NEWS OF THE WEEK
FULLERENES IN NATURE: C60 and C70 found in ancient Russian rock A n unusual carbon-rich rock be/ % lieved to be more than 600 milJ L J L lion years old has yielded the first evidence that fullerenes occur in nature. The fullerenes C60 and C70 were discovered in a sample of shungite, a rock of uncertain origin found near the Russian town of Shunga, about 250 miles northeast of St. Petersburg. The serendipitous discovery was made by geochemist Peter R. Buseck and mineralogist Semeon J. Tsipursky, both of Arizona State University, Tempe, and mass specrrometrist Robert Hettich of Oak Ridge National Laboratory in Tennessee. They published their findings last week in Science [257,215 (1992)]. The discovery means fullerenes can no longer be thought of as purely artificial materials made in the lab using laser vaporization, carbon arcs, or combustion processes. Several groups previously looked for fullerenes in soot deposits and in meteorites, without success. And scientists searching for the fullerene spectral signature in interstellar dust clouds found no definitive evidence. The rock sample that changed all this was sent to Tsipursky, formerly a researcher at the Russian Academy of Sciences in Moscow, by a Russian colleague who knew of his and Buseck's interest in carbonaceous rocks. While examining high-resolution transmission electron microscopy (HRTEM) images of the shungite sample, Tsipursky saw some curious honeycomb patterns in the midst of graphitic and amorphous material. The patterns reminded him of HRTEM images of lab-made C60 he had seen several months earlier in Buseck's lab. Buseck was intrigued by his colleague's observation but could not believe the black Russian rock actually harbored fullerenes. Nevertheless, he, Tsipursky, and Hettich used several different mass spectrometry techniques to analyze the shungite sample, and they finally were able to confirm that it con6
JULY 13,1992 C&EN
Buseck: serendipitous discovery
tains trace amounts of C^ and C70. Hettich, who's experienced in mass analysis of synthetic fullerenes, performed control experiments to ensure fullerenes were not formed during the analyses. These experiments "definitively established the presence of fullerenes in our samples," the researchers note. "It's a very exciting discovery," comments chemistry professor Harold W. Kroto of the University of Sussex in Brighton, England, one of the discoverers of Cœ (buckminsterfuUerene). However, he tells C&EN, 'Tm not surprised that it forms in a terrestrial environment." After all, C^ has been found to form in a sooting flame, and Kroto suspects it is widespread in the environment. Kroto does find two aspects of the discovery surprising, however. First, the fullerenes in the shungite appear to be segregated from other carbonaceous material inside fractures in the rock. And the fullerenes appear to have been formed in the condensed phase. This is in sharp contrast to the usual methods for making C 60 in the lab,
which involve gas-phase reactions that produce many other carbon species as well. In the gas phase, the fullerene cages have plenty of room to grow. A solid environment, on the other hand, seems too crowded for this kind of growth to occur. In any case, Kroto believes the new discovery will spark a renewed search for better methods of making fullerenes—particularly in the solid phase. Like many discoveries in science, this one poses more questions than it answers. For example, although the sample of shungite dates from the Precambrian era (600 million to more than 4 billion years ago), no one can say whether the fullerene molecules in it formed in Precambrian times or much more recently. How the fullerene inclusions could have formed also is a mystery, particularly since the origin of shungite itself is a matter of controversy. Some scientists believe the rock is biogenic in origin (like coal), whereas others think it may be metamorphic or volcanic. "I know frustratingly little about the rock and about the geological setting," Buseck tells C&EN. He also isn't sure what percentage of the sample consists of fullerenes, although he thinks it is a very small fraction. Nevertheless, his group's findings are sure to trigger further searches for fullerenes in nature. "I should imagine people will be looking under every stone," Kroto remarks. And detecting C60 in space remains, for Kroto, an exciting possibility. The experiments in 1985 that revealed the existence of the carbon-cage molecule were aimed at elucidating the stellar chemistry that leads to the formation of long carbon-chain molecules. "The jury is still out" about the existence of fullerenes in interstellar space, Kroto says. But he, for one, is optimistic that buckminsterfuUerene, believed to form in the atmosphere of carbon-rich stars, will eventually be found in space. Ron Dagani
