What Are Crystals Made Of?

It’s summer, and my son and I are walking home from preschoool and he’s exploring the area and looking at everything. As he does, he stops at a smallish boulder that’s been left at the corner of a road by a landscaper. “Daddy!” he calls, “Look!” So I go and look. He’s pointing at a band of what I think is quartz, rippling through the stone. “What is that?”

“Those are crystals,” I tell him. “Like the ones we saw at the museum. Remember them?” We’d just recently been to the Cincinnati Natural History Museum, and one thing they had on display was a collection of different crystals and geodes.

“Oh,” he says, staring at the rock. “They’re pretty.”

“Yes,” I agree, “they are.”

“What are they made of?”

Uhm…

What is a crystal?

To begin with, let’s hit Merriam-Webster up. They define ‘crystal‘ as:

  1. quartz that is transparent or nearly so and that is either colorless or only slightly tinged
  2. something resembling crystal in transparency and colorlessness
  3. a body that is formed by the solidification of a chemical element, a compound, or a mixture and has a regularly repeating internal arrangement of its atoms and often external plane faces
  4. a clear colorless glass of superior quality; also : objects or ware of such glass
  5. the glass or transparent plastic cover over a watch or clock dial
  6. a crystalline material used in electronics as a frequency-determining element or for rectification

What is a Crystal, a page on University of California Berkeley’s College of Natural Resources site, says:

Something is crystalline if the atoms or ions that compose it are arranged in a regular way (i.e, a crystal has internal order due to the periodic arrangement of atoms in three dimensions).  Gems are described as amorphous if they are non-crystalline.

Crystals characterized by well developed crystal faces (external surfaces) are described as euhedral . Crystals do not always show well developed crystal faces seen on euhedral examples.

A crystal is built up by arranging atoms and groups of atoms in regular patterns, for example at the corners of a cube or rectangular prism.

The basic arrangement of atoms that describes the crystal structure is identified. This is termed the unit cell.

Crystals must be charge balanced.  This means that the amount of negative charge must be compensated by the same amount of positive charge.

 

So what are crystals made of?

Atoms.

More usefully, Crystal Structure of the elements says that the only elements that don’t form crystals are promethium, astatine, radon, francium, einsteinium, fermium, mendelevium, nobelium, lawrencium, rutherfordium, dubnium, seaborgium, bohriumhassium, meitnerium, darmstadtium, roentgenium, unubium, unutrium, unuquadium, ununpentium, ununhexium, ununseptium, and ununoctium. Of all of these, only radon is found naturally on Earth, and the idea that it has no crystal structure is contradicted by Elements Database which states it has a cubic crystal structure. So it’s quite possible that the others have them as well, and we just don’t know because they tend to fall apart before we can see what they do.

The most common crystals on Earth tend to be made out of the most common elements on Earth. Why? Because they’re available to make crystals. These elements are oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg) in the proportions seen below.

I’ll be honest here, and say that I expected carbon (C) to be much higher on that list. You know, what with it being so vital to every living thing we see. But no. Carbon is part of the 1.5% “other”, and makes up only 0.15% of the Earth’s crust. Go figure.

Most likely, the crystal that caught my son’s eye was either feldspar or quartz – the boulder was granite, after all, and granite is largely made up of those two crystals. Quartz is silicon dioxide (SiO2), it comes in a variety of colors depending on the impurities in the crystalline structure, and it ranges from transparent to opaque. Feldspar is actually a group of three related minerals (KAlSi3O8, NaAlSi3O8, and CaAl2Si3O8) which can resemble quartz. I’m certain a minerologist could figure out the difference, but I certainly couldn’t. Not from a purely visual inspection, anyway.

When Ice Is on Fire, Does The Ice Melt?

“Dad?”

“Yes, son?”

“When ice is on fire, does the ice melt?”

“…”

“…”

“…what?”

burning_ice_cube

It was one of those questions, the random sort of question a five-year-old (he just turned six last week, but he was five when he asked it) will ask. I don’t have any idea what prompted the question, or where it came from. But, the more I thought about it, the more interesting it sounded.

Can ice burn?

Well, it depends on what you mean by ‘ice’ and by ‘burn’. Here’s how Merriam-Webster defines it:

Full Definition of ice

  1. frozen water; a sheet or stretch of ice
  2. a substance resembling ice; especially : the solid state of a substance usually found as a gas or liquid <ammonia ice in the rings of Saturn
  3.  a state of coldness (as from formality or reserve)
  4. a frozen dessert containing a flavoring (as fruit juice); especially : one containing no milk or cream; British : a serving of ice cream
  5. slang : diamonds; broadly : jewelry
  6. an undercover premium paid to a theater employee for choice theater tickets
  7. methamphetamine in the form of crystals of its hydrochloride salt C10H15N‧HCI when used illicitly for smoking —called also crystal, crystal meth

For these purposes, we’ll stick with the first two definitions. ‘Frozen water’ and ‘the solid state of a substance usually found as gas or liquid’.

