VIII. RESULTADOS Y DISCUSIÓN
8.4 Tasa de crecimiento
(Russell 1987). This cannot be seen as a distinct feature in the site samples, but it may be contributing to a broad strong band between1030-910cm-1, which is seen in all the ATR spectra for the sample sites. The lack of characteristic features again suggests very little kaolinite minerals are present or it is present in the disordered form.
The feldspar mineral IR spectra have strong bands between 800cm-1 and 700cm-1. The spectra for the reference clay and eaten sites (Figure 3.32) have a distinctive band at 788/4cm-1, a feldspar characteristic. This may be due to residual unweathered feldspar in the CRM, as feldspars may weather to produce kaolinitic clays (Wada 1987, Wilson 2004). Quartz is a common constituent of most clay minerals, having sharp well defined characteristic bands at 800 and 781cm-1. These bands are not seen in the material from any of the sites. This suggests the samples have a large degree of amorphous material.
3.5.5.1 IR Determination of organic content and structural water.
The IR spectra were examined for absorption bands associated with biological molecules (Table 3.26) as consumption of soil for antimicrobial products derived from microbial organisms has been suggested as a possible geophagy function (Ketch et al. 2001)
Table 3.24 Characteristic Infra Red absorption bands of organic groups (Madejova et al. 2001, Madejova 2003, Skoog et al. 2006).
Figure 3.33 (a) Site 7A, 7B and Site 8 organic or structural water content; (b) Sites 2, 9 and 10.
The samples were dried at 1050C overnight and allowed to cool in a desiccator, to remove unbound atmospheric water. Material of biological origin will have absorption bands due to amide, carboxylic acid and aromatic groups (Table 3.24).
The spectra for Sites 7A (Figure 3.33a) has a broad band from 1740-1518cm-1 whereas 7B and 8 and spectra for Sites 2, 9 and 10 (Figure 3.33b) only shows a weak feature. There is little variation % T, over the
80
bands cm-1 Functional groups Absorption bands cm-1
O-H aliphatic and aromatic 3600-3000 CONH2 amides 1720-1640
H-O-H vibrations of water 3430-3395 C=C- alkene 1670-1610
NH2 also secondary and tertiary 3600-3100 O-H water 1638-1627
COOR ester 1750-1700 Ø-O-R aromatic 1300-1180
COOH carboxylic acid 1740-1670 R-O-R aliphatic 1160-1060
C=O aldehydes and ketones 1740-1660
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region 1700 -1600cm-1. This is the region where both adsorbed water and amides would have adsorption. The spectra for all the sites show a weak broad band with a maximum % T between 1650 and 1620cm-1. A broad peak centred on 1636cm-1 is associated with OH deformation of H2O. Peaks at 1648cm-1 may be associated with C=O, C=C and CONH2, organic groups.Site 7A has features potentially due to organic functional groups or structural water. LOI results suggested the presence of organic carbon compounds in sites 7A, 7B and 8. Structural water may be associated with the presence of the paracrystalline short-range ordered mineraloid opal or hydrated kaolinitic mineral, halloysite. Both were identified by the Rockwood XRD stick matching analysis (Table 3.22). The halloysite form is produced from weathering of igneous rocks (Galán 2006).
In summary:
o IR spectra suggest the presence of small and variable amounts of kaolinites o presence of feldspar and silicate minerals
o there is little evidence of the presence of montmorillonites.
o limited evidence of detectable amounts of organic material and very low structural water content.
3.5.6. XRF Analysis performed at Rockwood Industries
The results for analysis of the CRM standards (Table 3.25) show there is a high level of agreement between the measured and the published values, which permits confidence in the values obtained for the site samples. The differences between measured and reported was <3.5%, and may be due to the limited number of replicates. The reported values are quoted as oxides.
Table 3.25 XRF comparison of measured mean oxide results CRM, Kaolinite KGa-2 and SWy-2, Reference data*
from Mermut et al. (2001).
KGa-2 KGa-2 SWy-2 SWy-2
Measured Reference* Measured Reference*
Na2O (%) 0.05 0.06 1.57 1.47
MgO (%) 0.06 0.04 2.99 2.94
K2O (%) 0.02 0.02 0.2 0.2
CaO (%) 0.04 0.03 1.96 1.89
TiO2 (%) 1.98 1.91 0.11 0.09
Fe2O3 (%) 1.19 1.15 3.77 3.74 SiO2 (%) 44.49 43.49 63.46 61.46 Al2O3 (%) 40.14 38.14 21.05 22.05
SUM (%) 87.97 84.84 95.11 93.84
Using the TAS (Total-Alkali-Silica) classification system (Figure 3.34) and the calculated values obtained from the XRF data (Table 3.26) the (%Na2O+K2O): The %SiO2 ratio suggests the samples are close to the acidic, andesite-dacite composition junction (Figure 3.34). The mineral composition of dacite includes the plagioclase feldspars, oligoclase, andesine, quartz, silica and tridymite. Dacites are often grey in colour with dark crystal inclusions. This classification for the samples also agrees with the analyses by (Vogel et al. 2004), who describes the Guanacaste ignimbrite plateau as containing extensive rhyolitic and dacitic ignimbrite flows and lava flows, with small volume of andesitic pyroclastic flows.
