Silicon

Misha Glenny and guests discuss the physics, biology and chemistry of the element silicon which is at the heart of some of the most useful and beautiful objects on the planet. While it is still being created throughout the universe, the silicon we have here was made billions of years ago in dying stars. In its compounds we have long used silicon for glass and, more recently, purified silicon has become the foundation of modern electronics. Perhaps less appreciated is the role silicon compounds play in the biology of life on Earth, on the levels of carbon dioxide in the atmosphere and the cycling of elements between land, oceans and atmosphere that sustains us. With Kate Hendry Oceanographer at the British Antarctic Survey and Bye-Fellow of Queen’s College, University of Cambridge Andrea Sella Professor of Chemistry at University College London And Monica Grady Professor Emerita in Planetary and Space Sciences at the Open University Produced by Martha Owen Reading list: Christina De La Rocha and Daniel J. Conley, Silica Stories (Springer, 2017) Bernard Quéguiner, The Biogeochemical Cycle of Silicon in the Ocean (John Wiley & Sons, 2016) In Our Time is a BBC Studios Production Spanning history, religion, culture, science and philosophy, In Our Time from BBC Radio 4 is essential listening for the intellectually curious. In each episode, host Misha Glenny and expert guests explore the characters, events and discoveries that have shaped our world.

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  This is In Our Time from BBC Radio 4, and this is one of more than a thousand episodes you can find in the In Our Time archive. A reading list for this edition can be found in the episode description wherever you're listening. I hope you enjoy the programme. Hello. The chemical element silicon is at the heart of some of the most useful and beautiful objects on the planet. Created billions of years ago in dying stars, it's one of the most abundant elements in the Earth's crust and a building block of the universe. More recently, silicon has become the foundation of modern electronics and we live now in the silicon age.

• 2:25Speaker 03

  Less appreciated is the role it plays in the biology of life on Earth and the cycling of elements between land, oceans and atmosphere that sustains us. With me to discuss silicon are Kate Hendry, oceanographer at the British Antarctic Survey and bi-fellow of Queen's College, University of Cambridge, Andrea Sella, professor of chemistry at University College London, and Monica Grady, professor emerita in planetary and space sciences at the Open University. Welcome, Monica. Let's start at the very beginning. Can you take us to where it started, silicon, when it started and how it was created? What is this stuff?

• 3:10Speaker 04

  Well, silicon was created in one of the later generation of stars. If we go back 13.2 something billion years to the Big Bang, which is when hydrogen and helium and a little bit of lithium were created, and then the stars came and the original stars burnt hydrogen to helium, and then the next generation of stars were burning helium to carbon, and then carbon, nitrogen and oxygen were burning. And when I say burning, I'm not talking at a temperature of an ordinary fire, I'm talking billions, millions of Kelvins, very, very hot.

• 3:49Speaker 04

  And you get to a stage when oxygen has been created and that burns by adding a helium nucleus to it to produce silicon. And this is the penultimate stage before a star will collapse.

• 4:05Speaker 03

  And what part did silicon then play in the formation of planets, particularly in our solar system?

• 4:14Speaker 04

  Because we've been going for only, only 4.5 billion years, our solar system. We're very much sort of later generation from the big bang. And many, many stars have contributed to the interstellar medium. So this is the space between the stars. It's not space, it's full of stuff. It's full of, of dust, which is silicate, dust, silicon and oxygen grains together.

• 4:42Speaker 03

  Just explain to me, silicon, I understand is an element, silicate is what?

• 4:48Speaker 04

  Well, as you say, silicon is an element and that's just silicon atoms all bonded together. A silicate is, it's a compound where you have silicon bonded to oxygen, making long lattices and, and sheets. And then you've got other bits and pieces in there as well, magnesium, iron, calcium and so on. And these are minerals.

• 5:10Speaker 03

  And this stuff is, is floating around in the interstellar space, as you say?

• 5:15Speaker 04

  It is, it's tiny, tiny little grains, mixed with ice, water ice, and also still lots of hydrogen. And you have these big clouds, molecular clouds with all this stuff in. And, and they might start to collapse. And when they collapse, they, they fall in on themselves and they make a disk. And there are some wonderful pictures that you can see of this actually in operation that have been taken by some of our huge telescopes. And you can see these discs, that you, you look at them front on and you can see gaps in the disk where planets are forming, where the, the dust is clumping together. And as it clumps, it gets hotter and it starts to melt.

• 5:59Speaker 04

  And we've got these planet forming processes. And, and like the earth, it was originally made of these silicates. They've started to melt. Iron goes into the metals, there's iron in the silicates, goes to the centre to form a core, and we've got the crust. Which is, it's mainly a material which is left over from this melting process. And it goes on in asteroids as well.

• 6:26Speaker 03

  So that means without silicon or without silicates, the Earth wouldn't exist.

• 6:31Speaker 04

  Absolutely.

• 6:32Speaker 03

  Right, okay. Andrea Sella, can you take us now down to silicon at the atomic level? What's it comprise and why does that matter?

• 6:44Speaker 05

  So silicon is element number 14, which means that it has 14 positively charged protons in its core. And then different numbers of neutrons. But the key thing is that the organizing principle for chemistry in a way is the periodic table. And silicon sits directly below carbon. And so one of the key things when we talk, for example, you know, when we hear about silicates, is the fact that silicon is almost invariably connected to four things. So if you have the the element itself, and the element is sort of a strange, it's a metalloid. It sits somewhere between a nice electrically conductive metal.

