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Clay minerals within the fine fraction (<2 µm) from surface sediment and worm cast samples were identified using X-ray diffraction techniques, supplemented with information from FTIR spectroscopy. 3.5.2.1 X-ray diffraction

Due to the large number of samples analysed, the clay mineral identification described below was completed on eight surface sediment samples that were considered to be representative of environments and have a good geographical spread in the estuary. This was done to confirm the identities of clay minerals present; the mineralogy of remaining samples was then identified utilising this information as a guide.

XRD scans were performed on subsamples after each procedure: potassium saturation (air-dry), glycolation, 300, 400, and 550 °C heating, then on a separate subsample: magnesium saturation (air- dry), 300, 400, and 550 °C heating (fig 3.5 A&B; Table 3.3). Glycolation is commonly used as an indicator of expandable phases such as smectite or vermiculite; during this procedure ethylene glycol is adsorbed onto inter-layer cations between tetrahedral-octahedral- tetrahedral (TOT) sheets resulting in the swelling of the mineral and the development of a peak around 16-17 Å (Harward and Brindley, 1965; Mosser-Ruck et al., 2005). Progressive heating is another technique commonly used to identify clay minerals, where dehydration and

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Figure 3.5 - XRD diffactrograms of sample 26 indicating peak positions during various treatments (see Table 3.3 for reference). (A) Magnesium-saturated sample. (B) Potassium-saturated sample. See text and table 3.3 for full outline of treatments. In both A & B the three peaks that are measured are evident at ~14Å (chlorite), ~12Å (inter-layer vermiculite) and 10Å (smectite).

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migration of inter-layer cations results in the progressive collapse of inter-layer zones at different temperatures (Starkey et al., 1984; van Groos and Guggenheim, 1986; Tucker, 1988; Kloprogge et al., 1992).

In this study, magnesium-saturated, air-dry samples (fig. 3.5A) have two distinct peaks at 14.4 Å and 7.2 Å. Upon glycolation, a component of the 14.4 Å phase, shifts to 16.5Å; this indicates the presence of two phases within the initial 14.4 Å peak. The shifted peak at 16.5 Å is an expandable phase, such as smectite or vermiculite (Moore and Reynolds, 1997).

At the 300°C heating step, the expandable clay mineral has collapsed back toward the 14 Å region. The 400°C heating step results in the partial collapse of the peak around 14 Å, and there also appears to be a broad shoulder between 14 Å and 10 Å. The final 550°C stage produces three broad humps at 14.4 Å, 12 Å and 10 Å; these three

Magnesium saturation

Procedure Chlorite Inter-layer vermiculite Smectite

air dry 14.4Å & 7.2Å 14.4Å 14.4Å

glycolation 14.4Å & 7.2Å 14Å-17Å 17Å

heating to 300 14.4Å & 7.2Å 14Å 14Å

heating to 400 14.4Å partial collapse& 7.2Å 14Å-12Å 14Å-10Å

heating to 550 13.8Å reduction, and 7.2Å

collapse 12Å 10Å

Potassium saturation

Procedure Chlorite Inter-layer vermiculite Smectite

air dry 14.4Å & 7.2Å 14.4Å 14.4Å

heating to 300 14.4Å & 7.2Å 14Å 14Å

heating to 400 14.4Å partial collapse& 7.2Å 14Å-12Å 14Å-10Å

heating to 550 13.8Å reduction, and 7.2Å collapse

12Å 10Å

Table 3.3 - XRD clay mineral identification table. Table shows position of each clay mineral identified on the basis of sequential treatments

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humps indicate that there are three component clay minerals. Complete collapse of expandable clay minerals at 550°C produces a peak at 10 Å (Starkey et al., 1984; van Groos and Guggenheim, 1986; Kloprogge et al., 1992). The broad 12 Å hump is indicative of a partially inter-layered 2:1 clay mineral such as hydroxy inter-layered vermiculite (Meunier, 2007). The last clay mineral is identified by the behaviour of the 7.2 Å peak. The first two heating steps only result in the partial degradation of this peak. At 550°C the residual peak at 14Å peak drops in intensity, and there is a coincident collapse of the 7Å peak; this is behaviour diagnostic of chlorite (Starkey et al., 1984; Tucker, 1988; Moore and Reynolds, 1997). Well-ordered interstratified clay minerals would potentially yield super-order peaks in the 24-32Å in air-dried or glycolated samples (Moore and Reynolds, 1997), but a check on this region and the poorly crystalline nature of the traces indicated that this type of clay mineral is not present in the Leirárvogur Estuary.

