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Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A.
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ABSTRACT |
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 Top Abstract IntroductionMethodsResultsDiscussionAcknowledgmentsReferences cited |
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), additional broad peaks of lower intensity at approximately 4.328 Å (20.5 °2) and 2.50 Å (35.9 °2), and lacking diagnostic cristobalite peaks at 3.14 Å (28.5 °2) and 2.84 Å (31.5 °2). At present, the most widely used classification of disordered silica is a threefold system that recognizes an amorphous phase (opal-a), a cristobalite-like end-member (opal-c), and intermediate structures incorporating both cristobalite and tridymite domains (opal-ct) (Jones and Segnit 1971). This classification does not accommodate a dominantly tridymite-like structure as is observed in the Twiggs clay. The silica phase in these samples is more accurately described as "disordered tridymite." A new term "opal-t" is proposed to describe this end-member phase.
Key Words: Opal • disordered silica • X-ray diffraction • cristobalite • tridymite • silica diagenesis
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INTRODUCTION |
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TopAbstractIntroductionMethodsResultsDiscussionAcknowledgmentsReferences cited |
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The diagenetic transformation of amorphous silica is the subject of many studies, including Murata and Larson (1975), Mizutani (1977), Keller (1984), Williams and Crerar (1985), and Rice et al. (1995). The general diagenetic sequence is transformation from an amorphous phase to progressively more ordered structures. Intermediate disordered structures are believed to consist of both cristobalite and tridymite domains. Increased structural order is achieved through the loss of tridymite stacking, as the structure becomes more cristobalite-like. With continued burial, metastable cristobalite stacking gives way to crypto-crystalline quartz. These transformations are believed to occur through solution-precipitation reactions occurring at relatively low temperatures (50–110 °C) (Murata and Larson 1975; Keller 1984; Williams and Crerar 1985), although evidence for solid-state transformation of amorphous silica to a partially ordered phase was presented by Mizutani (1977).
Most often, X-ray diffraction (XRD) provides the basis for identifying the low-temperature silica polymorphs; however, very small crystallite sizes, disorder within the framework, and the presence of multiple interfering phases present challenges in interpreting the characteristically broad, low-intensity XRD reflections of partially ordered structures. Techniques other than XRD, including transmission electron microscopy (TEM), infrared microscopy (IR), and nuclear magnetic resonance spectroscopy (29Si MAS NMR), have been utilized (Wilson et al. 1974; Jones and Segnit 1975; Sanders 1975; de Jong et al. 1987; Rice et al. 1995), but these efforts have resulted in differing and sometimes conflicting interpretations, and the structure of disordered silica remains imperfectly understood.
The low-temperature polymorphs of silica are most often classified according to the threefold classification scheme developed by Jones and Segnit (1971). This scheme uses crystal structure as revealed by XRD to classify disordered silica into three categories. The first category, opal-a, is a highly disordered phase that exhibits a single broad XRD hump centered near 4.1 Å (21.6 °2) (Fig. 1a
). Biogenic opal and many gem opals fall into this category (Jones and Segnit 1971). Silica of the second category, opal-c, is the most ordered structurally, as evidenced by four easily defined XRD reflections that correspond to the four most intense reflections of cristobalite at 4.04 Å (22 °2), 3.14 Å (28.5 °2), 2.84 Å (31.5 °2), and 2.47 Å (36.1 °2) (Fig. 1c). The third category, opal-ct, is intermediate to opal-a and opal-c in terms of structural order, and is the most common form of disordered silica in marine sediments (Kastner 1979). The XRD signature of opal-ct consists of three broad reflections at approximately 4.05 to 4.10 Å (22 to 21.6 °2), 4.25 to 4.35 Å (20.9 to 20.4 °2), and 2.50 Å (36 °2) (Fig. 1b). These positions correspond roughly to reflections produced by cristobalite and opal-c, but also include additional reflections that correspond to the positions of the stronger tridymite lines (Figs. 1d and 1e). The reflections of opal-ct are more diffuse and less intense than opal-c, owing to its relatively less ordered structure (Jones and Segnit 1971).
