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Simon Redfern

Simon Redfern is a mineral physicist at the University of Cambridge. He studies the properties of materials in Earth, from biominerals in seas shells to the nature of Earth's inner core. He uses neutron and synchrotron light sources to study these properties at the atomic scale, and links the results to phenomena at the global scale. Tweets as @Sim0nRedfern.

Music of the spheres

722px-Liquid_water_hydrogen_bondI am sitting in a hot lecture theatre in the Università deli Studi, Florence, two days before the start of the proper business of Goldschmidt 2013, a meeting of thousands of geochemists from across the globe. Before discussions of the latest news and views in the geochemical world, a number of pre-conference workshops and short courses are taking place. It’s a chance to get an introduction and overview of the problems and methods in a particular domain.Duomo

The thermodynamic properties of geothermal fluids control how chemical elements are transported and concentrated, relevant to formation of metal ores, how chalky precipitates form in your kettle or central heating system, how energy can be extracted in engineered geothermal plants, how fluids flow from subducted crust to volcanoes in subduction zones, how CO2 might behave as it is pumped into carbon capture and storage sequestration sites, and even how organisms can survive and flourish in crustal rocks.

The starting point for much of our understanding lies in how water molecules respond to temperature, to pressure, and to changes in their chemical environment as salts are added to watery fluids. Fundamentally, it all depends on how the atoms interact in the fluid.

There are certain inherent problems of describing atomic interactions at the molecular scale. One is linked to the “three-body problem”. The properties of a geofluid or mineral can be calculated by considering interactions between all the atoms present. But those interactions can only be calculated between pairs of atoms. In practise, interactions between more than two atoms are approximated as being due to pairs, and the pairwise interactions are then summed up.

It is rather like trying to calculate the movements of the Earth, Moon and Sun with respect to each other. There are three astronomic bodies present, but the movements of the spheres are treated as the sum of the Earth-Moon, Moon-Sun, and Earth-Sun interactions. Geomaterials are treated in the same way, at a far smaller scale.

Galileo_Tomb_Santa_CroceWhile reflecting on this, I was reminded of the earlier notable work on those larger-scale interactions, carried out in this city, that transformed our view of Earth.

On my way to the lecture theatre this-morning I had stopped off at Basilica di Santa Croce. It is the last resting place of Galileo, and I took a look at his tomb. It is a wonderful monument. It is interesting to see how it contrasts with some of the others around him in the church (including Machiavelli, Rossini and memorials to Dante, Marconi and Enrico Fermi).

Striking features include the depiction of Galileo holding a telescope and globe, rather in the style of the orb and sceptre of a King or Emperor. Figures to his side hold geometric charts. A golden Sun is shown with circling planets. And above his head is a ladder (pointing to heaven?!) – his family symbol.IMG_5718

Galileo realised, from the movements of the tides, that the moon circled Earth and Earth orbits Sun. His views, famously, put him on a collision course with the Church’s then view of the Universe. The rest of this week is rather likely to be somewhat geo(chemically)-centric, but not, I think, in a sense that would cause Galileo any grief.

 

Spinning a yarn about perovksite

Magnesium silicate perovskite is the most abundant silicate in our planet. Never given a mineral name in its own right, it is unstable at Earth’s surface and has only been observed directly in the lab, rather than the field. So it fails to meet the criteria set down by the masters of mineral names, the International Mineralogical Association. Instead it adopts that given to calcium titanate, a rather rare and obscure mineral named after a similarly obscure Russian count and nineteenth century mineral collector.

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Natural perovskite from Perovskite Hill, Magnet Cove, Hot Spring County, Arkansas, USA, photo credit: Kelly Nash, wikimedia commons

The adopted family of compounds named after perovskite have, however, taken an importance far beyond that minor titanate accessory mineral. As well as dominating the geophysical and geochemical properties of Earth’s lower mantle, a whole raft of technological materials with the perovskite molecular structure have found applications in our modern world. First recognised for as transducers in submarine microphone systems their first applications were during the second world war, when they formed an important part of sonar detectors, for the atomic dance that perovskites perform converts stress to electric charge, and vice versa. Perovskites are used as transducers in sound systems, microphone pickups, electronic loudspeakers, and are essential components in your mobile phone. But oxides with the perovskite structure have also been used in non-volatile computer memory (PlayStationII memory cards are perovskite), in magnetic devices, and even in the humble gas lighter, generating the ignition spark.

Most recently, methylammonium perovskite has been proposed as a light harvester for dye-sensitised solar cells, the next generation of cheap, thin film, flexible, low-embedded-energy photovoltaic devices for solar energy production.

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Dr Julia Percival adjusts the knitted perovskite model at Surrey University, UK. Photo credit: Simon Redfern

Now the University of Surrey Department of Chemistry have given the perovskite story a new take. Their “Perovskite Project” aims to build a knitted version of the molecular structure of perovskite. Knitters and crocheters from across the world can contribute to this model, with knitting patterns for the perovskite structural units … the oxide octahedra and cation central sphere, available to download . They will be collecting contributions in August and assembling the giant knitted perovskite later in the summer.

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Interviewing for BBC radio.

I interviewed Dr Julia Percival, leader of the Perovskite Project, for BBC World Service Radio earlier in the month, hear more here, and Get knitting!