Educator How-To: Crystals, Geometry and Chemistry

Math is beautiful and inescapable. Especially in nature, patterns and equations just keep showing up.  The path of an orbiting planet, the growth of a nautilus, arrangements of leaves on a stem, the efficient packing of a honeycomb; we can find rules and algorithms and make predictions from them.

Crystals, with their obediently repeating structure, are an elegant manifestation of the ‘rules.’  To be a crystal, your building blocks (atoms, molecules, or ions) must follow patterns over and over and over and over and over.  Atoms, being predictable, simply do what their chemical properties and the conditions (temperature, pressure, etc.) indicate.  So what exactly does it take to go from a mess of elements and compounds to this[picture of green “Crystals of India” crystal specimen] example from the Crystals of India exhibit at HMNS Sugar Land?

If you’ve ever tried making rock candy from sugar water or ornaments from borax solution, then you have some idea what it entails: something dissolved that is capable of making crystals has to slowly come out of solution – usually the longer you give it, the bigger it can grow and the slower it grows, the more perfect the crystals.

Freezing water into ice also gives you crystals; they just don’t stick around and let you handle them conveniently at room temperature. Water and solutions in water aren’t the only way to get crystals; molten rock cooling (slowly) can also give crystals, but that’s a little tricky for home experimentation.

So time is your friend for crystal growth, pressure is a factor, and it needs to be easier for atoms to attach to the forming crystal than to stay in solution.  Having a solution that is saturated or supersaturated so it can barely hold all of the dissolved material helps. It also helps to have places for the crystals to start forming; a tiny ‘seed’ crystal or sometimes even just a rough spot on a surface can provide the nucleation sites to kick off crystal growth. Are there other ways crystals and the things we consider ‘gems’ can form? Yes!

For those of us with shorter attention spans, a cool way so see the process is with crystallizing hand warmers – a pouch holds a saturated solution of sodium acetate. When you flex a metal disk inside the pouch, you kick off a chain of crystallization and end up with solid material (and released heat energy).  Because the process is so fast in the hand warmer, the individual crystals are very small and jumbled up (polycrystalline); oriented in all different directions, and as a mass they are opaque (light is refracting all over the place) and relatively dull rather than shiny and smooth as slower-forming large crystal faces can be.  The structure of most metals is also polycrystalline, and things like plastic and glass (even the kinds misleadingly labeled “crystal!”) are amorphous.

The external crystal shapes we see are related to the internal structure – there are a lot of different ways atoms can pack together.

Practically, there will always be some disruption in a crystal structure, no matter how perfect it may appear, which allows for some very cool effects – crystals “twinning,” impurities that alter the color; the reason ruby and sapphire (both corundum crystals) appear different.

Crystals aren’t always pretty! Sometimes we want to prevent crystallization to avoid things like kidney stones, but crystals are useful for all kinds of things; optical equipment and lasers, X-ray crystallography to figure out structures of proteins (and once upon a time, DNA), and silicon chips used in electronic devices. 

Whether you prefer your crystals practical or decorative (photo of white spikey crystal from Crystals of India), they are amazing!

Can’t get enough crystals? Check out the Crystals of India exhibit at HMNS Sugar Land (free for members!)

 

 

Crystals are Cool: Making Rock Candy

Wish I Had Noted the Name
Creative Commons License photo credit: biggertree

There are so many different kinds of crystals all around us, but just what are they anyway?

Put simply, a crystal is a grouping of molecules or atoms that is organized in a specific way.  Every crystal has a unique shape and properties that make it recognizable.  In today’s experiment, we will be working with sugar crystals which are oblong and slanted on the ends.

There are a couple of things going on that contribute to the growth of sugar crystals in this experiment.  First, you will be creating a supersaturated solution by heating a saturated sugar solution and allowing it to cool.

Supersaturated solutions are solutions that are so full (of sugar in this case), that they are unstable.  The solution you will create will contain more sugar (the solute) than it can hold in a liquid form.  Therefore, the sugar must come out of solution – forming what is called a precipitate (also known as yummy rock candy).  The second mechanism that helps to form the sugar crystals is evaporation.  Slowly, the water evaporates from your solution.  As this happens the solution becomes even more saturated with sugar and the sugar will continue to come out of the solution and form sugar crystals.