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), represents a composite of cristobalite (101) and tridymite (404) reflections, with the position of the peak reflecting the relative proportion of the two polymorphs. A progressive decrease in the d-value from near 4.11 Å (21.6 °2) to approximately 4.04 Å (22 °2) is documented by Murata and Larson (1975) and Mizutani (1977). Concurrent with this shift is progressive sharpening of the peak and appearance of cristobalite peaks at 3.14 Å (28.5 °2) and 2.84 Å (31.5 °2). These changes are believed to represent a progressive increase in structural order during diagenesis that is achieved through preferential growth of cristobalite relative to tridymite (Williams and Crerar 1985). This structural model is challenged by de Jong et al. (1987), who examined eight natural opals and one synthetic opal using XRD and 29Si MAS NMR. They observed that the broad opal-ct NMR spectra more closely resembled that of amorphous silica rather than crystalline silica polymorphs, suggesting that the local silica arrangement in opal-ct is comparable to amorphous silica. The observed opal-ct NMR spectra could not be duplicated by superposition of the spectra of cristobalite and tridymite. They propose that the XRD pattern of opal-ct is not produced by line broadening due to cristobalite and tridymite microcrystallites, but indicates long-range ordering of O atoms. This long-range ordering of the oxygen array is achieved before short-range ordering of silicon is established.
The results of subsequent investigations are consistent with the earlier hypothesis that opal-ct consists of cristobalite and tridymite intergrowths. Guthrie et al. (1995) simulated XRD patterns of interstratified cristobalite and tridymite. Simulated XRD patterns corresponded closely to patterns produced by natural opal-ct samples. Their work indicates that both random and ordered interstratifications are found in disordered silica, and that the 41 to 45 °2 (2.2 to 2.0 Å) band is sensitive to the ordering state, with a peak produced near 43 °2 (2.1 Å) with ordering.
Elzea and Rice (1996) examined 24 opals using HRTEM and XRD. They found a continuous range of d-values from 4.03 to 4.11 Å (22 to 21.6 °2), consistent with intergrowth of cristobalite and tridymite. However, they found no evidence of a clear separation between opal-c and opal-ct as suggested by Jones and Segnit. Rather, they propose that the structure of opal-ct is a continuous series of disordered intergrowths from dominantly cristobalite-like to a "hypothetical tridymite-like end-member phase" (Elzea and Rice 1996, p. 495).
This paper presents evidence of a tridymite-like end-member identified in silica rich samples of the Twiggs Clay of Georgia. The Twiggs is a late Eocene marine claystone found in central and eastern Georgia (Fig. 2). Mineralogically, the Twiggs varies from smectitic and calcareous to opaline (Shearer 1917; Huddlestun and Hetrick 1979). This study examines samples of an especially siliceous facies of the Twiggs originally described by Hetrick (1992). These samples consist predominantly of smectite, disordered silica, and quartz (Eversull 2005). The opal occurs as spherical aggregates, typically <5 µm in diameter, and is clearly biogenic in origin. Molds of diatom frustules and sponge spicules are easily visible with SEM (Fig. 3) where the arrangement of lepispheres preserves the morphology of diatom frustules.
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METHODS |
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). Pairs of samples were collected from the base of the exposure through the entire clay section. Approximately 1 m separated the horizontally paired samples. Vertical spacing between sample pairs was also approximately 1 m. To avoid recent weathering, samples were taken from approximately one foot behind the exposed surface. The samples represent four distinct clay zones (Eversull 2005), but the upper stratum lacks disordered silica and is not discussed here. Fifty-four samples were examined by XRD as whole rock powders. These were prepared by processing approximately 2 g of sample in 10 mL ethanol in a McCrone Micronizing Mill for 3 min. Micronized samples were dried at 60 °C, and then disaggregated using an agate mortar and pestle. Randomly oriented samples were prepared by back-loading the powder into aluminum sample holders, using a procedure similar to that described in Poppe et al. (2001).
