Every science – actually every discipline any person can study – has some fundamental basics that are absolutely important.
You can’t study language without knowing the difference between a noun and a verb (and how that works with adjectives and adverbs). You can’t study biology without knowing what a species is. And you can’t understand geology without knowing what the difference between a rock and a mineral is.
In teaching an introductory geology class, you might guess I spend quite a bit of time discussing the latter. I always think it should be obvious. But the only obvious thing is that it isn’t obvious.
Rocks and minerals are two different things. Most people realize that, but have no idea what that difference is. The source of the problem is that rocks and minerals are almost always considered together at the same time.
You can buy a book called “Rocks and Minerals,” that has pictures of rocks and pictures of minerals in it. But what these books don’t do well is actually explain what is different about rocks and minerals.
The result is that in peoples’ minds, they are the same thing. But they can’t be the same thing. Otherwise, why would there be two different words? So people start making up differences, and I groan when they show up on exams.
Rocks are bigger and harder than minerals. Minerals can't have iron or magnesium in them. Minerals may be gasses or liquids, but rocks are always solid. Rocks are rounded; minerals are all jaggedy.
These are just some examples that I’ve seen and heard. The creativity people show under these circumstances is amazing!
However, I assure you, rocks and minerals are different. It’s unfortunate that this difference isn’t properly emphasized in field guides and classes. I know it’s in textbooks, and it’s in field guides, too. But that doesn’t mean that it’s properly covered.
So here it is, so you know:
A mineral is a material that meets these four criteria:
- Naturally occurring
- Inorganic solid
- Characteristic crystalline structure
- Specific chemical composition (either unchanging, or variable within defined limits)
Minerals are what a person might call ‘crystals’ when their at a rock and mineral store. Like a single quartz crystal. You know, the one on the chain that you dangled over your crush’s photo in the yearbook to see how many children he was going to have. (You know you did that!)
These are just crystals. That form naturally.
Snow is a mineral. It’s natural. It’s solid. It’s got that characteristic crystalline structure. And it’s always made of H2O.
If it’s man-made, like these bismuth hopper crystals, it’s not a mineral, no matter how pretty it is!
All right then, Doctor Smartypants. What’s a rock?
A rock is an amalgam of lots of individual crystals of minerals. Little (or big) minerals are stuck together to make a rock.
The minerals can be stuck together because they crystallized at the same time from cooling molten rock – making igneous rocks like granite.
Or the minerals can be stuck together because they were glued together by cements like silica or calcium carbonate. This is how we get neat things like sandstone.
Intense heat and pressure can cause minerals to grow and change shape in metamorphic rocks.
The point is that a rock has more than one crystal of a mineral in it. The crystals can be different minerals (like in granite), or they can be the same mineral (like in quartzite). As long as there’s more than one crystal, it’s a rock.
If that fist-sized, heavy thing you’re about to lob at your instructor is all one crystal of one mineral, then it’s a mineral.
If that fist-sized, heavy thing you’re about to lob at your instructor is many crystals of one mineral, it’s a rock.
If that fist-sized, heavy thing you’re about to lob at your instructor is many crystals of more than one mineral, it’s also a rock.