Burning, more properly called a combustion reaction, is a little more complicated. There’s an entire subfield of chemistry called thermochemistry that deals with burning (or, more properly, the energy release from a combustion reaction). In general, though, you need a compound to combust and an oxidant to react with the combusting compound, and some energy to get it started. The oxidant and the combusting compound then combine in a chemical reaction to produce one or more new compounds, and since the reaction is exothermic the process of making the new compound(s) generates more energy than it gives off.

Yes, that does mean that once you get a combustion reaction started it will continue as long as it has combustable compounds and oxidants. That’s why fire spreads.

So, can ice burn?

Water does not burn particularly well, because it’s already a product of a combustion reaction. Burning just about anything with hydrogen produces water. Burning hydrocarbon (sugar, wood, meat, alcohol, whatever) produces carbon dioxide and water and heat, burning hydrogen generates water and heat, burning acetylene gas forms carbon dioxide and water, and so on. Because it is the product of a combustion reaction, water is already at a low energy. You’d have to add energy (breaking the chemical bonds between the hydrogen and oxygen) to burn it, and then you’re not actually burning the water. You’re burning the hydrogen gas you released from the water.

Other ices can burn, however. Ethanol (a hydrocarbon) burns, and it freezes at -114 degrees Celsius (-173.2 degrees Fahrenheit). Gasoline (another hydrocarbon) certainly burns, and will freeze between -40 degrees and -60 degrees Celsius (-40 to -76 degrees Fahrenheit), depending on the exact properties of the substances in the gasoline. Acetone (nail polish remover) is another hydrocarbon, which freezes at -95 degrees Celsius (-139 degrees Fahrenheit) and which burns. Other liquids that burn also exist, obviously. And most of them require dangerously cold temperatures to freeze, and then will have fire. Use caution, lots of caution, if you actually plan to try this at home. And then, having exercised caution, you probably shouldn’t try this at home.

But, I really want burning water ice. For reasons.

Well, there’s a couple of different things you can do. The first is to put a layer of (water) ice cubes on top of a layer of calcium carbide. As the ice melts, the water reacts with the calcium carbide to produce hydrogen and acetylene gas, both of which will burn – old fashioned mining lamps actually used this reaction (something I learned when I went spelunking as a Boy Scout). The results look like this:

Another option is to pour alcohol on top of your ice, and light it up. The ice won’t burn, but there will be flames on the ice.

Regardless of which one of these you do, if you do one of them, please exercise caution. Lots of caution, because you’re playing with fire.

But… does the ice melt?

Yes. Because there is something hot near the ice, which will cause it to melt.

Why doesn’t it glow for days and days?

It’s Halloween, and like most parents here in the US I had my son out for trick-or-treating. One of the things he got was a couple of glow sticks, which he insisted on taking to bed with him. That’s when he found the one he got a few days ago, and with some alarm he told us it wasn’t glowing. We told him that they stop glowing after a day, and that’s when he asked his question.

Honestly, I don’t know. Beyond it being a chemical reaction of some sort. So, let’s find out!

The construction of a glow stick is simple enough:  there’s a tube containing one chemical inside a larger tube that contains a different chemical.  The inner tube is fragile, the outer flexible, and when you break the inner tube to mix the chemicals the glow happens. But why?

Inside the larger tube is actually two chemicals. One is a fluorophore, which is a chemical that emits light when excited. The other is diphenyl oxilate, a chemical that released enough energy to excite the fluorophore when it oxidizes. And the chemical inside the brittle tube is hydrogen peroxide. So when you break the inner tube, the hydrogen peroxide oxidizes the diphenyl oxilate, causing the fluorophore to become excited and glow.

The color of the glow depends on the exact fluorophore used, and both the brightness and duration depend on the ratio of hydrogen peroxide to diphenyl oxilate- the brighter the glow, the shorter the duration. And that thing about making it last longer by putting it in the freezer does work!  Heat speeds the reaction up, and cold slows it down.

So why doesn’t it last for days and days?  Because there’s a limited amount of diphenyl oxilate to oxidize, and when it’s finished the fluorophore stops getting excited.

Happy Halloween’