For investigation of the clay mineral assemblage, a representative sample split was dispersed in a 0.1 wt% sodium phosphate solution. The size fraction equivalent to a <2 µm spherical diameter quartz particle was extracted from the upper 5 cm of the suspension after settling for 3 h and 10 min as determined by Stoke’s law. An IEC-HT high-speed centrifuge was used to concentrate the extracted clay. The clear supernate was poured off and the clay paste was thoroughly blended with an aluminum spatula. Oriented sample smears were prepared on glass slides using a technique similar to that described in Moore and Reynolds (1997). Four XRD patterns were generated from each oriented smear: (1) in air-dried condition; (2) after saturation with ethylene glycol; (3) after exposure to 300 °C for 1 h; and (4) after exposure to 550 °C for 1 h.
All XRD data were collected on a Siemens D5000 diffractometer operating at 40 kV and 20 mA and equipped with a copper anode, Kevex solid-state detector, one-degree divergence slit, and spinning sample stage. The scan rate was two seconds per 0.02 °2 increment for whole rock and clay mineral analysis. Whole rock samples were scanned from 2 to 70 °2, and the clay minerals were examined from 2 to 35 °2.
Interpretation of XRD data were facilitated by the use of MacDiff version 4.2.5 (Petschick 2001). Peak positions on whole rock diffractograms were corrected to the quartz peak at 3.34 Å (26.7 °2). Mineral phases were identified by comparison of XRD data with IUDD standards. Clay minerals were identified by comparing peak position and intensity in air-dried and glycolated states, and after heat treatments, as outlined in Poppe et al. (2001).
Peak analysis was accomplished through the peak fit function of MacDiff, using the Split Pearson VII peak shape function and unsmoothed data. Peak boundaries were user-defined for greater consistency among samples. In addition to a single peak fit, MacDiff allows for discrimination of up to seven overlapping reflections. For each reflection, the peak shape function approximates the peak position, intensity, and FWHM. The fit is improved through least squares refinement. Up to 100 refinement iterations are permitted; however, for this investigation, refinement was limited to 20 iterations because little, if any, improvement was noted after 20 iterations. The quality of the fit is measured by the residuum, the discrepancy between the modeled curve and the observed curve, expressed as a percent.
It was evident from the shape of the curve that at least three reflections contribute to the 19 to 24 °2 (4.7 to 3.7 Å) band. Multiple attempts were made to model the observed whole rock XRD curve between 19 and 24 °2 (4.7 and 3.7 Å) by inserting a minimum of three peaks up to the maximum permitted seven peaks. Increasing the number of reflections often produced a lower residual value (R%), but sometimes resulted in unjustified shifts in peak positions. In selecting the overall best representation of the observed data, primary consideration was given to rational peak positions for mineral phases identified in the samples, and secondary consideration to the value of R%.
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RESULTS |
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. Three prominent reflections are characteristic: a well-defined peak at 3.34 Å (26.7 °2), a broad reflection centered between 13 and 15 Å (6.8 and 5.9 °2), and a broad band with multiple peaks extending from approximately 19 to 24 °2 (4.7 to 3.7 Å). The 13 to 15 Å (6.8 to 5.9 °2) peak expands to 17 Å (5.2 °2) following saturation with ethylene glycol and collapses at least partially to 10 Å (8.8 °2) after heating to 300 °C. Total collapse occurs with heating to 550 °C. The 26.7 °2 (3.34 Å) peak and 19 to 24 °2 (4.7 to 3.7 Å) band are not affected by glycolation or the heat treatments.
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) and 7.2 Å (12.3 °2) are observed in approximately one-half of the whole rock diffractograms and virtually all of the clay mineral diffractograms. Neither peak is affected by glycolation. The 7 Å (12.3 °2) peak is destroyed by heating to 550 °C. The 10 Å (8.8 °2) peak increases in intensity after heating.
