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Minerals

Meteorite impact turns silica into stishovite in a billionth of a second

Meteorite impact turns silica into stishovite in a billionth of a second

The Barringer meteor crater is an iconic Arizona landmark, more than 1km wide and 170 metres deep, left behind by a massive 300,000 tonne meteorite that hit Earth 50,000 years ago with a force equivalent to a ten megaton nuclear bomb. The forces unleashed by such an impact are hard to comprehend, but a team of Stanford scientists has recreated the conditions experienced during the first billionths of a second as the meteor struck in order to reveal the effects it had on the rock underneath.

The sandstone rocks of Arizona were, on that day of impact 50,000 years ago, pushed beyond their limits and momentarily – for the first few trillionths and billionths of a second – transformed into a new state. The Stanford scientists, in a study published in the journal Nature Materials, recreated the conditions as the impact shockwave passed through the ground through computer models of half a million atoms of silica. Blasted by fragments of an asteroid that fell to Earth at tens of kilometres a second, the silica quartz crystals in the sandstone rocks would have experienced pressures of hundreds of thousands of atmospheres, and temperatures of thousands of degrees Celsius.

A meteroite impact event would generate shock waves through the Earth.
NASA

What the model reveals is that atoms form an immensely dense structure almost instantaneously as the shock wave hits at more than 7km/s. Within ten trillionths of a second the silica has reached temperatures of around 3,000℃ and pressures of more than half a million atmospheres. Then, within the next billionth of a second, the dense silica crystallises into a very rare mineral called stishovite.

The results are particularly exciting because stishovite is exactly the mineral found in shocked rocks at the Barringer Crater and similar sites across the globe. Indeed, stishovite (named after a Russian high-pressure physics researcher) was first found at the Barringer Crater in 1962. The latest simulations give an insight into the birth of mineral grains in the first moments of meteorite impact.

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Simulations show how crystals form in billionths of a second.

The size of the crystals that form in the impact event appears to be indicative of the size and nature of the impact. The simulations arrive at crystals of stishovite very similar to the range of sizes actually observed in geological samples of asteroid impacts.

Studying transformations of minerals such as quartz, the commonest mineral of Earth’s continental crust, under such extreme conditions of temperature and pressure is challenging. To measure what happens on such short timescales adds another degree of complexity to the problem.

These computer models point the way forward, and will guide experimentalists in the studies of shock events in the future. In the next few years we can expect to see these computer simulations backed up with further laboratory studies of impact events using the next generation of X-ray instruments, called X-ray free electron lasers, which have the potential to “see” materials transform under the same conditions and on the same sorts of timescales.

The Conversation

This article was originally published on The Conversation. Read the original article.

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!