• 7:30Speaker 05

  And on the other hand, a rather non-conductive, let's say, non-metal. It sort of sits in a sort of sweet spot in the middle. In all of its structures, whether it be the element or in its compounds, you've always got silicon attached to four things. And that's the motif which really underpins an awful lot of the chemistry that it undergoes.

• 7:54Speaker 03

  What does that four things imply when you say it underpins the chemistry? Why is that number special?

• 8:00Speaker 05

  So the number is special essentially because you have four electrons in the sort of outermost part of the silicon, which are available for bonding. And so what that means is that you can easily form four links. It's possible to make more. Um, but certainly silicates, uh, you know, other compounds. And we will perhaps come to silicones and things like that. But you've always got this number four, which recurs. Now, one of the beauties of silicon is that on the one hand, you've got this metalloid type behavior. But you've also got loads and loads of chemistry. And one of the things that that allows you to do is to obtain silicon in extraordinary

• 8:45Speaker 05

  levels of purity. And that is a whole area of technology, which is underpinned on the one hand, as you said before, the electronics industry. But also to help us, um, define how we measure stuff. And the element silicon, because of this sort of strange combination of forming a very, very nice crystalline structure, and at the same time, having lovely chemistry that allows you to purify things. It has allowed us to define one of the key seven units that are used to measure everything in our world, and that's the M O L, the unit of chemistry.

• 9:27Speaker 03

  The M O L M O L.

• 9:28Speaker 05

  The M O L E, as in the Italian word mole, as meaning the quantity, from which molecule, as in a little quantity, comes.

• 9:39Speaker 03

  That's very useful, thanks. And, uh, just so we clarify, silicon, the element, silicate, it's silicon mixed with other things. What about silica?

• 9:52Speaker 05

  So silica is sometimes referred to as quartz. It means silicon with just oxygen. And so I've got a beautiful sort of little clump of crystals of silica or quartz. And you can see that they're very clear. They, they look rather glossy, although they've got the odd crack. But crucially, what they've got, they've got points. And you can see that they've got very clear facets. And the interesting thing about quartz is that you have left-handed crystals and you have right-handed crystals. And that's because of the structure of the silicon oxygen.

• 10:35Speaker 05

  Aggregates is that the silicon is surrounded by four oxygens and these form spirals which run through the structure. And what it means is that you've got left-handed crystals and right-handed crystals. They're quite odd.

• 10:50Speaker 03

  Kate Hendry, Andrea was talking then about purity and silicon, but we don't find pure silicon very often. What forms occur naturally on the earth?

• 11:03Speaker 06

  So we've heard a little bit about silicates already. So silicon likes to bond with atoms of other elements, normally oxygen. So when there are four oxygens around silicon, that's the sort of basic silicate structure. What can happen then is that the oxygen can bond with other metals, magnesium and iron. And in fact, in some cases, when these build these big framework structures that we've heard about, aluminium can sort of swap in for the silicon, if you like. And those are the sort of silicate minerals. There's lots of different ways those silicate shapes can come together to form these crystals. And there's a whole array of different minerals. But that's probably one of the most common ways that we find silicon on the earth. And we've heard about silica as well.

• 11:47Speaker 06

  We've heard a bit about how sometimes what happens when silica forms a structure is that rather than one silicon surrounded by four oxygens, the silicons sort of start to share the oxygens. So the ratio of silicon to oxygen changes and you get just usually one silicon to two oxygens. So that's what happens in quartz, as you've heard about already. But sometimes those silicon and oxygen units can come together in a more higgledy-piggledy way. And that's what we call an amorphous silica. And that's basically glass. So that can happen in nature. So if you call a lava, a molten rock, very quickly, it doesn't form a crystal, it will form this glass. So it's volcanic glass. But also, something that I am really interested in is that biology can do this too.

• 12:31Speaker 06

  So biological processes can actually make this amorphous silica, which we call biogenic silica.

• 12:37Speaker 03

  Well, I was going to ask you, what's the link between silicon and life on earth?

• 12:42Speaker 06

  Well, every living organism needs at least a little bit of silicon, including us. So we need a trace amount of silicon in our diet for our bones to be healthy. We don't normally have a problem getting silicon from our diet. There's plenty in drinking water, for example. There's quite a lot in beer, as it happens. So it's actually quite a good source of silicon. But in the plants that we eat as well. So we're generally fine with getting enough silicon. But some organisms actually need a lot more silicon in their diets, if you like. And those are the organisms that make this biogenic silica. It's an absolute requirement for them to have it. So plants produce silica in their leaves. Sometimes if you rub a leaf and you can feel it being quite rough, that's the little blobs of silica

• 13:27Speaker 06

  that they produce in their leaves, thought to be a sort of defence mechanism against herbivores. Animals can produce silica too, very basic animals. So it's the sponges. So these are the sort of really ancestral animals that live on the seafloor and in freshwater environments. They make their skeletons out of little needle-like elements called spicules, which in many groups are made out of silica.

• 13:48Speaker 03

  Are they actually, are they sort of gathering the silica up, or are they producing silica itself?

• 13:54Speaker 06

  So they take in silicon from their environment. And this is the silicon that's dissolved in the water that they're living in. So dissolved silicon in the earth's oceans and in fresh waters is in the form of silic acid. You can just think of that as dissolved silicon. It's a sort of dissolved silicate in its own right, really. It's got the same sort of structure. But they, sponges and other organisms, can take that dissolved silicon in through their cells in a sort of active process.

