On the eastern coast of Iceland, the land of ice and fire, there’s a small manmade cavern in the volcanic rock, which is home to a surprising treasure trove of perfect crystals like these. It’s called Iceland spar. They look a bit like squashed ice cubes, but these ultra-clear crystals have some unusual properties that might have been key to an entire civilisation’s success.
Because about 1000 years ago, as the Vikings dominated the North Atlantic thanks to remarkably advanced navigation at sea, there’s a good chance that this crystal was their secret weapon. [intro music] Iceland spar might seem mystical, but it’s made of a fairly common mineral called calcite, otherwise known as calcium carbonate. Now, our Rocks Box subscribers will know all about calcium carbonate from our video on aragonite.
Aragonite and calcite are both polymorphs of calcium carbonate, meaning they have the same chemical formula but with slightly different crystal structures. In general, calcite is the more stable of the two, and it’s the most common form you’ll find on the Earth’s surface. And you can find calcite pretty much anywhere.
It’s the major component of limestone rocks, which make up about a quarter of all the sedimentary rocks on Earth. Metamorphic rocks cooked rom these calcite-rich limestones make calcite-rich marble, and you can find it deposited in just about any other kind of rock, when calcium carbonate enriched fluids form veins through the crevices. Calcite itself has a trigonal structure, which means its simplest crystal shape is a rhombohedron - basically just a skewed cube!
But there are a ton of different shapes and colors that the crystals can take, depending on impurities present and the space and time the crystals had to grow. They can be clear, white, yellow, brown, or a whole rainbow of other colors. And they can grow as prisms, spikes, pyramids, globules, or as smooth or fuzzy coatings like on stalactites and cave walls.
Iceland spar is just one of these crystal forms. It’s super pure, making it clear and colorless, and it grows very slowly in plenty of space, letting the crystals take their natural rhombohedral form. As well as its common name, which honors the place where it was originally found, Iceland spar is also known as optical calcite, because of what it does to light as it passes through.
Take a look through one of these crystals and you’ll see what I mean - you get two slightly offset images of whatever’s on the other side. Don’t worry, you haven’t had too much Brennivin. That’s Iceland’s national liquor, and it’s very tasty if you like licorice and, well, alcohol.
This double vision effect is caused by birefringence in the crystal. and to understand birefringence, we need to talk about the polarization of light. Light essentially consists of electromagnetic waves, which oscillate in a direction that’s perpendicular to their direction of travel, like waves travelling along a string.
The string goes up and down, and you can see the peaks traveling forward. But with natural light, the oscillations don’t just go up and down, they also move left and right, and top right-bottom left, and so on. They can oscillate in any direction that’s perpendicular to the one they’re traveling in.
And because there’s no favored direction, we say that these natural light sources are unpolarized. Now, when light travels through transparent materials like glass or water, it gets bent, or refracted. That means when it exits the other side, the image is shifted from where you’d expect it to be if the light had continued in a straight line.
The amount it bends depends on the refractive index of that particular material, which is the ratio between how fast light moves through a vacuum and through the new material that’s now bending it. But birefringent materials like Iceland spar do things a little differently. Birefringent materials have different refractive indexes for light of different polarization angles, so these components of the incoming light wave end up being bent differently.
The result is two separate light rays traveling through the material, polarized perpendicular to one another. We can’t see the polarization directly, but we can see the separation of the two light paths because they produce an offset double image. And Iceland spar has this polarizing birefringence because of an asymmetry in its crystal structure.
You can get birefringence in other materials too, like quartz, ruby, and, weirdly, cotton, but none of these are quite so striking as in this pure, optical calcite. In fact, the whole concept of birefringence was first documented way back in 1669, and they used crystals of Iceland spar to study it. But there’s a good chance that the crystal’s optical trickery was known about far earlier than the 17th century, and might just have been used to help ancient Viking seafarers find their way.
The Vikings were a society originally from Scandinavia, who dominated the open seas of the North Atlantic between 800 and 1050 CE. They were known for raiding and settling many of the islands and coastlines of the area, including Iceland, England, Greenland, Svalbard and Newfoundland. But exactly how they accomplished this is a bit of a mystery.
Navigating the open ocean when you can’t see the shoreline requires you to be able to keep track of what direction you’re headed in. These days we use magnetic compasses to do that, but 1000 years ago, people didn’t fully understand the magnetic effect, and usable compasses didn’t arrive in Europe until the 14th Century, which is way after the end of the Viking Era. So since they couldn’t use magnets, scientists think that the Vikings would have used the sun to keep them on course.
Naturally, this is a bit trickier than relying on the magnetic north pole, because the sun moves in the sky during the day and through the year. But it seems that they had a few workarounds. For instance, archaeologists found a wooden, disk-shaped device in Greenland called the Uunartoq disk, which they think was a kind of sundial.
This disk could have helped seafarers keep track of their direction or latitude, based on shadows cast by the sun. That’s great and all, but there’s another problem with relying on the sun to navigate. You can’t always see it.
At the high latitudes of Iceland and Greenland, there are many months when the sun is just skirting the horizon or even stays below it all day long. Plus, cloudy conditions would make it hard to know exactly where the sun was, even when it was above the horizon. So how did these people solve this problem?