The most prominent characteristic of the whole rock sample diffractograms is the broad band extending from approximately 19 to 24 °2 (4.7 to 3.7 Å). Peak decomposition of this region reveals five overlapping reflections, as illustrated in Figure 5. The largest reflection is a relatively broad peak centered at approximately 4.11 Å (21.7 °2). A much sharper reflection is located at 4.25 Å (20.9 °2), and two additional broad reflections are centered near 4.32 Å (20.5 °2) and 4.47 Å (19.9 °2). Typically, the reflection at 4.32 Å (20.5 °2) is subordinate in peak intensity and area. The fifth reflection is typically a very broad, low-intensity hump centered at approximately 3.87 Å (23 °2). The single broad reflection produced by opal-a (Fig. 1a) was not necessary to produce a good fit of the XRD data. Residual values average 11.6% and range from 8.1 to 18.5%. Higher R% values generally correspond to more asymmetrical reflections, typically produced by samples from higher stratigraphic position.
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(4.7 to 3.7 Å) is more diverse. In general, samples from lower stratigraphic position produce broader, more complex reflections, and samples from relatively higher stratigraphic position produce smaller, simpler reflections. In all cases, the position, shape, and intensity of the reflection are essentially unchanged by glycolation or temperatures of 550 °C (Eversull 2005).
Peakfitting of the clay mineral diffractograms reveals from one to five peaks contributing to the observed 19 to 24 °2 (4.7 to 3.7 Å) band, with more peaks required to fit broader reflections. In all cases, a broad peak is centered near 4.10 Å (21.6 °2). All but two samples produce an additional, smaller reflection at approximately 4.32 Å (20.5 °2). Additional peaks, if present, are located at approximately 4.25 Å (20.9 °2), 4.47 Å (19.9 °2), and 3.87 Å (23 °2); however, these peaks are always subordinate to the 4.10 Å (21.6 °2) peak. Peak position data for peaks 3 and 4 (Fig. 5), the major disordered silica peaks, are reported in Table 1. The table also lists the 
d with respect to the (404) and (112) tridymite peaks. Many of the d values are negative suggesting a systematic increase in the unit cell when compared to tridymite. The range, mean, and standard deviation of the d-values are 4.090 to 4.141, 4.105 ± 0.003, 0.011 and 4.311 to 4.352, 4.324 ± 0.002, 0.007 Å, respectively.
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DISCUSSION |
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) and 4.25 Å (20.9 °2); kaolinite and illite/mica correspond to the 7.2 Å (12.3 °2) and 10 Å (8.8 °2) peaks, respectively. Smectite is identified by the behavior of the peak at approximately 14 Å (6.3 °2) (Poppe et al. 2001). The characteristic diffraction effects due to disordered silica are discussed below. Mineral composition of the samples and quantitative phase determinations are presented in Eversull (2005).
Quartz and smectite are unquestionably present in all samples, and both of these phases produce reflections in the 19 to 24 °2 (4.7 to 3.7 Å) region. An acceptable model of the XRD curve must include a relatively broad peak near 4.5 Å (19.7 °2) attributed to smectite and a relatively sharp peak near 4.257 Å (20.9 °2) attributed to quartz. These peaks are identified as peaks 1 and 2 in Figure 5.
Scrutiny of the 21.5 to 22 °2 (4.7 to 3.7 Å) region is critical to this investigation because the most intense reflections of the low-temperature silica polymorphs are found there. Cristobalite produces its most intense peak at 4.0397 Å (22.0 °2), and tridymite at 4.107 Å (21.6 °2) (Figs. 1d and 1e). For this investigation, the initial assumption was that both cristobalite and tridymite domains constitute the disordered silica phase in these samples. Accordingly, the initial attempt to model the observed curve incorporated two peaks that could be attributed to tridymite (404) and cristobalite (101), respectively. However, this produced generally undesirable results. For the majority of patterns, the peaks shifted to irrational positions (Fig. 6). In other patterns, modeled peak positions remained close to 4.04 Å (22.0 °2) and 4.107 Å (21.6 °2), but the quartz peak at 4.257 Å (20.9 °2) became unreasonably broad and asymmetrical (Fig. 7). For the majority of these samples, the best model of the XRD curve between 21.5 and 22 °2 (4.13 and 4.04 Å) is achieved with a single broad reflection (Fig. 5, peak 4). In individual samples, the center of the reflection varies from 4.090 to 4.141 Å (21.7 to 21.5 °2) and on average is located at 4.105 Å (21.6 °2). In all cases, the position of this peak corresponds most closely to the d-value of tridymite (404) at 4.107 Å (21.6 °2).