• 14:18Speaker 03

  Right, okay. So we've had silicon, we've had silica, we've had silicates. We'll come on to silicones later. But Monica, before we do, let's just stick with silicates and how they develop on earth. And in particular, I'm interested in what they produce as minerals.

• 14:36Speaker 04

  Well, this is these compounds where you've got silicon bonded to oxygen, bonded to another silicon, bonded to another oxygen, and you get these enormous, great big, you can make sheets, you can make rings, you can make cages of silicon and oxygen. And occasionally, you add a magnesium, you add an iron, a calcium, and aluminium.

• 15:00Speaker 04

  And you make lots and lots of different minerals. And you also make very beautiful minerals. Andre had his colourless quartz. But if you put a bit of, you know, a pinch of something else in there, you get amethyst, the beautiful purple semi-precious gemstone. And you can get citrine and tiger quartz, yellow and brown and all these different colours. And it's just what you've added in to the silicon and the oxygen.

• 15:27Speaker 03

  So jewellery basically depends upon silicon?

• 15:33Speaker 04

  Well, jewellery depends upon elements from the period number four in the table. I don't know, it's not, it's the group four. Because a lot of our jewellery, of course, is diamonds, which is carbon. And we know that silicon is just below. And where carbon bonds to carbon, bonds to carbon, bonds to carbon, to make the diamond structure, here we have silicon bonded to oxygen, bonded to oxygen, bonded to silicon, to make the silicate structure, to get those beautiful crystals as well.

• 16:03Speaker 03

  Now, let's get back to the purification of silicon. And there's a reason for this, Andrea, because for most ordinary people, let's face it, silicon is associated with three things, with sealants that you put around your bathtub, with

• 16:20Speaker 05

  Silicons.

• 16:20Speaker 03

  Silicons. Then there is the gel that some people use to enhance various body parts.

• 16:28Speaker 05

  Again, silicone.

• 16:29Speaker 03

  It's good. So we're really hitting the silicons here. And then finally, there are semiconductors and silicon valley. Now, first of all, can you explain what a silicone is? And secondly, how do we purify silicon so that it can be used in microchips, in semiconductors, essentially?

• 16:52Speaker 05

  So at the heart of all of this is the chemistry of the element silicon. And in the case of silicons, what you're doing is you're exploiting the fact that first of all, silicon likes to form long chains or even rings.

• 17:11Speaker 05

  Alternating silicon and oxygen. Now, of course, that means that if you have a silicon, you are attached to two oxygens. That means you've got room for two more things. And those two more things will typically be carbon. And so now you can start to make long chains, i.e. plastics. And those plastics are quite interesting because they can be made extremely elastic, deformable, rubber-like. And so the silicones are, you can make kind of gelatinous ones, you can make quite stiff ones, you can make doingy ones, to use the technical term, right? The kind of thing that you'll have in your kitchen, right?

• 17:56Speaker 05

  The sort of kitchen spatulas, that sort of thing. And that's really because of the availability of the chemistry and the fact that we've refined that so much. But to actually get to the element is quite interesting. And so, like so many metals, you've got to smelt it. And the way in which that smelting is done turns out to be using carbon. So you start by mixing essentially coke or coal, and you take quartz, right? The very nice, fairly pure silicon oxide. You grind them together, and then you heat them very hot.

• 18:38Speaker 05

  The result is you get carbon dioxide coming off, and you are left with pretty good pure silicon at the end of it. That silicon is about, you can get it 98, 99% pure, but that's nowhere near good enough for what you really want if you want to do that semiconductor industry, that...

• 19:03Speaker 05

  Microprocessor, all that kind of technology. For the extreme purity you need, you go one step further. And the one step further is an extraordinary process that was invented by a Polish metallurgist called Chokralski. And he was one day idly working at his desk and he had his inkwell on the desk and also a crucible containing some molten metal. And without realizing it, he dipped his pen in the molten metal, pulled it out, and out came an extraordinary thread of beautiful crystalline metal.

• 19:48Speaker 05

  And this is the inspiration for how silicon is purified. What you do is you take a crystal of ultra pure silicon and you dip it into a bath which contains molten silicon. Now silicon melts at 1400 degrees, so this is not a process for the faint-hearted. You've got to keep oxygen away because it'll instantly turn to the oxide. And now what you do is you rotate both the crystal and the pot and you pull the crystal upwards.

• 20:26Speaker 05

  And as you do so, the silicon atoms, as they solidify, as they crystallize onto the surface, they crystallize in a perfect crystal. They form a single enormous crystal, several, I mean, tens of centimeters across. And above all, because it's quite slow, if there are any impurities that stick to the crystal, they have time to come off and be replaced by the silicon itself. And so now you get to levels of purity which are one in a billion. It's extraordinary.

• 21:01Speaker 03

  We're going to, we're going to come back a little bit later and work out what the implications of that were for electronics in, in particular. But Kate, I want to move on to something else here, back to life on earth and, and silicon.

• 21:20Speaker 03

  Diatoms. Tell us what diatoms are in the oceans and how they regulate our atmosphere and what's silicon got to do with it?

