets show compositions characterized by small but persistent amounts of K (in the order of 0.1 a.p.f.u.), as observed in Table 4 (analyses 4–6). An increase in the proportion of mica layers interstratified in the sudoite structure generates mica-sudoite interstratifications with dominant 24 and 38 Å periodicities, corresponding to one sudoite + one mica layer and two sudoite + one mica layer, respectively (Figure 7b). The SAED patterns show a dominant 24 Å periodicity. The composition of these interstratified structures is characterized by an increase in K content (Table 4, analyses 7–9). The formulae have also been calculated for O10(OH)8. These formulae are unrealistic but permit the comparison with sudoite formulae. Chemical variability in these structures mainly affects the Fe/(Fe+Mg) ratio (0.44–0.62).
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). The SAED patterns obtained from these packets commonly show reflections of sudoite and pyrophyllite. Most of the analyses of these packets (Table 4, analysis 10–12) are characterized by an increase in Si content relative to sudoite and mica-sudoite interstratifications coupled with a decrease in Fe+Mg content. Some of these packets, however, show a low K-content, suggesting the presence of both pyrophyllite and mica layers inter-stratified with sudoite.
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). The composition is almost stoichiometric, with only 0.01 a.p.f.u. of Fe (Table 5).
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). The analyses show little variability and are characterized by a high Si content (~3.4 a.p.f.u.) (Table 5). In sample CL-26, the SAED patterns of illite (Figure 9c) correspond to a two-layer polytype with some stacking disorder. The AEM data (Table 5) reflect a decrease in Si relative to the analyses of illite from sample CL-64, and either a K-rich composition or a Na-K composition. Finally, the SAED patterns of illite from sample CL-34 are characterized by the presence of 0kl or hhl reflection rows typical of a well-ordered 2M1 polytype (Figure 9d). The AEM data correspond to K-rich particles (Table 5).
Pyrophyllite (only present in sample CL-34) forms coarse packets, and the SAED patterns correspond to an ordered two-layer polytype (Figure 9e). Some of the analyses obtained correspond to the ideal formula, whereas others show a slight decrease in Si, accompanied by some K, probably reflecting the presence of layers of white mica interstratified in the pyrophyllite structure.
Finally, trioctahedral chlorite appears in small amounts in samples CL-26 and CL-34. Both optical microscopy and TEM observations indicate that trioctahedral chlorite forms discrete grains, without an apparent relationship with sudoite. Trioctahedral chlorite shows SAED patterns characteristic of either two-layer polytypes or disordered polytypes (Figure 9f). Chlorite analyses show homogeneous Si and Al contents, and considerable variation in the Fe/(Fe+Mg) ratio, as observed in Table 5.
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DISCUSSION |
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TOPAbstractINTRODUCTIONGEOLOGICAL SETTING AND MATERIALSANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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, 3). Similarly, the structural characteristics of sudoite, as observed by TEM, also show differences among the several types of samples, mainly affecting the degree of ordering and the presence of mixed-layer phases. Some of these differences can be related to the different chemical compositions of sudoites, particularly to the influence of increased Fe contents. The AEM data for sudoite from sample CL-26 reveal a composition near the ideal formula of sudoite, with a very low Fe content. In contrast, the AEM data for sudoite packets from coarse-grained rocks display notable variations, with Fe contents greater than those measured in typical sudoites. Indeed, sudoites with such a composition have never previously been reported. An increase in Fe content is also accompanied by an increase in the Fe3+/Fe2+ ratio, which is especially evident in sample CL-34. In addition to the higher Fe contents, the analyses of sudoites from samples CL-64 and CL-34 reveal other chemical differences with the microprobe analyses of microscopic sudoites reported in the literature, the most significant being the decrease in the VIAl content. Nevertheless, these chemical deviations from the ideal sudoite formula are not reflected either in the tetrahedral charge, or in the octahedral occupancy, both parameters being within the range observed in typical sudoites.
These differences can be explained mainly by the substitution of Fe3+ for Al in the dioctahedral sheet and of Fe2+ for Mg in the trioctahedral sheet. Indeed, the AEM data of sudoite from both coarse- and fine-grained samples show some clear correlations between Fe content and Al and Mg contents (Figure 10). In the second plot in Figure 10, some points corresponding to sample CL-64 deviate clearly from the general trend. These points, characterized by the low (Fe+Mg) content and the high Al content, correspond to analyses of dickite-sudoite mixed-layer phases.
