Australia’s rich ore deposits were created millions to billions of years ago in conditions that some would describe as hell on earth: highly toxic chemical brews heated to 400-500°C under pressures as high as several 1000 times atmospheric pressure.
Similar conditions are found on the ocean floor about three or four kilometres down, where hot springs called ‘black smokers’ form metal-rich 'chimneys' and sediments.
Knowing the chemistry behind ore-forming reactions helps the minerals industry develop more effective exploration techniques and mineral processing methods. Recreating the extreme conditions is difficult, but studying the reactions themselves is even harder. Geologists have therefore tended to rely on subjecting appropriate mixtures to extreme temperature and pressure – and analysing the reaction products afterwards so they can make educated guesses as to what chemical species were present during the high-temperature, high-pressure reactions. This provides information on the elemental composition of the reaction species – but not their structure.
A collaborative team from The University of Adelaide (UA) and CSIRO, led by Associate Professor Joël Brugger and Dr. Weihua Liu, have a different approach. They’re using x-ray absorption spectroscopy (XAS) at the Australian Synchrotron to tell them what’s happening inside a purpose-built high-temperature, high-pressure spectroscopic cell. Only available at synchrotrons, XAS is ideal because it can be used to obtain in-situ information on solubility and structure under extreme conditions, including conditions close to the critical point of water (where liquid water and steam become indistinguishable), A further advantage is the time taken to obtain synchrotron data: a matter of days, rather than the weeks or months typically required for traditional laboratory-based solubility experiments.
The team’s autoclave has been christened mAESTRO for Australian Extreme SpecTROscopy. It was built in collaboration with the CNRS, France’s national scientific research agency, based on a similar cell developed for the European Synchrotron Research Facility (ESRF).
Using the French autoclave at the ESRF, Joël and his collaborators have shown that metals such as iron, copper, cobalt and bismuth adopt different structures at high temperatures – explaining the different behaviours of these metals in ore deposits. The team has also identified a tetrahedral structure at high temperatures and pressures rather than the octahedral structure previously assumed for transition metals such as iron, zinc and cobalt. This difference in structure has important implications for of the geochemical modelling of the formation of ore bodies and for optimisation of mineral processing conditions.
The French and Australian autoclaves can also be used to study aspects of carbon dioxide sequestration (which would store power-station carbon dioxide emissions deep underground at high temperatures and pressures) and geothermal power generation.
In October 2009, Joël and his colleagues spent a week at the synchrotron setting up and commissioning their highly specialised equipment.
“It took us a long time to reach this stage, with all sorts of safety issues to address as well as the scientific ones,” Joël says. “However, it’s easier than building a synchrotron on the seafloor – and our early results are very promising.”
After testing mAESTRO to 400 bar (equivalent to four kilometres below the sea) and 500°C, the group began examining the behaviour of nickel under high pressures and temperatures, and successfully mapped the evolution of the composition and structure of nickel chloride complexes as a function of temperature and pressure, a piece of crucial information in understanding of hydrothermal nickel ore formation.
“We want to learn enough about how metals behave under extreme conditions to be able to make useful predictions based on room temperature observations,” Joël says. “This will also involve studying metal sulphides and sulphides in general, which have very complex chemistry as well as being quite smelly and rather toxic.
“The beauty of mAESTRO is that the amount of sample it holds (equivalent to a few drops of liquid) is small enough to make safety precautions manageable, but large enough to make it easy to load.
“We’re looking forward to bringing mAESTRO back again for some more high-pressure beamtime at the Australian Synchrotron next year.”