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Table 3.26 Rockwood Analysis XRF Mean % oxide results samples from Sites 1, 2, 6, 7A, 7B, 8, 9 and 10
Site 1 Site 2 Site 6 Site 7A Site 7B Site 8 Site 10
Wt. % 1 9 11 13 14 15 40 46 56 60 62 70 75 77 100
Na2O 3.50 3.57 3.38 3.25 2.95 3.69 3.21 3.27 2.56 2.06 2.39 2.49 3.26 3.36 2.54 MgO 0.27 0.27 0.32 0.2 0.2 0.17 0.31 0.25 0.43 0.43 0.52 0.63 0.25 0.25 0.21 K2O 3.05 2.96 3.08 2.95 2.84 3.23 2.99 3.29 2.51 2.1 0.58 2.36 3.14 3.09 3.08 CaO 1.52 1.63 1.52 1.33 1.41 1.27 1.64 1.32 1.37 1.23 2.63 1.67 1.24 1.33 1.16 TiO2 0.67 0.65 0.65 0.66 0.66 0.59 0.68 0.62 0.67 1.26 0.84 0.7 0.63 0.64 0.7 Fe2O3 4.16 3.86 4.18 4.07 4.08 3.63 4.42 3.62 3.86 11.96 6.38 4.94 3.9 3.9 4.23 Al2O3 13.76 16.92 17.62 18.2 18.7 16.97 18.42 17.1 19.63 17.96 25.74 18.03 17.6 17.49 18.47 SiO2 65.47 64.64 65.88 64.99 64.09 66.82 63.71 65.86 60.84 55.13 50.83 59.5 65.26 65.49 63.89 SUM 92.4 94.5 96.63 95.65 94.93 96.37 95.38 95.33 91.87 92.13 89.91 90.32 95.28 95.55 94.28 Na2O+K2O 6.55 6.53 6.46 6.2 5.79 6.92 6.2 6.56 5.07 4.16 2.97 4.85 6.4 6.45 5.62 The data for the site samples Table 3.26 show a characteristic, mean SiO2 62.83% and mean %(Na2O+K2O) 5.78.
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Figure 3.34 TAS (Total-Alkali-Silica) Classification of Volcanic rocks, from Streckeisen et al (2002).
Figure 3.35 illustrates the small difference in aluminium content of the Cebus (Site 9) and Ateles eating sites (Sites 2, 9, 10). There were insufficient data points to permit statistical analysis of the aluminium values.
Figure 3.35 XRF mean aluminium content, calculated as oxide
3.5.7 Determination of sample pH
The ASTM definition for values of pH classifies the samples as predominantly acidic in character (Natural Resources Conservation Service 2004). Soil pH affects the solubility and hence availability of minerals or nutrients and mineral leaching. Most minerals and nutrients are more soluble or available in acid soils than in neutral or slightly alkaline soils.
The comparison of KCl pH with H2O pH i.e. the Δ pH provides an assessment of the nature of the net charge on the colloidal system. Aluminium, displaced by K+, reacts with OH– ions and increases H+ concentration. As a result, the solution pH is lowered. Commonly, exchangeable aluminium is present if the KCl pH is 5.2 or less (Natural Resources Conservation Service 2004).
0 4 8 12 16 20 24 28
1 9 11 13 14 15 40 46 56 60 62 70 75 77 100
Site 1 Site 2 Site 6 Site 7A 7B Site 8 Site 9 Site 10 mean % Al2O3 XRF Rockwood data
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Table 3.27 Results for pH determinations and value of = Δ pH
Site Sample pH in
* American Society for Testing and Materials
When this difference is negative, the soil solution has a net negative charge, which is characteristic of acidic material. Santa Rosa samples had a negative value of ΔpH (Table 3.27). With the exception of Site 6 samples Santa Rosa samples had a pH <5.2 and therefore would provide exchangeable Al (Table 3.27). Acidic soils between pH4.0-5.0 may have high concentrations of soluble manganese which is also potentially toxic.
3.5.8 Laser Diffraction Particle size Analysis
For the initial determinations the sieved <2mm fractions were dispersed in deionised water/ sodium hexametaphosphate solution 0.5%w/v, with a measured pH 6.7.
Data for each site was pooled for analysis and plotting and calculations were performed using GraphPad Prism version 4.00 for Windows, GraphPad Software, San Diego California USA, (www.graphpad.com). 1-Way ANOVA with Bonferroni Multiple Comparisons tests were used, where there was sufficient data. The Bonferroni method is a method for correcting for multiple comparisons. It can be used to correct any set of P values for multiple comparisons (Bonferroni was chosen as it was more conservative than Dunnett’s test for multiple comparisons-Graphpad). 1-Way Anova results and Bonferroni Multiple Comparisons tests results for the comparison of individual sites are presented in Table 3.28.