The fulgurite sample described in the study. The quasicrystal was identified in the metallic part at the centre. Credit: Università di Firenze.
Scientists have found a very rare mineral, which they call a dodecagonal quasicrystal, which probably formed when lightning struck near a fallen power line in a sandy region of the United States. The discovery is surprising, because until now experts doubted that such structures could form on Earth in natural conditions.
Quasicrystals are made of atoms arranged in an ordered fashion, but without the periodic repetition of a simple geometric form that is found in normal crystals. They only form in extreme temperature and pressure conditions. Because of their structure, they have magnetic and electric properties that are not found in either crystals or amorphous solids and could prove useful for many applications.
The rock was found in 2008 in Sand Hills, Nebraska, after a storm and close to a downed power line. It contains a mineral called fulgurite, a melted tube of sand with traces of conductor metal found in the power line.
Luca Bindi from the University of Florence has been studying quasi-crystals since 2009. He suspected that fulgurites, that often form when lightning hits sandy soils, could harbour quasicrystals. “I started asking colleagues from all over the world to send me samples of fulgurites with different origins” he explains “and one of these contained the quasicrystal described in this study”.
The sample was studied using a combination of methods including scanning electron microscopy and transmission electron microscopy, which revealed that it contains quasicrystals where atoms are arranged in layers of dodecagonal clusters, an arrangement that would be impossible for ordinary crystals. Further studies were carried out by comparing the sample with other quasicrystals of various origins. Bindi led the work along with Paul Steinhardt, from Princeton University, and the study is now published in Proceedings of the National Academy of Sciences1.
The researchers concluded that an electrical discharge created temperatures above1,700 °C, forming a quasicrystal with a symmetry and a composition that were never observed before. It is possible, as other experts have argued2, that the mineral was formed solely by the electrical discharges from the downed power line. However, the researchers favour lightning as an explanation, because the Sand Hills fulgurite extended well beyond the contact point with the aluminum wire, and has a morphology similar to other fulgurites caused by lightning.
Until now, quasicrystals had only been found in debris from impacts of extraterrestrial bodies and in residuals of nuclear explosions. These extreme conditions are difficult to reproduce, but now new insights from this study may pave the way for the synthesis of artificial quasicrystals.
“Beside new technological applications of quasicrystals that we do not image yet, our study can foster research in fields like condensed matter physics and solid-state chemistry,” says Bindi.