The potassium-saturated air-dry sample (fig. 3.5B) shows a similar progression to the magnesium-saturated sample. Potassium- saturation of the inter-layer spacing results in the dehydration of the clay mineral phase at lower temperatures. Potassium-saturation and 550°C heating produces more distinct peaks for the main mineral phases than magnesium-saturation and was therefore performed on the remaining fifty-six samples.

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In summary (Table 3.3), clay minerals identified in the Leirárvogur Estuary are: (i) 14 Å chlorite phase; (ii) a phase that expands on glycolation and collapses to 10 Å at heating to 550°C; (iii) hydroxy inter-layered 2:1 structure vermiculite (Meunier, 2007).

Clay mineral nomenclature can be unwieldy and complicated. For simplicity and ease of reading in the remainder of the text the ‘expandable’ clay mineral will be referred to as ‘smectite’, and the ‘hydroxy-inter-layer vermiculite’ as ‘inter-layered vermiculite’. The 14 Å-type phase will continue to be referred to as chlorite.

3.5.2.2 Infrared spectroscopy

The OH-stretching region (3800-2400 cm-1) of representative sample locations is shown in figure 3.6. The broad band centred at 3425cm-1 _is

attributed to the stretching mode of adsorbed water. The bands at 2850-3000 cm-1 _{and 2300-2400 cm}-1 _{are due to organics in the}

sediment and gas phase and carbon dioxide in the infrared spectrometer chamber respectively. The broad nature of the bands indicates that the clay minerals probably have a disordered structure (Farmer, 1974; Hornibrook and Longstaffe, 1996). The dominance of the water peak even after heating suggests the presence of adsorbed water in inter-layer spacings (Madejova, 2003).

Sample 44 has a distinct band at 3619 cm-1_{, this relates to an Al-OH}

bond and is common in smectites that have a high proportion of Al in the octahedral sites (Farmer, 1974; Madejova, 2003). The band

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Figure 3.6 - Infrared spectra for representative estuary and hinterland samples. The broad nature of the bands after heat treatment may indicate that the clay minerals present are poorly crystalline (Farmer, 1974; Hornibrook & Longstaffe, 1996). The band at approximately 3560 cm-1 _{is indicative of an iron–rich smectite}

such as nontronite. Bands at 3617 and 3697 cm-1 _{in sample 31 suggests that low}

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located around 3560 cm-1 _{in all but one of the samples (sample 44) is}

at the approximate position for the FeFeOH where this grouping dominates the octahedral sheets in the smectite clay mineral nontronite (Madejova, 2003). Nontronite is an Fe-rich dioctahedral smectite and is known to be a weathering product of pyroxene in basalt (Eggleton, 1975); this supports the evidence for a dioctahedral expandable phase identified using XRD. For vermiculite, an OH stretching band of Mg3OH unit would be expected at 3677 cm-1

(Farmer, 1974), although there are small peaks evident in this region, it is not distinct enough to confirm its presence, thus supporting the interpretation of the presence of nontronite smectite. Sample 31 (fig. 3.6), has very small peaks at 3617 and 3697 cm-1 _{and this may indicate}

the presence of a kaolinite. The size of these peaks indicates that kaolinite is present at trace concentrations; such low concentrations are unlikely to be solely responsible for the significant 7Å peak seen, as XRD data indicate that chlorite is also present.

In summary, infrared spectroscopy data indicate that the smectite mineral within the fine fraction is likely to be an Fe-rich smectite such as nontronite; there is no evidence for the expandable phase being a vermiculite clay mineral. There is a suggestion that kaolinite is present only in trace quantities within the fine fraction.

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