Blue Sugar
Creative Commons License photo credit: karsten.planz

What you are left with is a delicious science treat!  Make sure to only eat a little at a time and keep the rest sealed in a baggie.  Also, don’t forget to brush your teeth; it is pure sugar after all!  Have fun in your kitchen lab and don’t forget to be safe!  Always include an adult when trying new experiments.

Grab a handy adult; you will need one to do this activity!

Materials:
Granulated sugar – 1 cup
Water – ½ cup
Saucepan
Food coloring
Two canning jars
Spoon

What to do:
1. Dump one cup of sugar and ½ cup of water into your saucepan.  Don’t stir it!
2. Find your adult and have them help you put it onto the stove over medium-high heat.  Wait for the mixture to come to a boil and let it boil for one minute without stirring.  If you want colored rock candy, you may add some food coloring while it boils.
3. Instruct your handy adult to pour this mixture into the two canning jars.
4. Find a place on your counter that you can let the two jars sit undisturbed for two weeks.
5. Observe them once a day.  Slowly, crystals begin to form.  When you see a crust form on top of the jars, use a spoon to carefully break the crust so the water can continue to evaporate.  Don’t do anything else to your jars other than this!
6. When you feel like you have enough crystals of the right size, have an adult help you remove them from the jar using a dull table knife.
7. Eat and enjoy!  Don’t forget to brush your teeth, it is sugar after all!

100 Years – 100 Objects: Zoisite

The Houston Museum of Natural Science was founded in 1909 - meaning that the curators of the Houston Museum of Natural Science have been collecting and preserving natural and cultural treasures for a hundred years now. For this yearlong series, our current curators have chosen one hundred exceptional objects from the Museum’s immense storehouse of specimens and artifacts—one for each year of our history. Check back here frequently to learn more about this diverse selection of behind-the-scenes curiosities—we will post the image and description of a new object every few days.

This description is from Joel, the Museum’s President and Curator of Gems and Minerals. He’s chosen spectacular objects from the Museum’s mineralogy collection, which includes some of the most rare and fascinating mineral specimens in the world, that we’ll be sharing here – and at 100.hmns.org- throughout the year.

Zoisite

Zoisite (var. tanzanite)
Merelani Hills near Arusha, Umba Valley, Tanzania

This magnificent, near-flawless crystal of tanzanite (the gemstone variety of the mineral zoisite) was found in Tanzania in 1991. As seen here, the specimen is an exceptional example of trichroism, whereby the same crystal exhibits three distinctly different colors, depending on the viewing angle. 
 
Marvel at the world’s most spectacular collection of natural mineral crystals in the Cullen Hall of Gems and Minerals at the Houston Museum of Natural Science.

You can see more images of this fascinating artifact – as well as the others we’ve posted so far this year – in the 100 Objects section at 100.hmns.org.

100 years – 100 Objects: Azurite

The Houston Museum of Natural Science was founded in 1909 - meaning that the curators of the Houston Museum of Natural Science have been collecting and preserving natural and cultural treasures for a hundred years now. For this yearlong series, our current curators have chosen one hundred exceptional objects from the Museum’s immense storehouse of specimens and artifacts—one for each year of our history. Check back here frequently to learn more about this diverse selection of behind-the-scenes curiosities—we will post the image and description of a new object every few days.

This description is from Joel, the Museum’s President and Curator of Gems and Minerals. He’s chosen spectacular objects from the Museum’s mineralogy collection, which includes some of the most rare and fascinating mineral specimens in the world, that we’ll be sharing here – and at 100.hmns.org- throughout the year.

azurite

Azurite
Tsumeb Mine, Tsumeb, Namibia

 The Tsumeb mine has produced the world’s finest azurite crystals, of which this large 11-cm crystal group on green smithsonite is one of the best examples. The highly lustrous, elongated crystals with perfect terminations, on a contrasting base, admirably fulfill the requirements of connoisseurship.

Marvel at the world’s most spectacular collection of natural mineral crystals in the Cullen Hall of Gems and Minerals at the Houston Museum of Natural Science.

You can see more images of this fascinating artifact – as well as the others we’ve posted so far this year – in the 100 Objects section at 100.hmns.org