Crystals

I always love learning about crystals! It's so fascinating to watch them grow. We didn't make hot ice this time, sadly, because that's one of the best things to do. But we grew sugar crystals and left them in the solution for a lot longer than we've had the patience to before. So we got some nice big crystals.

They are so pretty!

We grew borax crystals too

Here we were attempting stalactites with epsom salts, but got more like…cave popcorn?

Still, lots of growth!

These are salt crystals (ideas for growing them here)

I love seeing the tiny cubes they make

And the pretty fern-like patterns!

We still have a lot of unbroken geodes from our last , so we broke one to see what crystals we could find inside!

The girls were very patient and didn't just smash the thing as we've done sometimes in the past, and it was worth it because it ended up splitting neatly in two:

like this!

Beautiful!

I love the group of big crystals within the matrix of tiny ones in this half of the geode!

Crystal-Growing Activities

For years now I've been wanting to repeat this activity because it's just SO COOL. But I've also been nervous because it's a little tricky to get right. Sometimes it doesn't work and then you have to patiently boil down the solution and try again. We did need to do that a couple of times, but it was so worth it! 

A couple weird stages our hot ice went through when we overheated it or crystallized it accidentally. It's okay. You can always melt it down, add a little more vinegar, and try again.

Unfortunately, the video tutorial for making hot ice which we watched last time was no longer online, but I kind of remembered what to do, and we watched this video to refresh our memories on the details. I also explain the process in my earlier blog post about it.

Crystals are a great lead-in to talking about the relationship between temperature and pressure (you can also talk about this with respect to geysers). This video is cool, about how a pressure cooker or Instant Pot works. The same principle applies deep inside the earth, and allows water to become superheated or supersaturated with mineral solvents.

Another activity I've always wanted to do is growing a crystal from a seed crystal. It is pretty easy. First, you just grow some small alum crystals, and then pick one of them and suspend it in the supersaturated alum solution. It is all outlined in this tutorial. 

Here are the small alum crystals. They're so pretty--like ice that doesn't melt, or like little diamonds. My children loved playing with them.

We picked out a nice biggish one to serve as our seed crystal. Then we suspended it with fishing line in the alum solution, like this:

We watched it for several days, and it grew a little each day. It was really cool! When we finally took it out of the solution, it was so big and beautiful! Here it is:

We loved the flat faces it developed—like a diamond!

We learned about how crystals grow bigger when left to grow slowly and undisturbed (intrusive vs extrusive mineral growth). Here's an example: we grew salt crystals on two pieces of cardboard (you just soak the cardboard in a supersaturated salt water solution). (See also here.) One was left outside to dry/crystallize in the sun and wind, and one was protected inside a warm oven. You can see the one that grew slower (on the right in this picture) had much bigger crystals! This is like what happens with basalt vs. granite.

Close up of those salt crystals on the right—I love how nicely cubic they are!

And here are some under a microscope. Beautiful!

Looking at crystals under a microscope is such an interesting thing to do. They become even more beautiful when you put a polarizing filter over your eyepiece so you can see the colors (birefringence). You can learn more about this and find instructions here.

These are citric acid crystals

Epsom salt crystals

And sugar crystals.

Epsom salt crystals, not under the microscope. You can learn how to grow these here or here. They grow really fast!

Another example of "intrusive" and "extrusive" crystal growth, this time with sugar. Instructions for this experiment are here, and you can learn more about the process in geology here. 

Here is the hot sugar solution while it's still cooling—you can see the surface has cooled more quickly than what's underneath, forming these cool surface wrinkles. Sometimes pahoehoe lava has wrinkles like this too!

And here we have the smoother, small extrusive crystals on the left, and the bigger intrusive ones on the right.

You can also make melted sugar that cools so quickly, it doesn't have time to form crystals at all! This is similar to obsidian. You can learn how to do that here. It is so pretty! And the children love to break off pieces and eat it. :)

Crystals are so cool. Find more crystal-growing activities here!

Igneous Rock

We have done a lot with rocks and the rock cycle before, so we didn't go all-out with it this time. There are tons of fun activities to do, though—igneous fudge, the chocolate rock cycle, and geologic brownie sundaes, to name a few. :) This one is similar to the "igneous sugar" activity we've done before, and demonstrates how crystal formation is aided by greater periods of time and warmer temperatures.

Sugar that cools very quickly forms very small crystals, or no crystals at all. To demonstrate, you melt sugar with a little water in a pan until it is just golden brown. Then pour the molten sugar in a very thin layer onto a pan you've had in the freezer. The sugar will cool almost immediately into this smooth, glassy substance, similar to the way obsidian or volcanic glass forms.

It's really quite beautiful.

On the other hand, if you make a supersaturated sugar solution that will cool very slowly (put it in a tall jar or glass, with a lid if possible), the crystals will have time to grow in a much larger and more definite form.

Like this. Ours took days and days to cool, so be patient! :)

This was a demonstration of how different types of magma flow at different speeds. The viscosity of magma affects what types of volcanoes will form in a particular location. We talked about this more during our volcano unit.