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(4.13 to 4.04 Å) XRD region produces rational peak positions and acceptable R% values; however, equally acceptable models can be produced by fitting the XRD curve with two discrete peaks assigned to cristobalite (101) and tridymite (404), as illustrated in Figure 8. In all cases, peak decomposition reveals a smaller reflection near 4.32 Å (20.5 °2), consistent with tridymite (112). The contribution of tridymite-like domains to the XRD curve is evident; however, the presence of cristobalite domains in these five samples cannot be excluded on the basis of peak decomposition.
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are consistent with a tridymite-like structure in the disordered silica phase. Peak 3 is located at 4.324 Å (20.5 °2) on average, but in individual samples may vary from 4.311 to 4.352 Å (20.6 to 20.4 °2). This is a reasonable fit with the ideal tridymite (112) peak at 4.328 Å (20.5 °2). A reflection at this location cannot be attributed to any other mineral phase identified in the samples. Kaolinite produces a (110) reflection at 4.366 Å (20.3 °2), but this position does not yield a good fit with the observed data. More significantly, the primary kaolinite reflection at 7.2 Å (12.3 °2) is not observed in many patterns, and where the (001) reflection is observed, it is typically a small reflection. If the peak at 4.324 Å (20.5 °2) is attributed to kaolinite, then the (110) reflection is disproportionately large relative to the 100% intensity (001) peak.
The fifth peak in the model is located at 3.87 Å (23 °2), on average. This peak is characteristically very broad and low intensity, and often is asymmetrical. Some of the opal-ct samples examined by Guthrie et al. (1995) produced a slight shoulder on the high angle side of the 19 to 24 °2 (4.7 to 3.7 Å) band. They successfully modeled this shoulder with a small degree of ordering. Alternatively, the high angle side of the 19 to 24 °2 (4.7 to 3.7 Å) curve could represent the interaction of multiple minor reflections including tridymite (402) and (311), smectite (004), and possibly kaolinite (021). Precise resolution of reflections contributing to the 22 to 24 °2 (4.0 to 3.7 Å) region is not possible within the seven-peak limit of MacDiff’s profile fit function; neither is it essential to the goal of distinguishing cristobalite and tridymite reflections. The presence, or absence, of a low intensity peak on the high-angle side of the modeled band has negligible effect on the location of the four other peaks in the model, although it does affect the shape and integrated area of the main 4 Å (22.0 °2) peak. Based on the mineralogy of the samples, it is reasonable to expect some excursion of the XRD curve between 22 and 24 °2 (4.0 to 3.7 Å), and including peak 5 in a model of the observed XRD curve is beneficial because it results in a lower R% and less skewed 4 Å (22.0 °2) peak.
Deconvolution of the XRD curve between 19 and 24 °2 (4.7 to 3.7 Å) produces a plausible fit with four of the five most intense reflections produced by tridymite and there is little indication of cristobalite-like domains. Corroborative evidence is found in the remainder of the XRD curve outside of the modeled band. In addition to a main peak at 4.04 Å (22.0 °2), cristobalite produces less intense reflections at 3.14 Å (28.5 °2), 2.84 Å (31.5 °2), and 2.49 Å (36 °2) (Fig. 1d). There is no convincing evidence of reflections near 3.14 Å (28.5 °2) or 2.84 Å (31.5 °2) in these samples. Every sample produces a broad reflection at approximately 2.5 Å (35.9 °2), but this reflection is common to both cristobalite and tridymite and is not diagnostic of either polymorph. The strongest tridymite reflections outside of the 19 to 24 °2 (4.7 to 3.7 Å) band are at 2.975 Å (30.0 °2), 2.5 Å (35.9 °2), 2.49 Å (36.0 °2), and 2.308 Å (39.0 °2) (Fig. 1e). Most samples produce a reflection near 2.97 Å, (30.0 °2) but the peak intensity is often less than expected relative to the 4 Å (22.0 °2) peak. A reflection at 2.3 Å (39.0 °2) is not detected in most patterns.