• 21:30Speaker 06

  So diatoms are a group of organisms I've not yet mentioned. They're really important in terms of the silicon cycle. So they're one of these organisms that make their shells out of silica. What diatoms are is that they're algae, so they're photosynthetic. They live in the surface of the oceans and our fresh waters and they use the energy from the sun to make organic matter out of CO2 from the atmosphere essentially. So they essentially in the oceans, they form the same, perform the same role as plants. They are the primary producers of the ecosystems. And they're responsible for, we estimate up to about forty percent of the formation of organic matter in the oceans are these diatoms. So

• 22:13Speaker 06

  they're incredibly important for locking up carbon. So not only do they fuel ecosystems, but they are really important for carbon cycling and that's that link with climate because they draw that CO2 down from the atmosphere. When diatoms die, they sink, they're actually fairly efficient at sinking out of the surface ocean where it's light, where they grow, down into the depths. So they take that organic matter with them and that can be buried away from the atmosphere for thousands of years. So they're really critical for carbon.

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• 24:59Speaker 06

  There's a lot we still need to understand about how diatoms make silica, to be honest. Scientists are working very hard on trying to understand those pathways. What we do know is that they take and dissolve silicon from their environment. They transport the silicon across their cell membranes. If the concentration of silicon in the water is pretty high, it can diffuse, but normally it's this active uptake that they use. From there, they use a special organelle, special device within their cells called a silica deposition vesicle, SDV. And what they do is that they concentrate the silicon, the dissolved silicon, so high that it starts to essentially spontaneously precipitate out of solution to make this silica.

• 25:42Speaker 06

  And like I say, they make their shells out of silica and they are absolutely beautiful. They have ornate structures, they have holes and spikes in all sorts of beautiful patterns and all sorts of shapes and sizes. That's all very much genetically controlled, so every species is identifiable by its shape and by its ornate structures.

• 26:01Speaker 05

  I mean, this point about beauty is really important. I mean, in the 1860s, a man called Adolf Schmidt produced an extraordinary atlas of the structures of these things. And they're just amazing. They're so geometric, they're just wild. But may I just add something to what Kate just said, because on the one hand, you've got the problem of kind of concentrating the silica, having, you know, a mechanism to be able to pull it out of the water, shove it into the cell. Then you've got to get it concentrated enough that you can precipitate it. But then the other thing you've got to do is to put it into the right shapes.

• 26:41Speaker 05

  And one of the things which is quite mysterious and a huge area of active research is how does biology control crystallization? And this is done with these, essentially with soft biological structures, self-organizing and providing templates. And how that works in detail is something which people are working on, but it's amazing.

• 27:06Speaker 03

  And Kate, you've got one which looks like a pair of trousers that you're fond of. What's that do?

• 27:11Speaker 06

  So, well, it's a species called Eucampia antarctica. It lives in the waters around Antarctica, as its name might suggest. And it's a type of diatom called a centric diatom. So these are sort of flat discs, if you like, in their very basic form, but they can have all sorts of ornamentation to them. And these ones in particular grow these sort of almost horn-like structures. So if you sort of orientate it in the right way, it does look a bit like a pair of trousers. And sometimes they're a little bit shorter, so they look a little bit more like a pair of shorts instead. But it's just a child in me, I just quite like that. I think they're quite fun.

• 27:42Speaker 03

  And they produce a lot of oxygen, is that right?

• 27:45Speaker 06

  So algae around the oceans will also produce oxygen, so that's another link with with life, absolutely.

• 27:53Speaker 06

  As I say, it's also this really strong link with carbon cycling in particular that we're interested in at the moment. And I said right at the beginning that diatoms control the silicon cycle, they're really important for the silicon cycle. They've essentially revolutionised how silicon is cycled on the planet. They evolved, well, they appeared in the fossil record very patchily in the Mesozoic, so this is a time of the dinosaurs. But then after that, after the dinosaurs died out 65 million years ago, the diatoms really took off then. So, you know, you might think we're living in the age of mammals, but in my view, we're living in the age of the diatoms. And since then, because they're so good at taking up that dissolved silicon from the ocean, they've essentially stripped silicon out of the surface ocean entirely.

• 28:37Speaker 06

  So if you go in most parts of the ocean today, you'll find pretty low levels of silicon in the surface ocean. And that means that the availability of that dissolved silicon can actually limit how much they grow. So how silicon gets into the oceans and how it moves around is really important. And I mention that because that's actually my research area, so just needed to get that in there. But especially in the polar regions where we're seeing a lot of climatic change, a lot of those processes are linked with that supply of silicon. So be it things like glacial weathering, or how the big Arctic rivers are releasing silicon into the ocean, for example, a lot of this is changing very quickly and could actually be impacting diatoms and how they grow.

• 29:17Speaker 03

  Thank you very much. Monica, we're going on to another compound now, and that's silicon carbide.

• 29:25Speaker 04

  Right.

• 29:26Speaker 03

  Tell us about silicon carbide and its long journey, and what does it do?

• 29:32Speaker 04

  Well, I also have something with me. It's nowhere near as beautiful as quartz crystals. What I've got is a small piece of a meteorite, and it just looks a bit like a piece of coal. It's a sort of matte black in colour. And it's a very special type of meteorite. It's called carbonaceous chondrite because it's got a lot of carbon in it. And these meteorites also have got grains within them.

