Thanks to Generation Genius for sponsoring
this episode of SciShow. Go to generationgenius.com and check out their
next-generation science videos for grades K through five. [♪ 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 — Greek
for “many colors” — 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
— which, appropriately, means “same 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 — 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 — better
known as ruby — 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 — if light of the right wavelength hits one of the iron’s
electrons — 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
— from like a candle flame or incandescent light bulbs — 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. If you want more science content — especially
for the kids in your life — you can also check out Generation Genius. They make science content for kids in kindergarten
through fifth grade, and their videos are both really informative and fun to watch. They feature an award-winning scientist named
Dr. Jeff, who is also known as the Dancing Scientist. Each video covers a specific topic that’s
aligned with state science standards — things like the Moon and its phases, weathering and
erosion, and chemical and physical changes. Besides videos, the service also includes
lesson plans, quizzes, reading material, and DIY activities. And they have plans for parents, teachers
and schools. So whether you’re looking for something
for your classroom or a fun weekend activity at home, there’s something designed for
you. If you want to learn more, you can try Generation
Genius for free — no credit card required. Click the link in the description to head
to GenerationGenius.com [♪ OUTRO]
