Name: 5種迷人的變色礦物 (5 Delightful Color-Changing Minerals)
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From the bedrock that supports buildings, to the bones that let us dance the night away,

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

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.

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

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

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

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

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

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

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

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

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

is why rubies and blue sapphires only show two colors.

They can be trichroic, or can show three colors.

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.

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.

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

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.

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

The brown is especially interesting, though.

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

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

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

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5種迷人的變色礦物 (5 Delightful Color-Changing Minerals)

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