SEM based analyses

Scanning electron microscopy (SEM) was used in conjunction with numerous analytical techniques to determine the physicochemical characteristics of dome rock and ash samples.

In an SEM, an electron beam is emitted from an electron gun and focused onto the sample.

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The beam electrons interact with electrons in the sample and produce various signals that can be detected (Figure 3.2). These signals contain information about the topography and composition of the sample and include, amongst others: secondary electrons (SE), surface topography; backscattered electrons (BSE), atomic number and topography; X-rays, chemical composition at the surface; cathodoluminescence (CL), internal structures and chemistry. The spatial resolution of SEM depends on the source and instrument optics (both a field-emission gun (FEG) SEM and a tungsten filament SEM are used here) as well the interaction volume with the sample, but is generally < 5 nm in the current study.

3.3.1.1 Sample Characterisation: FEG-SEM

Imaging and analysis of all samples was carried out on a Hitachi SU-70 (FEG) SEM in the GJ Russell Microscopy Facility, Department of Physics, Durham University. Operating voltage and working distance for individual images varied depending on the mode of operation (see below) and are recorded on the image.

Images were recorded at various magnifications for a representative analysis of the distribution, size, and shape of particles sampled.

Figure 3.2: Schematic representation of the energies produced from incident electron beam interaction with solid matter in an SEM.

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Secondary electron (SE) imaging was used to collect topographical and morphological data for dome rock stubs and ash samples. Dome rock stubs were principally used to image vapour-phase crystalline silica crystals in situ to identify morphological differences among samples. Ash samples were primarily imaged for the identification of fibrous particles, which would raise fibre-related respiratory concerns (see Chapter 2).

Backscatter electron (BSE) imaging was carried out on carbon-coated polished thin sections of samples. BSE imaging is ideal for obtaining high-resolution compositional images of a sample, allowing for quick discrimination among different phases. The interaction of an accelerated electron beam with a sample target produces a variety of elastic and inelastic collisions. The number of backscattered electrons reaching a BSE detector is proportional to the mean atomic number of a sample, with larger atoms (high atomic number) having a higher probability of producing an elastic collision. Therefore, brighter BSE intensity correlates with a greater atomic number and darker areas have a lower atomic number.

Differences in mineral polymorphs (e.g., crystalline silica) are not distinguishable by BSE due to crystals having the same mean atomic number.

Energy dispersive X-ray spectroscopy (EDS) was used to semi-qualitatively determine elemental phases for locations of interest in sample thin sections and to produce lower magnification elemental maps. All measurements were performed with an Oxford Instruments EDX system (INCAx-act LN2-free analytical Silicon Drift Detector) and interpreted using Oxford Instruments INCA software.

Wavelength dispersive X-ray spectroscopy (WDS) was used to quantitatively analyse elemental phases for locations of interest in sample thin sections and to quantitatively map single elements using combined EDS/WDS mapping. All measurements were performed with an Oxford Instruments WDS system (INCAWave 700 spectrometer) and interpreted using Oxford Instruments INCA software.

3.3.1.2 Elemental Analysis: Electron Microprobe

Electron probe micro analysis (EPMA) was used to quantify elemental substitutions in volcanic cristobalite. EPMA uses an incident electron beam to generate X-rays with energies and wavelengths specific to the elements present in the sample, allowing quantification of elements of interest. Analyses were carried out on a CAMECA SX100 SEM equipped with five vertical crystal WDS detectors and a PGT Spirit EDS analyser at the Grant Institute of Earth Science, University of Edinburgh.

Specific locations on carbon coated (25 nm) thin sections were quantitatively analysed for Na, Si, Mg, Al, K, Ca, Ti, Mn, and Fe. Measurements for Na and Si were analysed at 4 nA, 43

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and the rest of the elements at 80 nA. Sodium was measured first to minimize the effects of sodium drift on the results. Instrument calibration was carried out by analysing the following reference materials of known compositions: Na on Jadeite-BL7, Si on Quartz, Mg and Al on Spinel-Bl-8, K on Orthoclase-BL7, Ca on Wollast-BL8, Ti on Rutile-BL8, Mn on PuMn-BL8, Fe on Fayalite. The crystals used for sample measurements were: Spectrometer 1 TAP (Si, Al), Sp2 LLIF (Mn, Fe), Sp3 LPET (K, Ca, Ti), Sp4 TAP (Al), Sp5 LTAP (Na, Mg).

The large analysing crystals enabled higher analytical precision, lower detection limits, and faster analysis times. Spatial resolution of the set up conditions was approximately 1 μm and detection limits of <0.009 to <0.002 wt. % were obtained.

3.3.1.3 Structural Determination: Cathodoluminescence

Cathodoluminescence is the emission of light from a solid following excitation by an electron beam; and, when applied to geological samples, can provide information on provenance, growth fabrics, diagnostic textures, deformation and defects, and mineral zonation.

The structural composition of crystalline silica phases in dome rock as well as the identification of any internal structures that could elucidate crystal growth history was determined by CL. Cristobalite can be distinguished from the other crystalline silica polymorphs by its excitation wavelength – cristobalite is at 400 nm (blue), quartz is at 650 nm (red), and tridymite has peaks at 430 and 650 nm – as well as from groundmass feldspar (680-740 nm range) and glass (285, 460, 650). However, Al³⁺, Mn²⁺ and Fe^2+/3+ defects within crystalline silica can lead to spectra that are broader and/or displaced from ideal as well as the presence of minor peaks which are difficult to de-convolve. Incorporation of elemental defects allows for discrimination of different generations of the same mineral, e.g., zoning in zircon and plagioclase. As is the case with quartz, cristobalite luminescence was determined here to degrade with time, making long-duration and multiple measurements of excited areas difficult.

Monochromatic imaging and serial-CL analyses were carried out at the Durham GJ Russell Microscopy Facility using a Gatan MonoCL cathodoluminescence system and Digiscan II image processing software (Gatan, Inc., Pleasanton, CA, USA) attached to the Hitachi SU70 FEG-SEM. True-colour images were constructed from three consecutive grey-level analyses using a red, green and blue series of colour filters.

3.3.1.4 Structural Determination: Raman Microscopy

Raman spectroscopy provides information about molecular vibrations that can be used for structural identification of minerals. The technique involves shining a monochromatic light

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source (i.e., a laser) on a sample and detecting scattered light. A very small amount of the scattered light is shifted in energy due to interactions of the incident laser (electromagnetic waves) and the vibrational energy levels of the molecules in the sample. The frequency shift corresponding to the energy difference between the incident and scattered photon is termed the Raman shift. This shift provides information about vibrational, rotational, and other low frequency transitions in molecules, thus providing crystallographic and mechanic data for the sample of interest. Plotting the intensity of this shifted light (Raman shift) versus frequency results in a Raman spectrum. Different materials have different molecular and macroscopic vibrational modes, and therefore different characteristic Raman spectra.

Crystalline silica polymorphs were distinguished in uncoated polished thin sections using a Renishaw SEM-Raman Structural and Chemical Analyser, which couples Raman spectroscopy (Renishaw inVia Raman microscope, 785 nm laser) with SEM-EDS (JEOL JSM-6060 LV SEM with Oxford Instruments INCA energy dispersive X-ray analysis) at Renishaw plc, UK. This allowed for the identification of silica polymorphs for individual crystals. Reference spectra were obtained from the online Handbook of Minerals Raman Spectra (http://www.ens-lyon.fr/Raman/index.php), which uses reference spectra from Kingma and Hemley (1994).

3.3.2 X-ray Diffraction: Identification of crystalline silica phases in bulk material