By Assoc. Prof. A Gerson and Dr R J Hill FTSE
The earliest applications and development of conventional X-ray technologies at the beginning of the last century were strongly focused on earth science materials. Over the past 20 years minerals have continued to play a central role in the development of the new generations of synchrotron radiation techniques and the range of applications of synchrotron X-ray techniques to the study of the earth sciences has grown enormously. In this article we focus on the analytical possibilities arising from the application of synchrotron X-ray radiation as opposed to conventional cathode-tube based X-ray sources, using examples drawn from mineral characterisation and mineral processing.
Conditions in the Earth's lower Mantle
Knowledge of the dynamics of the Earth's mantle is important for understanding the global changes in the history of our planet. Recent work1 at the European Synchrotron Radiation Facility at Grenoble (reproduced here from its Website) has focused on the crystal structures of ferropericlase (Mg,Fe)O and (Mg,Fe)SiO3-perovskite at high pressure. These minerals are considered to be the main constituents of the Earth's lower mantle. Their properties are best studied with synchrotron radiation, where the X-ray beam's unprecedented intensity, collimation and tunability can be used to good effect in the constricting environment of the high pressure cells required for 'in-situ' experiments.
Under ambient conditions, the end members of the MgO-FeO solid solution - periclase (MgO) and w»stite (FeO) have the same crystal structure and they form a complete solid solution. However, at pressures above 17 GPa and ambient temperature, w»stite transforms into a phase with rhombohedral structure. With increasing pressure above 100 GPa at 300 K it transforms to the NiAs or the anti-NiAs structure, whereas periclase retains the NaCl structure at least to 227 GPa.
The X-ray studies of the externally heated samples in the diamond-anvil pressure cell show that magnesiow»stite partially dissociates into a lower density, magnesium rich, phase and a higher density, iron rich, phase at 85 GPa, corresponding to a depth of 1900-2000 km (PREM). Such dissociation of magnesiow»stite provides important insights into the heterogeneity of the lower mantle.
The Production of Smelter-grade Alumina by the Bayer Process
Filtering, drying and aging of crystalline solids can affect both crystalline phases and amorphous structures and therefore ex situ X-ray measurements are not necessarily indicative of in situ structure. The application of high intensity synchrotron X-radiation has enabled the formation of aluminium hydroxide (gibbsite) particles in caustic aluminate liquors to be followed in real time in situ. The crystallisation of gibbsite is one of the critical unit operations in the Bayer process for the production of smelter grade alumina and cannot be studied on a conventional X-ray source. An example of in situ diffraction measurements taken at the Australian National Beamline Facility at the Photon Factory synchrotron using image plates is shown in Figure 1.2 These diffraction patterns show that the phase of the precipitate change as a function of solution preparation conditions. These measurements are of direct application to the Australian alumina industry.
Figure 1: Powder diffraction patterns of a solution containing NaOH/Al = 1.37, 6.0 M NaOH (a) made by dissolving Al(OH)3 dissolved at atmospheric pressure. Only gibbsite (G) is observed. (b) Solution made by heating the solution to 160oC for 16 h. Both bayerite (B) and gibbsite (G) are observed. (c) Solution made as for (b) but seeded with gibbsite. The flattened top of the G(002) diffraction peak is due to saturation of the image plate by the diffracted X-rays. G=gibbsite, B=bayerite.
Elemental mapping
With the use of microbeam focusing3 and the high X-ray intensity available at synchrotron X-ray sources, highly sensitive XRF mapping of mineral assemblages can now be carried out routinely. An example of a gold fluorescence from an arsenopyrite grain is shown in Figure 2.4
Figure 2: X-ray fluorescence map of an entire arsenopyrite grain. A 6 x 7 µm beam was used in 5 µm steps in x-y direction, collecting the Au fluorescence for 2 s. The incident energy was set to 12.0 keV, well above the Au edge. Grain is ~100 micrometers across.4
Understanding Acid Mine Drainage
One of the most common and frustrating forms of contamination around mining sites is the formation and spread of acidic water on the surface and into groundwater near mines. It occurs when metal-sulfide ores are exposed to air and water and the sulfide is transformed to sulfuric acid. Moreover, some of the associated metals, such as zinc, are toxic and can leach into groundwater and contaminate wells and other drinking water supplies.
Recent studies on beamline 2-ID-D (SRI-CAT) at the Advanced Photon Source (APS) at Chicago by scientists from the University of Wisconsin-Madison, the Australian Geological Survey Organisation5 and Argonne National Laboratory have found compelling evidence that micro-organisms play a central role in the formation of certain mineral deposits. These results shed light on the basic question of biology's function in the formation of some metal ores, and hold out the promise for applications in mining-site remediation. They take advantage of the extremely high intensity of the synchrotron X-ray beams and the ability to focus them onto very small sections of the sample for chemical and other analyses.
The studies were performed on biofilms (excretions from bacteria clinging to a surface in an aqueous environment) collected by SCUBA divers in a flooded zinc and lead mine in Tennyson, Wisconsin. The X-ray fluorescence microbeam studies revealed the process by which minerals accumulate within natural biofilms populated by species of sulfate-reducing bacteria. The reduction of sulfate to sulfide by the Desulfobacteriaceae bacteria is linked to the oxidation of organic matter and releases sulfide ions into the solution around the biofilm. This leads to saturation in the neighbourhood of the biofilm and, as a result, zinc from the surrounding groundwater rapidly forms microscopic crystals of zinc sulfide in the biofilm.
