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?”
What is a crystal?
To begin with, let’s hit Merriam-Webster up. They define ‘crystal‘ as:
- quartz that is transparent or nearly so and that is either colorless or only slightly tinged
- something resembling crystal in transparency and colorlessness
- 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
- a clear colorless glass of superior quality; also : objects or ware of such glass
- the glass or transparent plastic cover over a watch or clock dial
- a crystalline material used in electronics as a frequency-determining element or for rectification
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?
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.