Although Jones and Segnit (1975) assert that most disordered silica yields an XRD signature similar to opal-ct and distinctly different from that of tridymite and cristobalite, the signature of the Twiggs samples appears to match that of tridymite. These samples contain disordered silica of sedimentary origin with a structure dominated by tridymite-like domains. While this is unique, it has been reported previously.
Mitchell and Tufts (1973) published one of the first reports of a disordered tridymite polymorph. They used XRD to examine 32 samples of fossilized wood. With only two exceptions, they found the silica phase to be most like disordered tridymite. Bayliss (1978) observed that the XRD pattern of Australian opal published by Petruk et al. (1977) differs from that of opal-ct, opal-a, and opal-c, and more closely resembles disordered tridymite.
Wilson et al. (1974) used XRD, electron microscopy, and IR spectroscopy to probe the structure of a deep-sea chert and a bentonite. The XRD patterns of both samples were characterized by a broad reflection at 4.1 Å. Although this is close to the most intense reflection of tridymite, the authors did not consider this to be diagnostic. However, electron diffraction produced a "hexagonal pattern consistent with tridymite" (Wilson et al. 1974, p. 5), and the infrared spectra of both samples produced bands consistent with tridymite. No diagnostic cristobalite bands were detected. Additionally, the platy, hexagonal morphology of their samples is more similar to tridymite than octahedral cristobalite. They propose that the structure of these two samples is essentially that of disordered tridymite in which the structure is disrupted by random transverse displacement of the sheets normal to the c-axis.
Sanders (1975) examined volcanic gem opals by electron microscopy and diffraction. They determined that the opals are a mixture of amorphous and crystalline silica, and that the crystalline phase of some of the opals has a tridymite structure. Iijima and Tada (1981) reported disordered low-tridymite forming as pore cement and veins in chert, porcellanite and silica tuff in sediments from Japan. Elzea and Rice (1996) reported that tridymite stacking was common, and possibly dominant, in some opals they examined.
The long-held interpretation of opaline silica as disordered intergrowths of cristobalite and tridymite implies the existence of end-members of dominantly cristobalite and tridymite stacking. The currently accepted model for diagenetic transformation of amorphous silica also implies that first-formed structures will be dominated by tridymite stacking. Early recognition of the cristobalitic end-member, opal-c, was facilitated by its relatively sharper XRD signature. By contrast, the tridymite end-member remained "hypothetical." XRD examination of a large number of Twiggs clay samples supports the existence of such an end-member. The XRD signature of these samples is consistent with that of line-broadened tridymite, with a broad primary reflection near 4.107 Å (21.6 °2), additional broad peaks of lower intensity at approximately 4.328 Å (20.5 °2) and 2.50 Å (35.9 °2), and lacking diagnostic cristobalite peaks at 3.14 Å (28.5 °2) and 2.84 Å (31.5 °2).
These samples appear to represent a disordered tridymite end-member analogous to the cristobalite end-member, "opal-c." The current classification of disordered silica does not accurately describe the disordered silica phase in these samples because the classification does not accommodate a dominantly tridymite-like structure. Rather than broadly categorize these samples as "opal-ct," it is more accurate to describe these samples as "disordered tridymite" as proposed by Wilson et al. (1974), and incorporate a new term, "opal-t," into the classification to describe this end-member phase.
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ACKNOWLEDGMENTS |
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Footnotes |
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MANUSCRIPT HANDLED BY LAURENCE GARVIE
MANUSCRIPT RECEIVED February 8, 2007; MANUSCRIPT ACCEPTED October 9, 2007
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REFERENCES CITED |
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