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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.
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