When you think of where crystals form, you might imagine some cave
deep in the Earth, or maybe like, actual magma, or something.
But as it turns out, the ocean is full of crystals, too.
And among them is aragonite,
which is probably the most important crystal you have never heard of.
Because aragonite does a lot more than just look pretty on rock collectors’ shelves.
It’s a vital part of marine ecosystems,
and it tells us something about life in the ocean millions of years ago.
I promise you this: You're never going to think about the words
"crystal blue water" the same way again.
[♪ INTRO]
Okay, to start, let’s talk about what’s going on in the ocean.
Ocean water is notoriously salty, thanks to a bunch of sodium chloride.
But it’s not in the same form in the water as it is in your pantry or on your fries.
The salt crystals dissolve in water and the charged ions
of sodium and chlorine float around separate from each other.
And there are tons of other ions floating around in seawater too, that came from
other things that go all Wicked Witch and get melty when they get wet.
Two of these ions are calcium, which is positive, and carbonate, which is negative.
Both of those ions contribute to the ocean’s mild alkalinity.
Which is like the opposite of acidity.
When the concentrations of these ions are high enough,
they can come together to form calcium carbonate.
And calcium carbonate is what aragonite is made of, where all the
calcium carbonate bits are arranged in a specific crystal structure.
The crystals have what’s known as an orthorhombic structure, but that’s not the
only shape you can make out of those calcium carbonate molecules.
Another very common form is called calcite.
Its crystals have a trigonal structure,
which looks a little different at the molecular level.
There’s also a more rare form of calcium carbonate called vaterite,
which has crystals shaped like hexagonal-based prisms.
Vaterite is more common in mineral springs than it is in the ocean,
and weirdly enough, it’s also found in gallstones.
It also isn’t stable over the long term,
so it’ll turn into aragonite or calcite when exposed to water.
Which is why when we’re talking about calcium carbonate in the ocean,
aragonite and calcite are the two main characters.
Whether calcium carbonate in the ocean forms aragonite or calcite crystals
depends on the amount of dissolved magnesium
that is hanging around in the water.
Magnesium ions tend to stop calcite from forming, so when there’s more
magnesium messing with calcite, aragonite crystals will form more easily.
And while aragonite crystals can form on their own,
they are most often built by helpful little marine organisms.
Molluscs, like mussels, oysters, and sea snails,
all make aragonite in order to form their shells.
Corals use it when making their reefs.
Even some kinds of seaweed build a kind of rigid skeleton,
using, you guessed it, aragonite.
Not only is aragonite the backbone of marine invertebrate ecosystems, it’s also
the building block for some of the most beautiful things we find in the water, too.
The mother of pearl inside oysters?
Aragonite.
The pearls themselves?
Aragonite too.
Though this one is plastic.
There’s even a rare kind of iridescent gemstone called ammolite,
that’s made from fossilised aragonite in ammonite shells!
Aragonite washes up on shore, too.
Lots of beautiful white sandy beaches that are on your vacation wishlist
are almost certainly made from aragonite.
Though, how they came to be might make you rethink your holiday mood board.
Because at least 70% of all carbonate sand in Hawaii and the Caribbean
has passed through the gut of a parrotfish.
The parrotfish crunch up hard corals to get to the tasty organic matter inside,
and poop out the indigestible aragonite in sand-sized chunks.
So just, like, try not to think about where
that lovely white aragonite sand came from when it’s between your toes!
Anyway, the ocean is a complicated system, and there is another crucial factor
in the ability for these organisms to make their aragonite bodies - ocean acidity.
See, carbon dioxide in the atmosphere dissolves into seawater
and makes something called carbonic acid.
As I mentioned before, calcium carbonate is alkaline,
and it reacts with any acid in the water.
This is actually the same thing that happens in your stomach when you take
antacids, because calcium carbonate is the main ingredient in those, too.
The problem is, this reaction effectively uses up the carbonate in the ocean,
stealing it away from the organisms that need it for their shells.
So when carbonate runs low, the waters become more acidic, which then starts
eating away at the aragonite in seashells and reefs that have already formed.
And acid isn’t the only thing in the water that can affect aragonite formation.
You might remember earlier when I was talking about magnesium.
And since the presence of magnesium in the water
can prevent critters from making calcite,
it’s the main thing that keeps the organisms making the aragonite crystals.
And scientists can use the record of aragonite formation
to reconstruct ocean conditions throughout Earth’s history.
The amount of magnesium in the ocean changes over long timescales,
largely because of changes in the rate of seafloor spreading.
When the movement of tectonic plates makes lots of volcanic rock, the sea water
reacts with the magnesium in that rock, causing it to leach out into the water.
Right now, we have an ‘aragonite sea’,
where magnesium is high and the aragonite is the go-to crystal form for critters.
But about 70 million years ago, during the Cretaceous period, magnesium levels
were lower and calcite was the preferred crystal, making it a ‘calcite sea’.
Other factors can affect the prevalence of aragonite or calcite
in ancient oceans too, including the sea temperature and the water depth.
So geologists can study the crystal structure of ancient organisms
to figure out whether they built their shells with aragonite or calcite.
Doing that reveals information about the ocean conditions and chemistry
when they lived, which can tell us about the history of our world.
All thanks to this little crystal.
This little crystal!
I’m talking to the mic.
So that I-
I felt like you probably couldn’t hear me when I was all the way down there.
Which goes to show how important the crystal hitting your mailbox this month is!
SciShow Rocks Box subscribers will be receiving
a beautiful specimen of aragonite.
Our Rocks Box subscription is a monthly delivery of a mineral or fossil
that we thought was neat enough to send to you.
They’re always collected using best ethical practices
for the people in that area, and for the world.
The subscriptions have been selling out fast, so don’t wait!
Visit SciShow.Rocks - and yes, that’s a real URL - for all the details
and to snag a subscription while you still can.
Thanks for watching!
[♪ OUTRO]