Well, we think something they called “sunstones” were part of the answer. An Icelandic legend, written in the 13th century about a Norwegian chieftain, mentions using a so-called sunstone to locate the sun in overcast, snowy weather. Unfortunately, the saga gives no details about what exactly the stone was, or how it worked, but researchers think that it let them see polarization in the sky.
Like we said, light coming from the sun becomes polarized when it passes through the Earth’s atmosphere. Gas particles that are smaller than the wavelength of light can scatter the incoming light, in what’s known as Rayleigh scattering. Different wavelengths are scattered by different amounts, which is what makes the sky appear blue.
But Rayleigh scattering in the atmosphere also polarizes the light coming from the sun, leading to a pattern of polarisation in the sky. Light that’s close to the line of the sun’s rays hasn’t been scattered much, so it’s not polarised. But at further angles from the sun, the scattering is greater, so the light is strongly polarized.
Effectively, that creates a ring of polarization that’s centered on the sun and hits its peak about 90 degrees away from it. We can’t see this polarization with our naked eyes, although there are some animals that can. Certain insects like bees and spiders have special eyes called ocelli which have light sensors that only allow certain polarizations of light through.
This means that certain areas of the sky would appear darker, allowing these insects to intuitively know the position of the sun, even when it’s hidden behind trees or clouds. So, if the Vikings found a way to visualise the polarization of the sky in the same way, then they too could intuit the position of the sun and use it for their navigation. And that’s where the Iceland spar comes in!
Because of these crystals’ birefringence, you can use them to figure out the changing degree of polarization, and even the angle of polarization in the sky. You might be able to see this effect yourself if your rocks box includes a really clear sample Or you have some other clear sample to experiment with Here’s how To start, you need to create a double image on the crystal itself You can do this by making a mark or sticking a piece of tape on one face or by blocking off the faces so that only a thin beam of light comes through looking through the crystal should then show you a double image By the way, if the sun is visible while you try this, don’t look at it. Because, you know.
I don't want you to hurt yourself Then if you hold the crystal up to the sky and move it side to side, and you’ll see that one of the images gets much darker than the other one, The direction you’re facing when it’s darkest is 90 degrees away from the direction of the sun, because of that Rayleigh scattering ring we talked about. if you're having trouble seeing it you can try it at a time of day when when there aren’t any buildings blocking out the sun Or from a location with good sightlines of the horizon So, if you find all the directions where there’s the maximum contrast between the two images, you’ll map out that ring, and in the center of that ring is where you’ll find the sun! And we have evidence that this works outside of your backyard, too.
A study in 2013 showed how people using a crystal of Iceland spar could estimate the position of the sun to just one degree of accuracy. That’s about the thickness of your pinky finger held at arm’s length. That said, some researchers argue we’re overthinking this whole navigation by polarization business, because you can usually see well enough where the sun is based on how the clouds or the horizon is illuminated.
However, a study in 2011 showed that, while this certainly gives you a rough idea, in practice people are pretty bad at estimating the sun’s position this way, and the results are really inconsistent, especially at twilight. Still, other researchers thought that the difference in refracted images you see in the crystal might be hard to make out in cloudy conditions. But a study in 2014 showed that while the visuals aren’t as well defined on cloudy days as they are in full sun, the technique could still be used to locate the sun, even at twilight when the sun is below the horizon.
So if it could work, what are the chances that Viking navigators actually did use these crystals as their wayfinding sunstones? The theory is there, then, but the physical evidence was still lacking. If these sunstones were so indispensable, you’d think we’d find them left and right at Viking sites.
Thing is, calcite is a pretty fragile crystal, and it’s possible that hundreds of years of wear and tear would render the crystals unrecognizable which might explain why they’re so hard to come by. It’s a 3 on Moh’s hardness scale, meaning you can scratch it with a copper penny. Not only that, but calcium carbonate is soluble in weak acids.
But in 2002, divers exploring a shipwreck in the English channel did find a white, opaque rhombohedral crystal near some other navigational tools. It became known as the Alderney crystal, and it was from the late 16th Century, so a bit after the end of the Viking era. Although this crystal was calcite, its opacity wouldn’t have made it much good for navigation.
That said, we don’t know if it was this opaque when the ship went down or if the seawater damaged it between then and now. Research in 2013 showed how exposure to seawater could alter the transparent crystal enough to make it opaque, so it may have been crystal-clear when the ship went down. So perhaps it’s not surprising that the Vikings’ iceland spar crystals haven’t survived the last 1000 years.
But the Alderney crystal does at least give archeologists an idea of what to look for in Viking settlements and shipwrecks. Whether or not these ancient seafarers actually used this calcite for navigation, our SciShow Rocks Box subscribers can act out their own Viking voyages with their own piece of Iceland Spar. Subscribers receive an ethically-sourced mineral every month, right to their door, along with a handy info card telling you all the fun facts about your new specimen.
If you want to sign up for the waitlist, head over to Scishow. rocks or click the link in the description, where you can also peruse our other offerings, like this cool geologist’s loupe you can use to study your collection. Thanks for watching!