• 30:02Speaker 04

  The most part of this meteorite, which has come from an asteroid, was made at the time the solar system was made, four point five, six, seven billion years ago. But some of the tiny grains in here were made in other stars. And they blew into the molecular cloud from which our solar system was made. And some of those grains are silicon carbide. And these have come from massive stars, red giants that are burning silicon. And it gets mixed up. And there's the most wonderful processes going on in stars. We have nuclear fusion, which is bringing the elements together. But also, you've got a lot of turbidity and you've got a lot of mixing.

• 30:47Speaker 04

  And when you've used up some of the fuel, part of the outer part of a star will collapse in itself. But then you get turbulence and material is dredged up. And you have some wonderful things called hot bottom burning. And you have dredge up. And the silicon carbide crystals are made in a procedure of a third dredge up of a hot bottom burning star. But it's these things that are then blown off the star when the star might explode or have its stellar wind. And you've got silicon carbide there. And we can tell that it's from a different star than our sun, because our sun is not burning to make silicon.

• 31:28Speaker 04

  But also, the carbon isotopes, we've talked about carbon, the atom, but we have different isotopes, different amounts of the particles in the core, in the nucleus of the atom. And most of the carbon in the solar system has ninety-two atoms which weigh twelve units to one atom which weighs thirteen units. In some of these stars, it's four twelves to one thirteen. And we can see that in these silicon carbides. We also have graphite there. We have aluminium oxides there. We have diamonds as well, all these different pre-solar grains. You can't see them in this little bit I have here, but they're there.

• 32:13Speaker 03

  And what does silicon carbide do?

• 32:17Speaker 04

  Doesn't do very much, really.

• 32:19Speaker 05

  Well, silicon, I mean, i- i- in a meteorite, I suspect it doesn't, it just is.

• 32:24Speaker 04

  It's there, yes.

• 32:25Speaker 05

  Um, but, but silicon carbide is technologically extremely important. When you think of the fact that carbon and silicon have the same structure, you can actually make a kind of alloy, but it's actually a fifty fifty mix of carbon and silicon together. And as you would expect for something that has a diamond structure, it is exceptionally hard. We sometimes refer to it as carburendum. And so it's very widely used for polishing and so on.

• 32:52Speaker 04

  It's, it's the grains that you find on Emery paper.

• 32:55Speaker 05

  Ah, I see.

• 32:56Speaker 04

  Carburendum. But also moissanite is silicon carbide, the mineral. And that can also be polished and be a, uh, some of the first industrial diamonds were made of moissanite.

• 33:08Speaker 03

  So, talking about applications, Andrea, I want to know about semiconductors, silicon's importance, and why from the nineteen fifty's onwards, the silicon age really blossoms.

• 33:24Speaker 05

  Yeah, I mean, the silicon age is, is kind of critical. It's, it's the moment when the world transformed from really being kind of steel-based and, and, you know, we were quite electrical with copper. But really, when silicon took off. And when I was a child, I remember that there were these things which were called solid-state. And, and I wanted a set of solid-state walkie-talkies that had, you know, the transistor radio. And all of that comes from the fact that, that silicon is not a metal. Now, I've got a wafer of silicon, of ultra-pure. So it's a, it's a beautiful little disc with a mirror-like finish.

• 34:04Speaker 05

  It looks metallic, but it has a faintly purple sheen to it. And when I said before that there's a difference between metals and these metalloids or semiconductors. You can think of a metal as being a material in which essentially the electrons are pretty well free to move. And so there are loads of motorways that allow them to travel around. And so they can be sloshed from one side to the other. Silicon, although it conducts electricity somewhat, there's quite a bad traffic jam in a sense, partly because the electrons are actually being used to hold the silicates together.

• 34:48Speaker 05

  But there's something quite interesting, and that is, sitting above our traffic jam turn out to be motorways. There's a sort of region where the electrons can move. And the fact that silicon has this kind of gap between where the electrons are kind of immobile and the region where they can, that's the thing which really makes all the difference, is that you can actually control the movements of the electrons, and that is often done by the addition of deliberate impurities into the silicon in places where you want them.

• 35:27Speaker 03

  Thank you, Andrea. Back to the diatoms which are made partially with silicon, because, Kate, one of the things you've looked at is how our studying of diatoms can tell us about long-term patterns in climate and climate change.

• 35:49Speaker 06

  Absolutely, and that all relies on a few different things coming together. So going back to what I mentioned earlier, that diatoms are very good at sinking. So when their cells die, their cells sink to the sea floor and they can get buried in sediments, and they can get preserved there. As sediments build up through time, it's a little bit like pages of a book. You can sort of go back through the pages and read back through the history of time, just like you would an old diary. So what we can do is we can go and take cores of those sediments, and we can figure out how old they are. First of all, that's important, we need to know how old the sediments are as they go down deeper into the sediments. And at each of those horizons, we can look at the diatoms that are there. And the other thing I mentioned as well before is that all those beautiful

• 36:34Speaker 06

  shapes they make are really distinctive in terms of the different species. So we can count the different species that have grown through time. Now, this is important because some species love to live in particular environments. Some of them live all over the place. We call them cosmopolitan diatoms, but some of them are very specific. And so one example of this is that, well, a couple of species of one genus called fragile Ariopsis grow in sea ice. They only really live in sea ice. So if we find those species back through time, we know that there's sea ice there. So this gives us what we call a proxy for understanding how, where sea ice has been in the past. And so if we're trying to figure out how sensitive the sea ice extent is around Antarctica, for example, we can do that. And this is what's done.

