Analytical techniques used for physical analyses

2.4.1.1 X-ray Diffraction (XRD)

X-ray crystallography is used to determine the atomic and molecular structure of the crystalline phases of minerals by analysis of the diffraction patterns produced when X- rays pass through the sample (Toby, 2005). XRD can be used to characterise heterogeneous solid mixtures and to determine if different crystalline components, or minerals are present (Toby, 2005).

XRD utilises a focussed X-ray beam approximately 20 mm wide that is directed at varying angles onto a sample. The focussed X-ray beam is partly transmitted and partly diffracted by the molecular layers in the sample. The angle of diffraction is dependent on the lattice spacing of the atoms or molecules (d-spacing): a simple equation (Bragg’s

ODZQȜ GVLQș) calculates the d-spacings of the crystal (measured as 2-theta) from the

detector as peaks on a chart (a diffractogram or XRD trace) of diffraction angle versus intensity (Tucker, 1988).

The X-ray machine used in this study was a GBC EMMA X-ray Diffractometer with copper CoKĮ radiation operated at 35 kV voltage and approximately 30 mA current in the Soil and Earth Sciences Laboratory of the Institute of Agriculture and Environment, Massey University. This X-ray machine was used for qualitative purpose and therefore no analytical standards were used. The machine was, however, calibrated according to manufacturer instructions.

Powdered whole-rock samples were mounted in aluminium sample holders and run through the X-ray machine and the diffraction patterns recorded. Slides were also made for samples subjected to the primary preparation steps for clay separation as discussed in sections 2.3.2 and 2.3.3 (samples from which carbonates, organic carbon and iron and aluminium oxides have been removed). To assist in the identification of the clay minerals different secondary pre-treatments (Table 2.2) were applied. These were necessary because different clays expand or collapse in response to different treatments.

AllVDPSOHVZHUHVFDQQHGEHWZHHQƒDQGƒșon the X-ray machine. Minerals were

identified using the mineral powder diffraction data book (JCPDS, 1980).

Four secondary pre-treatment methods were used (K-saturation, Mg-saturation, heating and glyceration) on each clay fraction separated from each sample to aid the identification of clay minerals present. Table 2.2 shows the diagnostic d-spacings of several common clay minerals and how these d-spacings change in response to the pre- treatments used.

Table 2.2. Selected diagnostic d-spacings (Å) of common soil minerals at specified conditions of cation saturation, glycerol solvation and heat treatment. Taken from Eslinger and Pevear (1926) and Harris and White (2008).

Mineral Diagnostic d-spacings (Å ) K, 25°C Mg K, 550°C (3 hours) Mg+glycerol Kaolinite 7.2 7.2 No peak 7.2 Illite 10 10 10 10 Montmorillonite 10-14 14-15 10 15-18 Vermiculites 10-12 14 10 14 Chlorites 14, 7 14, 7 14, 7 14, 7

Kaolinite is not affected by the Mg2+íglycerol treatment but collapses when heated to 550°C for 3 hours. The 2:1 clay mineral, illite, has a non-expandable lattice after Mg2+íglycerol treatment and a lattice spacing that is not affected by heat treatment. The peak at 14.0Å due to 2:1:1 mineral chlorite only slightly widens with potassium saturation but is not affected by any other treatment (Ghazi & Mountney, 2011; Harris & White, 2008; Reeves et al., 2006; Velde & Barre, 2010; Whitton & Churchman, 1987). According to Hussain et al. (1985) and Afzal et al. (1999), heat treatment causes an increase in the reflection at 14.0Å due to chlorite, with a slight shift from 14.0Å to 13.6Å giving the reason that this shift might be due to partial dehydration, but does not expand with glyceration. However, montmorillonite has an expandable lattice after glyceration and collapses when heated. Vermiculite also expands with glyceration and collapses on heating (Avery & Bullock, 1977; Whitton & Churchman, 1987).

Each of these pre-treatments was carried out according to the method of Whitton and Churchman (1987) and is described as follows:

Potassium saturation

Approximately 10 mL of the clay suspension (from section 2.3.3) were poured into a tube and 3 drops of 1M HCl and 3-5 mL of 1M KCl solutions were added. The tubes were stirred well with a Teflon rod and allowed to settle overnight. The next day the clear supernatant liquid was extracted with a pipette and a further 10 mL of KCl

solution was added and allowed to stand overnight. The clear supernatant was extracted and the tubes were filled with distilled water, shaken gently and allowed to stand overnight. The supernatant liquid was extracted again. This was repeated until the clays began to disperse (Whitton & Churchman, 1987). Then 1 mL of clay suspension of the clay fraction for each sample was sedimented onto aluminium discs for clay mineral analysis. The clay suspension was added drop by drop with a pipette to the disc which was then kept overnight to dry. These discs were then run through the XRD.

Magnesium saturation

About 10 mL of clay suspension were taken in a tube and 10 mL of 1M MgCl2 solution

together with 2-3 mL of 1M HCl were added. This mixture was stirred well with a Teflon rod and allowed to stand overnight. The clear supernatant liquid was extracted with a pipette the next morning and the tubes were refilled with distilled water, shaken gently and allowed to stand overnight. The next morning the supernatant was extracted. This was repeated until the clay began to disperse. Then 1 mL of clay suspension of the clay fraction for each sample was sedimented onto aluminium discs for clay mineral analysis. The clay suspension was added drop by drop with a pipette to the disc which were then kept overnight to dry. These discs were then run through the XRD.

Heating

The K+-saturated discs were heated to 550°C in a muffle furnace for 3 hours, cooled and another diffractogram was obtained (Whitton & Churchman, 1987).

Glyceration

The Mg2+-saturated slides were sprayed with a 10% glycerol and water solution using an aerosol spray bottle. Each slide was allowed to dry for 3-4 hours and another diffractogram was again obtained (Whitton & Churchman, 1987).

2.4.1.2 Organic petrography

Organic petrography is widely used to characterise organic matter of source rocks. The technique is used to identify the organic constituents (macerals) present in sedimentary rocks of all ages (Alpern, 1980), and is conducted on a polished section. The identification of macerals in a source rock is useful for source rock diagnosis, as macerals are mainly related to biological units and allow reconstruction of the environment of deposition of the source rock (Bertrand et al., 1993; Schiøler et al., 2010).

To investigate the macerals, polished grain mounts of four samples were examined by Richard Sykes at GNS Science (Lower Hutt) using a Leica DMRXA2 petrological microscope fitted with a Leica DC500 digital camera. The mounts were scanned at 500 and 625x magnification under both white light and blue light (fluorescence mode). No quantitative macerals analyses were undertaken.