Thread by @mikamckinnon

Ah, I see we're on "Crystals are Magic" day again already.

Crystals are minerals growing in specific HIGHLY ordered structures.

Crystals can be natural or synthetic.
Crystals can make me happy ('cause they're neat!)
Crystals are never magic.

Note: opalite is always synthetic.

I dislike it when science is reduced to magic because reality is so much cooler.

For all my griping about not wanting to memorize All The Things for exams, mineralogy & crystallography are badass and deserve more respect than getting waved away as Magical-Wagical Wooishness.

Are you ready to dive into hardcore science?

If you want to understand the properties of a crystal -- the real, physical properties of how it will behave when you zap it, polish it, wear it, build on it, break it, or keep it in your pocket -- you look at its structure.

Crystal structure is the geometry of a single unit cell which then repeats over & over & over.

Unit cells are parallelepipeds (3d parallelogram) described by:
lengths of edges (a, b, c)
angles (α, β, γ)
fractional coordinates of particles (xi, yi, zi)

It's... a lot.

But geology is shit you find irl! That means we don't need an infinite variety of possibilities, just actual crystal forms we can find in the wild or grow in labs.

Waaaayyyyyy easier subset.

Our biggest cheat? Crystals are symmetric. All of them.

You just have to pick the right axis.
Every crystal has translational symmetry: slide unit cell to repeat repeat repeat. Many ALSO have rotational or mirrored symmetry.

Like tessellations?
You'll love crystal structure.

Crystal structures are organized by their symmetry & lattice structure:
Triclinic
Monoclinic
Orthorhombic
Tetragonal
Hexagonal (trigonal or hexagonal symmetry)
Cubic

These 7 crystal systems divide into 32 crystal classes. And... that's it. That's every possible structure.

Q: But wait! I've seen crystals of different shapes! Did you lie to me?

A: Ahhh, but wait! Crystal structure influences shape, but what you see is the crystal habit. Crystal habit depends on:
crystal structure
if it's solitary or grows in packs
if it had room & resources to grow

Crystal habit is where things get super-fun because while it's still all driven by that technical core, it's prettttyyyyyy

Acicular crystals are fans of needles.
Globular crystals look like rock warts.
Cubic crystals look so fake it's awesome.
Dendritic crystals are fractals.

Actual crystal properties are:
Colour (which is usually a useless lie)
Habit (shape)
Streak (colour of powder)
Cleavage or fracture (how it breaks)
Lustre (shiny!)
Density
Hardness
Random other shit like if it's lickable & tasty; if it's magnetic; if it glows when you bash it.

All those properties come down to the crystalline structure -- what chemical elements are bound together into what structure, and what trace contaminants snuck in?

No magic necessary.

Diamond and graphite are the exact same chemical composition: carbon with carbon and more carbon.

But their crystal structures are totally different, and that makes all the difference between a mineral you can draw with and one you can drill into rock with.

Crystallization happens in more than just geoscience.

The science of caramelisation vs crystallisation of sugars is a huge part of baking desserts (and a challenge for how you could possibly make chocolate in space! qz.com/1680054/astron…).

It's still not magic.

Crystal properties can be SO COOL.

Quartz, boring ol' quartz, is piezoelectirc: put it under mechanical stress and that fucker can generate electricity. WHAT?!

Calcite is birefringemt: it splits light. You can use that trick to find the direction of the sun on cloudy days.

Galena isn't even a metal yet it's a freaking semiconductor, and was the crystal in early radio sets where you balanced the cat's whisker wire to convert radio waves to electric signals.

Alexandrite takes "colour is a lie" to new lows by changing depending on the light source.

I could keep going because it turns out that irl crystal properties are a goddamn nightmare of delight and confusion that fills me with joy and headaches.

But I'll save it for @MineralCup in September.

Until then, browse mindat.org & minerals.net

@MineralCup As of Nov 2018, we know of 5,413 minerals: ima-mineralogy.org/Minlist.htm

This shrinks & grows as we find new wonkiness (space diamonds! quasicrystals! Pandoraite, which totally isn't doom-inspired!) or realize something's actually a variety of something else (ruby is fancy corundum!)

@MineralCup Bonus:
Physics, chemistry, and geology are scientific processes that work on other planets.

Based on composition, landscapes (geomorphology), processes we can see now, & any other context we can find, we can extrapolate the properties of minerals on other planets.

@MineralCup Extrapolating mineral properties to other worlds gets REALLY fun when applied to fiction.

The season opener to Stargate: Universe has our heroes searching for carbonate rocks in desert-that-was-ocean to fix their spaceship air filter.

https://twitter.com/mikamckinnon/status/972552174847889408

@MineralCup Q: So collecting crystals won't make me happy?

A: I feel soothed by smooth pebbles that have undergone enormous turmoil yet survived, inspired by how rocks quietly tell so many stories, & mesmerized by the geometric beauty of crystals.

Magic isn't necessary to like crystals.