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  • K through five.

  • [β™ͺ INTRO ]

  • From the bedrock that supports buildings, to the bones that let us dance the night away,

  • minerals are pretty amazing.

  • And with their rainbow of colors, they also add some sparkle to our lives.

  • Normally, individual minerals are one color, that's not always the case.

  • Some look multi-colored upon a single glance, and others look different depending on whether

  • you're looking at them in the sun or under incandescent light.

  • Some minerals even look different if you rotate them.

  • When you do, they'll appear to change color before your eyes.

  • Sometimes it's from, say, light green to dark green, but for others, it's green to

  • blue, or even blue to violet to… burgundy?

  • It's a phenomenon known as pleochroism β€” Greek for β€œmany colors” β€” and it's not a

  • trick of the eye: It's physics.

  • So here are five pleochroic minerals, and why you and your friend might disagree on

  • their color if you're holding them just right.

  • The atoms in minerals are arranged in one of six crystalline structures that repeatover and over again.

  • Those structures are called unit cells.

  • The most symmetric of them is named isometric β€” which, appropriately, means β€œsame measurement.”

  • You might also hear it called cubic, because the unit cell is cube-shaped.

  • Diamonds, for example, have this kind of structure.

  • But diamonds, while often shiny, are arguably not as interesting as our first example: corundum,

  • which is made of aluminum and oxygen arranged in a hexagonal crystal lattice.

  • A hexagonal unit cell has three axes that meet each other at 120 degrees, and a fourth

  • axis perpendicular to those.

  • While isometric crystals are symmetric in all three spatial dimensions, hexagonal lattices

  • are symmetric in only two.

  • The third looks different.

  • That means that light is going to travel through it differently depending on the direction

  • the light is coming from.

  • This is the origin of pleochroism.

  • Essentially, when light passes through a crystal, some of it bends, or refracts.

  • And the amount it bends depends on what the crystal is made of β€” in particular, what

  • the situation of its electrons is.

  • When light enters a more electron-dense region, it slows down.

  • And the slower it travels, the more it bends.

  • So if you have a crystal with an asymmetric lattice, the electrons are unevenly spaced.

  • The light wave will enter and split into two rays that travel at different speeds and get

  • refracted different amounts.

  • In pure, transparent, corundum, this just produces an effect called birefringence, where

  • the two light paths create a double image.

  • You can see this in other clear crystals like calcite.

  • But things are different in red corundum β€” better known as ruby β€” or colored sapphires.

  • These things get their base colors from impurities in their crystal lattices, which absorb different

  • wavelengths of light.

  • But because of the asymmetric hexagonal lattice, rubies aren't just red.

  • They can vary between an intense purple-red and lighter orange-red depending on how you

  • hold them up to a light source.

  • Blue sapphires also switch from looking violet-blue to a lighter greenish-blue.

  • All thanks to the properties of light.

  • The structure of red and blue conundrum is only symmetric across two dimensions, which

  • is why rubies and blue sapphires only show two colors.

  • In other words, they're .

  • But other minerals are even cooler.

  • They can be trichroic, or can show three colors.

  • One of them is called cordierite.

  • Cordierite has an orthorhombic crystal structure, made up of this formula.

  • Basically, it's some magnesium, iron, aluminum, silicon, and oxygen.

  • And the lattice axes are all different lengths but meet up at right angles.

  • So it actually doesn't have any symmetry.

  • Its structure looks different along each dimension.

  • This mineral is usually found in metamorphic rocks like gneiss and schist, so is often opaque.

  • You may have even purchased a pizza stone made of it.

  • And, like, it'd be hard to observe trichroism there.

  • But transparent cordierite displays a different color for each dimension.

  • Along one, it's pale yellow or green.

  • Along another, it's light blue.

  • And along the last, it's violet or blue-violet.

  • It's the last of these that makes gem-quality cordierite, known as iolite, a common substitute

  • for blue sapphire or tanzanite gems in jewelry.

  • You just have to cut it so the top of your gem lines up with the violet axis.

  • Cordierite may even have a place in history for its pleochroism.

  • Medieval Norse sailors, what many might refer to as Vikings, used a stone to help navigate

  • when it was so overcast you couldn't see where the Sun was.

  • And according to research, it may have been cordierite.

  • This makes sense, too.

  • Trichroic minerals would be able to do this job, so long as you consistently held the

  • same plane of the stone up to the sky.

  • And if you cut the gem so that its length, width, and depth were all different, those

  • planes would be really easy to keep track of.