• 37:19Speaker 06

  It's been, it was done actually a couple of decades ago. And what scientists did is took lots and lots of cores around Antarctica and mapped out exactly how far sea ice got during the last ice age. And you can also do it for warmer periods, too. And that means we can understand that sensitivity. And that means that if we're trying to make future projections of where sea ice is going as a result of current global warming, we can actually test the models against what we understand from our observations.

• 37:46Speaker 03

  Thank you, Kate. Monica, we've been talking about the elements such as silicon travelling from distant stars, collapsing and so on. What do we learn about the stars from silicon and what it brings here?

• 38:03Speaker 04

  If you're in a dark place outside and you've got a really good view of the night sky, you'll see that the stars are different colours. You know, some are bluish, some are reddish, some are white. And the different colours are the different stars and they have their different sizes, their different brightnesses, luminosity, different temperatures, different ages. And by looking at stars and seeing what they're doing, is it a red giant star? Is it a blue giant? Is it a white dwarf? We can tell the processes that are going on in those stars and we can look at the

• 38:45Speaker 04

  different clusters of stars and it tells us about, it tells us about the ages of the stars and what's actually been happening as the galaxy evolves. And also when we look at other galaxies and try and Make judgements about the stars within them. What's really interesting though is that because we've learnt a lot about the behaviour of silicon in the solar system and in planet formation in the solar system, we can look at other stars that have got planets around them and we can make inferences about what those planets might might be doing. Now, some of the planets, some of the biggest planets that we've seen around other stars

• 39:28Speaker 04

  are what we call hot Jupiter's and these are stars that are as big or as big or bigger than Jupiter, but they are orbiting their stars much closer than Mercury orbits our star. And some of those have got really hot atmospheres which have got actually silicon gas or silicon hydrated gas or silicon oxide gas in their atmospheres. And it's like, well, there's not going to be any life on those, no chance. But as our telescopes get more powerful and we can see in greater detail planets around other stars, we're going to be able to see things like, oh, well, actually, this particular star has got a planet round it which which looks like Earth, maybe it's got oxygen in

• 40:11Speaker 04

  its atmosphere, you know, maybe it's, you know, got water there, maybe it's got diatoms in the water, we don't know. But the idea that the Earth is a singular planet has, I think, been put to rest with the the finding of so many Earth-like planets. Whether the Earth is singular in having life, we still don't know, but

• 40:37Speaker 03

  But the assumption

• 40:38Speaker 04

  I've been talking about physics, all right, the physics of silicon, which then moves on to the chemistry of silicon, Andrea talked about, which then moves on to the biology of silicon, which is what Kate's talking about. And then we bring all these together and we start saying, well, actually, what's the chance? The physics is is based on what happened in the Big Bang, the chemistry is based on the atomic structure and the biology is based on what those atoms do. And it's like, is it likely that it's only happened on Earth? I don't know.

• 41:07Speaker 03

  Kate, you wanted to come in then?

• 41:09Speaker 06

  Yeah, just an interesting sort of thing to note as well, which is that right at the beginning, when I said that biology can do this silica formation, I said that organisms taken silic acid dissolve silicon from their environment actively. And by that, I mean, there are special proteins in cell in the cell membranes that transport the silicon into the cells. And there are lots of assumptions that go into these calculations, but we're able to calculate how long ago it is that those proteins evolved. And they're incredibly ancient, these silicon transporters. They go back, if we're right, billions of years. So it's a very fundamental process within our planet's biology of moving silicon around.

• 41:48Speaker 04

  So were these some of the very earliest, you know, sort of pre-Cambrian type?

• 41:52Speaker 06

  Absolutely. We think they're shared by some of the, some bacterial groups, but they're also, we can find homologues for these genes throughout the, the tree of life, basically.

• 42:03Speaker 04

  Yeah, brilliant.

• 42:04Speaker 03

  Andrea, talking about the tree of life, our life is carbon based. Is it theoretically possible to have life that would be silicon based?

• 42:13Speaker 05

  That's funny. This is a question that comes up over and over again. And if you look at science fiction writers, Isaac Asimov and lots of others have always imagined that you might land on a planet where you find, you know, some weird life form. And people have certainly tried to see what they can do. I think what I would say is that, first of all, that the idea that, that life is carbon based is slightly annoying in that, yeah, carbon, but my God, if you haven't got silicon, if you haven't got sodium, if you haven't got calcium, if you haven't got molybdenum, iron, copper, nickel, you know, all of these things.

• 42:51Speaker 03

  So it being carbon based is a bit of a fallacy.

• 42:53Speaker 05

  And so.

• 42:54Speaker 04

  Oh, no, I disagree. It's based on carbon and it's got all these bits and pieces added in. But, you know, it's like saying bread is based on flour.

• 43:04Speaker 05

  Well, what I would, what I would say is that carbon certainly provides huge amounts of the machinery and the structures and in a way, the chemical intelligence associated with cells. And so carbon is absolutely fundamental. But to imagine that carbon, you know, is somehow stands out from the rest. I'm less sure. That said, one of the unique things about carbon is the fact that actually, many of its compounds are not very reactive, because the carbon is quite small and the atoms around it shield it. And that's one of the things which makes so

• 43:47Speaker 05

  much of chemistry possible and so interesting. Silicon's a little bit larger, it's a little bit more reactive. And so, actually, some of the structures that we think about when we think carbon-based structures, that we think about associated with life, start being more flexible, more floppy, less rigid. And so, things like information storage in DNA is entirely based on the rigidity of these structures. Whether that can be replicated with silicon-based things, I don't know. But there's certainly people trying.

