Diamonds are not only
beautiful and valuable gems, they also contain information of the geological
history. By using ultra-thin slices of diamonds, Dorrit E. Jacob and her
colleagues from the Macquarie University in Australia and the University of
Sydney found the first direct evidence for the formation of diamonds by a
process known as redox freezing. In this process, carbonate melts crystallize
to form diamond. The slices were prepared by Anja Schreiber of the GFZ German
Research Centre for Geosciences in Potsdam, Germany. The work is published in
Nature Communications. The study shows that the reduction of carbonate to
diamond is balanced by the oxidation of iron sulphide to iron oxides.
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| Siberia's Udachnaya diamond mine, by Stepanovas (Stapanov Alexander). (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons |
The researchers used the new
nano-scale technique of Transmission Kikuchi Diffraction to discover rims of
the iron oxide mineral magnetite just a few ten thousandths of a millimetre
thick around sulphide minerals inside the diamonds. The GFZ's Anja Schreiber
prepared these slices using a focussed beam of charged atoms (ions) to ablate
the surface. The already ultra-thin slices were re-thinned after being mounted
on a carbon-coated copper grid. This process was carried out for the first time
successfully on a grid and yielded the data set used for the study.
The results also solve a
puzzle that has occupied diamond researchers for decades, namely the
over-abundance of sulphide occurring as inclusions in diamond. Iron sulphides
are the most common inclusions in diamond even though there is only about 0.02%
of sulphur in the mantle: it now appears that the oxidation of the iron
sulphides directly causes the formation of the diamonds that include them.
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