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Origin and mechanisms of formation of sudoite
Mixed-layer phases involving 14 Å trioctahedral chlorite layers are common in both weathering (Brown, 1967; Herbillon and Makumbi, 1975; Proust et al., 1986; Banfield and Murakami, 1988; Murakami et al., 1996) and diagenetic environments (Kerrick and Cotton, 1971; Lee and Peacor, 1985; Ahn et al., 1988; Bettison and Schiffman, 1988). Moreover, as noted by Reynolds (1988), interstratified chlorite phases are frequently regular, and some of the 1:1 chlorite interstratified structures (e.g. corrensite) have been proposed to be a stable phase. The interstratifications described can represent either of two contrasting processes: (1) intermediate steps in the transformation of one of the constituents into the other; and (2) the incomplete evolution of a precursor phase towards the two constituents of the interstratification. The first process has frequently been observed both in weathering and in diagenetic environments. Examples of this process include the ‘vermiculitization’ of biotite during weathering and the ‘illitization’ of smectite during diagenesis, among others. ‘Vermiculitization’ of biotite commonly occurs through the formation of intermediate biotite-vermiculite interstratifications (Banfield and Eggleton, 1988), and the illitization of smectite, through the formation of illite-smectite mixed-layers (Peacor, 1992). In both cases there is, however, textural and structural evidence of the process. Thus, the presence of layer transitions or layer termination is common in the structure. The second process is illustrated, for example, by Lee and Peacor (1985) and Ahn et al. (1988) who described mica-chlorite interstratifications formed from precursor smectite, which would lead, at higher temperature, to intergrowths of discrete packets of illite and chlorite.
In the sequences studied, sudoite, muscovite and pyrophyllite coexist in a KI range between 0.30 and 0.42, suggesting that these three phases are stable in this metamorphic interval. Indeed, Fransolet and Schreyer (1984) concluded from hydrothermal experiments that sudoite is stable at approximately the same temperatures as pyrophyllite. On the other hand, textural evidence, observed by optical microscopy, suggests the transformation of dickite in sudoite, pyrophyllite and muscovite (Lázaro et al. 2003). A similar paragenesis was interpreted as formed from kaolinite by Daniels and Altaner (1990). Therefore, the dickite-sudoite interstratifications can be interpreted as intermediate steps in the dickite to sudoite transformation, whereas both the mica-sudoite and the pyrophyllite-sudoite interstratifications can be interpreted as intermediate, probably metastable phases, in the dickite 
muscovite + sudoite and dickite pyrophyllite + sudoite transformations. These two reactions, which probably occurred through a dissolution/precipitation process, appear to be complete in some of the rocks studied, given the lack of dickite and the presence of discrete grains of sudoite, muscovite and pyrophyllite. Nevertheless, the TEM study reveals that the progress of the reaction has been variable in several lithotypes, and is also dependent on the depth of the tectonic slices.
Three reactions can be proposed for these transformations:
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Although most pyrophyllite in these rocks appears to have formed through reaction 1, the presence of pyrophyllite-sudoite mixed-layers suggests the more complex reaction 2, which produces sudoite and pyrophyllite simultaneously. On the other hand, reaction 3, which is similar to that proposed by Daniels & Altaner (1990) for tosudite and sudoite formation from kaolinite, is suggested by the presence of mica-sudoite mixed layers. Reactions 2 and 3 are dependent on the activity of K, Mg and Fe in the fluid phase. K-feldspar, which is a common component in samples from the same sequence, is the more probable source of K for reaction 3, and the extent of this reaction would depend on the availability of K-feldspar. Both Fe oxides, very abundant in these rocks, and dolomite were, probably, the source of Fe and Mg for reactions 2 and 3. These reactions, which appear to be simultaneous, also explain the presence of packets containing the three types of layers, as indicated by the AEM data. The chemical differences between the sudoites from the three samples studied can be tentatively related to different initial mineralogy. Indeed, the lack of carbonate in sample CL-34 and the presence of dolomite in samples Cl-64 and Cl-26 suggest that Mg availability was an important factor controlling the amount and the composition of sudoite.
On the other hand, trioctahedral chlorite is very scarce in these rocks. Lázaro et al. (2003) noted that a notable increase of trioctahedral chlorite occurred in the underlying tectonic slices. The formation of trioctahedral chlorite can also be related to the dickite transformation in those microdomains locally enriched in Fe+Mg. Indeed, we have not observed intergrowths of sudoite and trioctahedral chlorite, similar to those described by Abad et al. (2003). Figure 11 shows graphically the composition of the phases coexisting in these assemblages. Whereas deviations of the sudoite composition towards the pyrophyllite, muscovite and dickite fields reflect the presence of interstratified phases, there is no evidence of phases intermediate between sudoite and trioctahedral chlorite.
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CONCLUSIONS |
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TOPAbstractINTRODUCTIONGEOLOGICAL SETTING AND MATERIALSANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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The presence of metastable mica-sudoite and pyrophyllite-sudoite mixed layers suggests that sudoite formed from dickite in both types of rocks studied, with proposed formation reactions as follows: dickite + K + Fe + Mg + H2O sudoite + white mica + SiO2 and dickite + Fe + Mg + H2O sudoite + pyrophyllite. The composition of sudoite was probably controlled by the initial mineralogy, especially by the availability of Mg.
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ACKNOWLEDGMENTS |
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TOPAbstractINTRODUCTIONGEOLOGICAL SETTING AND MATERIALSANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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Footnotes |
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(Received 4 February 2005; revised 30 June 2005)
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REFERENCES |
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TOPAbstractINTRODUCTIONGEOLOGICAL SETTING AND MATERIALSANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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: analyses from sample CL-34. x: analyses from sample CL-64. 
: analyses of Mg-rich sudoites from sample CL-26.