  • Andalusite is also orthorhombic, but it has a simpler chemical formula.

  • It's just some aluminum, silicon, and oxygen.

  • Like cordierite, it's usually opaque.

  • But when you've got transparent crystals with a bunch of impurities, andalusite varies

  • in color from yellow to green to brown.

  • The brown is especially interesting, though.

  • Because while it can be caused by vanadium or chromium impurities, andalusite can also

  • look brown thanks to its trichroism.

  • To see this, you'd have to hold it the right way, and iron and titanium would have to replace

  • some of the aluminum in the mineral's crystal lattice.

  • But there's something special going on here, too.

  • Because if you just put plain old iron and titanium in the lattice, your mineral won't

  • necessarily look brown.

  • Andalusite gets its color from a phenomenon known as intervalence charge transfer, or

  • . I know, this episode has lots of really good words.

  • Basically, the iron and titanium's electron shells are so close together that β€” if light

  • of the right wavelength hits one of the iron's electrons β€” it'll get knocked off and

  • fall around the titanium.

  • That causes both the iron and titanium to get a different electric charge, so they absorb

  • different wavelengths of light and produce that brown color.

  • Because light travels in a straight line through a medium, to view the effect you have to hold

  • the crystal so the titanium is lined up behind the iron.

  • It's actually moving atoms around!

  • When the mineral chrysoberyl has a chromium impurity interspersed throughout its orthorhombic

  • lattice, it's better known as alexandrite.

  • It's one of June's multiple birthstones, and was named after to-be Tsar Alexander II

  • for a sixteenth birthday present.

  • So that's one of the things you get when you are about to become an emperor.

  • You also later get assassinated, so it's not like it's all cracked up to be.

  • It's trichroic, and is colored green, orange, or purple-red depending on your point of view.

  • But it also displays a different kind of color-change that we couldn't not mention.

  • Both the amount of chromium and its distribution throughout the crystal make alexandrite look

  • green in daylight, and red by candlelight.

  • It happens because the chromium in its lattice has lost three of its electrons.

  • And that kind of chromium transmits most light at either the blue/blue-green 490 nanometers

  • or red at 600 nanometers.

  • So when it's exposed to more reddish light β€” from like a candle flame or incandescent

  • light bulbs β€” alexandrite transmits more red than green.

  • And when it's out in the Sun, which transmits much shorter, greener wavelengths, it transmits

  • more blue and green light.

  • Although, since our eyeballs are more sensitive to green, that's the main color we see.

  • Our final example is tourmaline.

  • It's not one specific mineral species, but is actually a group of them.

  • They're closely-related, dichroic minerals that all have a variety of hexagonal unit

  • cell called trigonal, but they have different chemical and physical properties.

  • They all have silicon, aluminum, and boron atoms in their lattices, but could have any

  • of sodium, lithium, calcium, magnesium, manganese, iron, chromium, vanadium, fluorine… and copper.

  • It's a lot.

  • This variety causes tourmalines to have a wide variety of colors, from greenish-blues

  • to reds, pinks, and yellows.

  • And since tourmalines come in a rainbow of colors, their dichroism comes in a rainbow, too.

  • We could talk about these things all day, but for brevity, let's stick with green tourmaline.

  • It's either pale green or dark green depending on how you're looking at it.

  • But on top of this, green tourmaline can also display color zoning, which is when your crystal

  • is just two different colors.

  • You might have even heard it referred to as watermelon tourmaline.

  • This can happen when different sections of the lattice have different impurities that

  • absorb different colors, which is pretty straightforward.

  • But green tourmalines specifically look like watermelons in a different way.

  • They exhibit a green-to-red color change that's kind of the opposite of what happens with alexandrite.

  • It's called the Usambara effect.

  • Is this the last great word in this episode?

  • I think so.

  • In a short enough crystal, the chromium in green tourmalines absorbs some of its usual

  • light, so the mineral looks green.

  • But in longer crystals, more of that short-wavelength light gets absorbed as it moves through the mineral.

  • So by the time it comes out only the red light is left.

  • In an asymmetric stone with a high enough concentration of chromium, you see both colors,

  • but only when there's a light source right behind it.

  • So here's to all the colorful minerals out there.

  • Thanks for giving us a good light show, and also for the lessons in chemistry and physics.

  • And also, also for all of these really good words, columdum.

  • Feel free to incorporate this knowledge when you're showing off your bling, and let us

  • know in the comments if you have any favorite pleochroic minerals we missed.

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