A group of researchers, including scientists from St Petersburg University, has brought to light two new minerals that are generated in volcanic gases – arsmirandite and lehmannite. It was previously thought that minerals with such structures and compositions could only be obtained under laboratory conditions. This landmark discovery will help in developing methods to synthesise new materials and also in studying the transfer of metals in geological systems.
A paper about this research has been published in Scientific Reports.
Polyoxometalates are compounds containing nanoclusters that are formed by metal atoms and oxygen. They are known for their high catalytic activity, interesting magnetic and pharmaceutical properties, and great structural and chemical diversity. This can explain why research into polyoxometalates is very popular in the world of chemistry today. Even so, very little is known about the role of polyoxometalate clusters in geochemical processes. Research into naturally occurring minerals with nanosized clusters is a relatively young field in modern mineralogy. The basic discoveries have only been made in the current century, within the past twenty years.
In the laboratory, polyoxometalates are synthesised, as a rule, from aqueous solutions, although they can be obtained from other fluid media, in particular from ionic liquids. The formation of polyoxometalates from gases is a rather unusual process, and for this reason the discovery of arsmirandite and lehmannite is of special significance.
Professor Sergey Krivovichev, Head of the Department of Crystallography at St Petersburg University and a member of the research team
Both minerals are exhalative in origin, and their crystals grow directly from volcanic gases. Rising to the surface from the depths of the Earth, these gases came into contact with the atmosphere and lower temperatures, as a result of which arsmirandite and lehmannite settled out. The minerals were discovered during investigations of fumaroles – rather small fissures through which hot gases escape – in the Plosky Tolbachik volcano on the Kamchatka Peninsula.
Stanislav Filatov, a professor in the Department of Crystallography at St Petersburg University and Lidiia Vergasova, a senior research associate at the Institute of Volcanology and Seismology, which is part of the Far Eastern Branch of the Russian Academy of Sciences, were the pioneers in this research. About ten years ago, Igor Pekov, a professor at Moscow State University and a Corresponding Member of the Russian Academy of Sciences, became involved in this work. It was he who, during one of his expeditions to Kamchatka, collected samples of arsmirandite and lehmannite for further investigation.
Sergey Britvin, a professor in the Department of Crystallography at St Petersburg University, carried out an X-ray diffraction analysis of these minerals. It revealed the unique character of their crystalline structure – the presence of metal-oxide nanoclusters.
‘The research into and discovery of these minerals was made possible only owing to the state-of-the-art equipment in the Resource Centre for X-Ray Diffraction Studies at the Research Park of St Petersburg University. Without the high-precision instruments here, these results could not be obtained,’ Professor Krivovichev noted. ‘But that was only half of the story. The other half was the highly skilled scientists at the University and the fruitful collaboration between mineralogists and crystallographers at two of the country’s leading universities and the Kola Science Centre of the Russian Academy of Sciences. Incidentally, the discovery of arsmirandite and lehmannite is only one of such joint efforts that have been undertaken by St Petersburg University and the Kola Science Centre.’
At first sight, the minerals are practically indistinguishable from one another and are an aggregation of emerald-green crystals. Only after a detailed analysis do the differences become apparent. Both consist of polyoxometalate clusters, in the centre of which is an iron atom (in arsmirandite) or a titanium atom (in lehmannite), surrounded by eight oxygen atoms. They, in turn, are surrounded by twelve copper atoms at the vertices of the cuboctahedron.
‘This is the so-called metal-oxygen core of the cluster, which is surrounded on all sides by eight arsenate groups, completing the construction of the whole cluster,’ Professor Krivovichev explained. ‘The clusters themselves are arranged in a three-dimensional staggered structure, with sodium and chlorine atoms between them. It can be said that the structure of the minerals is a well-formed arrangement of polyoxometalate clusters in a halite, or rock salt, structure.’
The story behind the names of the two new minerals is interesting. The name ‘arsmirandite’ reflects the presence of arsenic in the mineral (the prefix ars coming from arsenicum, the Latin name of the chemical element) and its miraculous properties (from the Latin mirandus, meaning ‘worthy of astonishment, breathtaking, surprising’). Lehmannite is named after the German scientist Johann Gottlob Lehmann, a member of the St Petersburg Imperial Academy of Sciences. A native of Saxony, he lived the greater part of his life in Dresden, but in 1761 he came to St Petersburg, where he was invited to become a member of the Academy of Sciences and chosen to be a director of the Imperial Cabinet of Natural Materials. On 22 January, 1767, while working in the chemistry laboratory, he got poisoned by arsenic compounds, which are a constituent of lehmannite.
The discovery of polyoxometalates that are crystallisable from a gaseous phase compels scientists to reconsider their ideas about the mobility of various elements, including strategic ores, in natural conditions.
‘It is also essential for explorative chemical synthesis,’ Professor Krivovichev pointed out. ‘The point is that we have been able to come up with such clusters for polyoxopalladates – polyoxometalates with the precious metal palladium – but we haven’t been able to obtain them with copper. It turns out that Nature has “invented” a way of synthesising them independently of humans and has shown us the way to imitate these conditions in the laboratory. By the way, we are working on this now in the Department of Crystallography at St Petersburg University and at the Kola Science Centre.’
The scientists are continuing their research on the minerals they discovered. They are also planning to publish a complete description of arsmirandite and lehmannite in the near future.