• 44:28Speaker 03

  But Monica, you seem to imply that it would be at least theoretically possible that somewhere else in the universe silicon-based life existed.

• 44:37Speaker 04

  Well, I wouldn't be surprised at anything that the universe throws at us, you know, I really wouldn't. But if we've got, if we've got the diatoms which require silicon, which can take silicon through a membrane, I can, I'm thinking more on the sort of neural networks type thing where, you know, you've got the idea that maybe we've got these networks we can make of silicon oxygen structures which can transfer signals in the same way

• 45:14Speaker 04

  the neurons in the brain transfer signals. Can we have that sort of integration of a silicate-based network with a carbon-based network to make some sort of being? I haven't got a clue. But it'd be really cool to find out.

• 45:36Speaker 03

  Well, on that fascinating unanswered question, it's time to bring this week's episode to a close. But my thanks go to Monica Grady, Kate Hendry and Andrea Sella. Next week, it's 1898 and the Spanish-American War, when Spain lost Cuba and the rest of its remaining empire, and the USA gained the Philippines and Puerto Rico. Thanks for listening.

• 46:04Speaker 12

  And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Misha and his guests.

• 46:11Speaker 03

  Anything else that you want to throw in there that we haven't mentioned yet? Monica?

• 46:16Speaker 04

  It's not a question, it's not something that I want to throw in, it's a question I want to ask. I want to ask Andrea. You're talking about the left-handed and the right-handed quartz molecules. What's, what's the significance of that? What does it change with quartz?

• 46:31Speaker 05

  So what it does is it changes two things. First of all, you see it in the, the shape of the actual crystal. And the interesting thing is you can see those facets on the crystal, which in fact are extremely important historically, because it was measuring the angles between the faces of quartz and other minerals, which led people to realise that there must be unique sort of fundamental chunks, unit cells, from which things were built. And, and quartz plays a very important role in that. But the second thing, and it turns out I've brought a prism,

• 47:07Speaker 03

  More show and tell.

• 47:08Speaker 05

  which is made of quartz.

• 47:10Speaker 04

  Right.

• 47:10Speaker 05

  But you can see that it's two kind of triangular pieces, which have been glued together with the legendary Canada balsam, which is a completely transparent adhesive that was used from the 18th through into the 20th century.

• 47:25Speaker 04

  And it goes horrible yellow colour when it ages.

• 47:28Speaker 05

  Eventually, yes, that's right, that's right. And it turns out that photons effectively travel in a spiral. And that means that if you have very precise optics...

• 47:39Speaker 05

  Using a quartz crystal, it's going to give you problems because it will behave slightly differently with the two kinds of handedness. If, on the other hand, you have two of these oppositely arranged quartz chunks together, one cancels out the other. Now, why would you want to use quartz optics? Quartz is a pain, right? You need to heat it to, you know, 1,200, 1,400 degrees to actually soften it and work with it. The thing about quartz is that it's transparent in the ultraviolet part of the spectrum. And so if you use quartz optics, you can look much further down at all kinds of interesting

• 48:24Speaker 05

  electronic properties.

• 48:26Speaker 03

  Kate, there was something I wanted to ask you because we recently did a program on archaea. And I learnt then that there are three domains of life: bacteria, archaea, and eukaryotic beings, multicellular beings. And now you tell me that this single cellular life, the diatoms, what are they?

• 48:49Speaker 06

  They sit in eukaryotes. Yep, they're nestled in there as well. So, yeah, all the algae groups sit in there. And those silicon transporters I mentioned, like I say, they've been found in new bacteria and in eukaryotes. Could well be there in archaea as well. Yeah.

• 49:05Speaker 04

  What fascinates me about the tree of life is when you look at it, is how close we are to slime moulds.

• 49:14Speaker 04

  We haven't come very far, have we?

• 49:16Speaker 03

  Well, that was the interesting thing about the discussion with archaea was just how much DNA we share with archaea or certain species of archaea, certainly.

• 49:27Speaker 05

  I have a terrible confession, Kate. And that is, you know, you talked before about how the, these diatoms, they sink once they die through the ocean and land at the bottom and they form these sediments. Well, as a chemist, I'm a big fan of those sediments and, and it's rather tragic, the fact that those sediments that are referred to as Fuller's earth, or bentonite, or diatomaceous earth even, are absolutely fantastic filters for doing chemistry. And so we will pour a bunch of these, an inch of, of some of these beautiful things, although all we see is dust, right, onto a filter and then pour our solutions through and we know that it

• 50:12Speaker 05

  is so fine that it will capture, you know, it will stop very fine salts and, and other bits from getting through.

• 50:21Speaker 04

  That's okay, there's plenty to go around.

• 50:22Speaker 05

  Yeah, well, I feel they're, they're sacrificed for a good cause.

• 50:27Speaker 04

  In that, I also need to make a, a confession then Kate, because we also take these sorts of things and stuff them in a test tube for purifying gases. So we, we cool it down with liquid nitrogen and condense different gases. And then when you heat them up, the different gases come off at different temperatures because these are fantastic filters, just they're really great, you know? I'm delighted.

• 50:51Speaker 03

  Monica, tell us a bit more about rocks and silicon and clay, all that relationship of...