Beyond possible applications in bioremediation, this work broadens the fundamental scientific understanding of the role that microbes play in the formation of mineral deposits.
Mineral Separation by Selective Flotation
Separation of valuable minerals from valueless gangue is often achieved by selective mineral flotation processing. Hydrophobic collector molecules are added to the mineral slurry. These molecules are designed to selectively interact with the valuable components of the ore thus rendering them hydrophobic. In some cases however, the collector molecules do not interact sufficiently with the mineral surface to induce hydrophobicity and the surface must first be 'activated'.
The activation mechanism in which sphalerite (ZnS) is conditioned with Cu(II) (normally as nitrate or sulphate), prior to adsorption of xanthate and flotation, has been extensively studied over more than three decades. Much is understood concerning the adsorption of Cu(II) from solution and the release of Zn(II) to solution. Until recently, the nature of the copper species formed on the surface of sphalerite was the subject of some controversy. XAFS (X-ray absorption fine structure) analysis (carried out at the Australian National Beamline Facility, Photon Factory, Tsukuba) has been used to clarify the form of the Cu on the sphalerite mineral surface.6 Figure 3 shows a Cu atom which has replaced a Zn atom on the (110) surface of sphalerite to form a slightly distorted trigonal planar structure bonded to three S atoms.
Figure 3: (a) A typical industrial froth flotation slurry and (b) an example of the structure of Cu activated sphalerite, one of the fundamental process that occurs during mineral processing. The Cu atom is represented as a black sphere, the S atoms are white and the Zn atoms are grey. The three S atoms bonded to the Cu atom are also shown as spheres.
 
Surface Sensitive Analysis of Sulfide Minerals
Figure 4 illustrates the enhancement in performance and surface chemical information provided by the combination of high flux, excitation energy control and X-ray line resolution from an analysis of the S 2p region of vacuum fractured pyrite (FeS2) surface.7 The conventional XPS spectrum (a) required 11/2 hours to acquire using the best resolution settings of the analyser. The fixed, high energy source limits the surface sensitivity for the S 2p such that the majority of the signal is derived from the bulk disulfide state. Only a small surface (top monolayer) disulfide contribution is observed. Spectra (b) and (c), were acquired in only 15 minutes with synchrotron excitation. The dramatic increase in resolution is evident and the use of variable excitation energy highlights the ability to tune the surface sensitivity. At 300 eV, fully 40% of the signal is derived from the top 11/2 monolayers of the surface. Surface disulfide is dramatically enhanced and surface monosulfide is now apparent. Increasing the excitation to 400 eV confirms the surface nature of these components by increasing the kinetic energy and hence the escape depth of the S 2p photoelectrons.
Figure 4: Conventional (a) and variable-energy synchrotron radiation XPS spectra (b and c) of the S 2p core line of vacuum-fractured pyrite, FeS2. Full scale counts are (a) 8x103, (b and c) 8x107.7
The Future
The unique and very powerful characteristics of synchrotron radiation provide wonderful new opportunities for increased understanding of earth science processes, in the natural environment and in the laboratory and industrial context. It is anticipated that in the next ten years not only will the current techniques continue to be developed in terms of elemental sensitivity, the range of environments available, mapping capabilities, surface sensitivity, etc, but also that new techniques will be developed. It is also anticipated that the remote operation of experimental stations, simultaneous multi-technique analysis, in-situ measurements, and a greater focus on industrial applications are likely to become a central focus of the future development of synchrotron facilities.
References
- L.S. Dubrovinsky, N.A. Dubrovinskaia, S.K. Saxena, H. Annersten, E. Hïlenius, H. Harryson, F. Tutti, S. Rekhi and T. Le Bihan, Stability of ferropericlase in the lower mantle, Science 289, (2000) 430-432.
- A.R. Gerson, J.A. Counter and D.J. Cookson, The influence of solution constituents, solution conditioning and seeding on the crysalline phase of aluminium hydroxide using in situ X-ray diffraction, Journal of Crystal Growth 160 (1996) 346-354. J. Counter, A. Gerson and J. Ralston, Caustic aluminate liquors: preparation and charactersiation using static light scattering and in situ X-ray diffraction, Colloids and Surfaces A 126 (1997) 103-112.
- G.E. Ice, Microbeam forming methods for synchrotron radiation, X-ray Spectrometry 26 (1997) 315-326.
- L.J. Cabri, M. Newville, R.A. Gordon, E.D. Crozier, S. Sutton, G. McMahon and D.T. Jiang, Chemical speciation of gold in arsenopyrites, The Canadian Mineralogist 38 (2000) 1265-1281.
- M. Labrenz, G.K. Druschel, T. Thomsen-Ebert, B. Gilbert, S.A. Welch, K.M. Kemner, G.A. Logan, R.E. Summons, G. De Stasio, P.L. Bond, B. Lai, S.D. Kelly and J.F. Banfield, Formation of sphalerite (ZnS) deposits in natural biofilms of sulfate-reducing bacteria, Science 290 (2000) 1744-1747.
- A.R. Gerson, A.G. Lange, K.E. Prince and R.St.C. Smart, The mechanism of Cu activation of sphalerite, Applied Surface Science 137 (1998) 207-223.
- W. Skinner and A. Pratt, Synchrotron XPS Studies of Cu-activated Pyrite, to be submitted to Surface and Interface Analysis (2002).
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