Mineral Analysis Facility
The Centre's Mineral Analysis Facility includes a range of microscopic and x-ray instruments from leading manufacturers.
The Cameca SX 100 Electron Microprobe is a fully digitised instrument with highly integrated electronics and full automation which can be set up for unattended analysis and evaluation. It has a standard tungsten electron emitter which produces a beam of electrons of tuneable energy focussed onto the surface of the sample, exciting a volume of ca. 1 µm3. The sample can be moved across the beam in the perpendicular plane, to analyse a selected number of sites. There are five wavelength dispersive detectors, each equipped with a choice of diffracting crystals, allowing simultaneous quantitative analyses of up to five different elements at chosen points across the sample, at a special resolution of ca. 1 µm.
The spectrometer can be programmed to run a series of scans for different combinations of elements, giving a comprehensive elemental analysis of the surface. There is also an energy dispersive detector, used to collect fluorescence spectra allowing identification at each point selected of all the elements present with a fluorescence line at lower energy than the electron beam. The spectrometer can also be set up to map the relative concentrations (non-quantitatively) of up to four elements at a time (or five elements with fewer pixels per map) across a raster of 512 x 512 pixels in a selected area.
The instrument is equipped with an optical microscope which can be centred on the focal point of the electron beam in order to identify areas to sample. It has continuous zoom (from 250 µm to 1.7 mm field) for ease of location of points of interest in inhomogeneous samples.
Samples should be flat, for example a thin section, and must be coated with a thin film of carbon before being introduced into the spectrometer. All measurements take place in a vacuum, therefore volatile samples cannot be used.
Digital Instruments NanoScope IIIA Scanning Probe Microscope have facilities for in air (or in fluid) scanning experiments. The instrument provides contact and tapping mode Atomic Force Microscopy (AFM) and Scanning Tunnelling Microscopy (STM). A maximum scan size available of 150x150 mm with a maximum Z resolution of 0.3 nm and X-Y resolution of ca. 2 nm for AFM studies (STM atomic imaging is possible with suitable samples).
The microscope enables the determination of surface morphology and structure (including electronic structures with STM) of solid materials at very high magnification (in some cases attaining atomic resolution). Sample size is limited (<1cm diameter, ~1mm thick) and a clean flat surface essential. Suitability of material and appropriate preparations must be discussed with staff in advance.
The FAST ComTec MCA-3 Mössbauer Spectrometer has a 25 mCi Co-57 source and is equipped with a liquid N2 cryostat with temperature controller for experiments between 77K and 300K. Recoil commercial software is utilised for fitting of data.
The spectrometer allows studies of the crystal chemistry of iron in minerals (oxidation state, spin state, co-ordination and covalence). Samples must be > 100mg of powder with at least 5% Fe content. Samples will be mounted by staff. Powder samples can be mounted in a sealed anaerobic package.
Booking is strictly by arrangement with staff, experiments can only be run by staff/trained users.
The FEI XL30 ESEM-FEG is used for imaging a wide range of samples, everything from polished rock sections to bacteria to Egyptian mummies. With carbon or gold coated samples, the ESEM can be used as a traditional SEM utilising either secondary electron or backscattered electron detectors.
Under ‘wet’ mode, the ESEM can be used to image uncoated samples, ideal for imaging biological materials and precious samples. The ESEM is equipped with an EDAX Gemini EDS system, so elemental analysis of samples can be carried out while imaging. The EDAX system can also be used to automatically map and analyse components. The ESEM is also equipped with a Gatan gold stage for cryo-imaging.
The RHK-VG UHV-Scanning Tunneling Microscope microscope enables the determination of surface morphology and structure, including electronic structures of solid materials at atomic resolution at room temperature. The LEED/AES sample analysis will yield surface order parameters and elemental surface ratios with contaminants identified to ca. 3 %.
Samples must be single crystal and may be prepared by in situ Ar+ ion cleaning and annealing or may be cleaved in situ to prepare a fresh clean surface. Facilities for gas dosing to assess sample reactivity are present on the microscope.
The Gemini 2360 Surface Area Analyser measures the surface area of samples by making use of the Brunauer-Emmett-Teller (BET) theory. Granulated or powdered samples are first purged of adsorbed water by heating under a stream of helium for several hours, using a Micromeritics FlowPrep 060. The purged samples are then evacuated and cooled using liquid nitrogen, before nitrogen gas is admitted to the sample vial and an identical empty vial. A differential pressure transducer measures the imbalance in pressure between the sample and blank vials, caused by the adsorption of gas onto the sample surface. The system uses this data to calculate the number of molecules of nitrogen required to form a monolayer on the sample surface and, since the molecular dimension of nitrogen are known, calculates the surface area of the sample in square metres per gram.
The X-ray diffraction laboratory deals with classic geological problems such as soil lithography and mineral phase identification, using two diffractometers: a Philips PW1730 and a Bruker D8Advance, with a 9 position sample changer which allows us to run batches of standard powder diffraction samples.
The Bruker is fitted with a Göbel mirror which focuses the X-ray beam, giving us beam intensity comparable to first generation synchrotron. It is this beam intensity that allows us to perform reflectometry experiments, measuring the X-rays reflected from mineral surfaces to deduce surface roughness.
We can also collect diffraction patterns from non-powdered samples, for example those too precious to powder, such as meteorites and archaeological samples.
Since the diffractometer is in theta-theta geometry, we can take further advantage of the precise control over sample height afforded us by the reflectometry stage to perform depth profiling of the surface 1-2 μm of samples, by varying the incident angle of the X-ray beam and hence the X-ray penetration depth.
These XRD patterns were collected using the PW1730. The patterns demonstrate that while the presence of PVS scale inhibitor during barite precipitation leads to widely differing morphology (shown in the accompanying SEM micrographs), the crystal structure is essentially the same, save for those changes in orientation, and hence peak intensity, which give rise to the shape of the particulates.
Our facilities are available for commercial use.