• 51:01Speaker 04

  All that stuff.

• 51:02Speaker 03

  Yes.

• 51:03Speaker 04

  Okay, so I talked a bit about when the, the planet formed, we got these, these dust grains, the silicates mixed with other stuff, clumped together, heated, we had the smelting reaction going on, so the iron went to the centre to make the core. And then we have on the surface, a layer, the, the crust of the earth, which is what we walk on, which has got, it, it's sort of depleted a little bit in iron compared to some of the other rocks. And we also have these in the meteorite collection. And, you know, here's another of my show and tell bits. This is a meteorite which has come from the asteroid Vesta. And it's really black and shiny on the outside. That's where it got heated up as it came

• 51:47Speaker 04

  through the atmosphere. But when you look at it on the inside, it's a very pale whitish grey colour. And this is because the minerals in here are mainly calcium, aluminium...

• 51:59Speaker 04

  Oxygen silicate based with calcium and aluminium, sodium and potassium in, but very, very little iron and magnesium. And this is also what we see on the surface of the moon. When you look at the moon and most of it's silver, but you've got the face, which is the darker bits. The darker bits have got the magnesium and the iron in, and the lighter bits are the same stuff as this. And it's where it's where the original silicates have melted and the iron and the magnesium has gone away. And you've just got these left. Now, the next thing that can happen is it hasn't happened on the moon, but it's happened on earth and it's happened on Mars, is if you get water there, you get those minerals can be altered.

• 52:39Speaker 04

  And they alter to clay minerals. We call them phyllosilicates and these are sheets which have got silicates there. And now they've got water mixed in there as well. And this is, it's a, a, a sort of, I don't know, what size is it? A walnut size? No, it's smaller than that.

• 52:56Speaker 03

  Smaller than a walnut.

• 52:57Speaker 04

  Hazel nut, hazel nut size, little nugget, which, and you look, look at it, it's a paleish green in colour. And this is a meteorite that has come from Mars. So this is a piece of Mars.

• 53:09Speaker 03

  You mean it's, it's broken off Mars?

• 53:11Speaker 04

  It's, uh, uh, an asteroid came and hit Mars and bits broke off it. And in here, we've got some of those sheet silicates which have got water in them. And we know there's been water on Mars. We know that there is water on the earth. We actually know that there is water on the moon now in, in some places. But, you know, this is the reason why we think that there could well be life on Mars. Because we've got these phyllosilicates there, those, these clay minerals, which some people have suggested might be a template for other molecules to sink down, to attach to, to form the membranes,

• 53:52Speaker 04

  which then enclosed cells to, to have the, the first earliest cells forming for, you know, whether you've got your nucleus in there and all that sort of stuff. So again, this is something else, which is the interface between chemistry and biology. Uh, and then going on, on for life.

• 54:10Speaker 03

  Andrew, I've got one final question for you about semiconductors and Moore's Law and why chips can get smaller and smaller and more and more powerful. Does that have anything to do with the properties of silicon that you can get so many transistors on a wee chip?

• 54:33Speaker 05

  I mean, in many ways, it's not so much the silicon as, I think, our ability to fabricate these things and that we have been able to

• 54:44Speaker 03

  Replicate.

• 54:44Speaker 05

  Sort of, well, essentially etch and to deposit and to put structures onto this silicon framework, let's say, in ever finer detail. And people keep saying we're going to hit the buffers because sooner or later, the little wires, you know, that are embedded in these things, the connections are getting so close together that you're going to start getting quantum talk. And yet, the semiconductor industry has managed to defy that over and over again. It's quite extraordinary.

• 55:27Speaker 03

  Well, thank you all very much. The conclusion that I have from this is, on the whole, silicon is a jolly good thing. Quite right, absolutely.

• 55:38Speaker 05

  I think a life without silicon would be close to a life without sunshine.

• 55:43Speaker 03

  Oh.

• 55:44Speaker 05

  Or cheese.

• 55:47Speaker 13

  Would you like tea or coffee?

• 55:48Speaker 05

  I'd love a cup of tea.

• 55:49Speaker 13

  Tea?

• 55:50Speaker 03

  Coffee, please. Coffee, please. Cup of tea for me.

• 55:53Speaker 04

  I have to get to King's Cross to get a train at five.

• 55:57Speaker 12

  In Our Time with Mischa Glenny is produced by Simon Tillotson and it's a BBC Studios production.

• 56:04Speaker 13

  I'm Jamie Bartlett and for BBC Radio 4, I'll be looking at how fakery took over the world. No, no, hang on, hang on, sorry. You're not Jamie Bartlett, I'm Jamie Bartlett. Oh, really? Well, who am I then? I'm afraid you're not real, pal. You're just an imitation chatbot I created to help me make this series on modern fakery and why it's everywhere. Sounds good. What's going to be in it? Well, there's a lot. 1980s professional wrestling, dodgy academics, AI psychosis, Covid vaccines, skeptics. What's it called? Everything is fake and nobody cares with me, Jamie Bartlett. And me, Jimmy Bartlett. Listen first on BBC Sounds.

• 56:51Speaker 03

  Hello, Misha Glenny here. Before you listen to this episode, I wanted to share some exciting news. You can now watch this episode and all future episodes of In Our Time in full video. Just search for In Our Time on YouTube or watch episodes 28 days early by subscribing to the streaming service BBC Select in the US and Canada.