Minerals. One of these things is not like the other…

photo of chrysacolla and ice cube
(Left) Chrysacolla, a mineral formed in the oxidation zone of copper deposits. (Right) Ice, taken from my freezer.

Or it is? In keeping with divulging science facts, I wanted to tell you how many minerals there are out there. And to examine what a mineral actually is. Are both of the items pictured above minerals? How are they alike? How are they different? Let’s explore:

Wikipedia defines a mineral as “a naturally occurring solid formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties.” In looking through other definitions of “mineral” (and there are many), this one seems fairly comprehensive, especially because it contains the “naturally occurring” and “formed by geological processes” components. A mineral must also have a crystalline structure—the orderly geometric spatial arrangement of atoms. A former geology major explained to me that ice, therefore, is a mineral when it occurs in nature, but is not a mineral when it’s made in one’s freezer. Hmm, I’m thinking we’ve all seen freezers where it seems geological processes could indeed be going on, but guess the basic idea can hold. So while the two items above both have a crystalline structure, and as we’ll see in a bit, some similar physical properties, the chrysacolla is a mineral, but the ice cube is not.

The International Mineralogical Association is responsible for approving and naming new mineral species. Wow. Really cool! I want to get to name a mineral, but think of the pressure… OK, OK. According to the IMA, there are over 4000 known minerals. One web site I found listed 4714 different species, of which 4349 are IMA-recognized. Of the over 4000 known minerals, only about 100 occur commonly, 50 are “occasional”, and the rest are apparently “rare” or “extremely rare”.

Now, put one or more minerals together in an aggregate, and you get a rock. The mineral calcite is a primary component of the rock limestone, for example. So common every day rocks can offer great opportunities for studying minerals. If you don’t have anywhere to go out and dig, the web is not all that bad a place for exploring minerals. There are cool alphabetical lists of minerals, some with pictures and accompanying geeky info. Here’s ice, for example.

One could spend a really long time perusing these lists, or a handy dandy mineral reference book like Eyewitness Handbooks Rocks and Minerals, which I acquired as a previously used copy last summer but have really not given the field time it deserves. These guides will help you understand minerals based on their properties—things like hardness, luster, color, streak, cleavage, and fracture. Hardness, for instance, is rated on a scale from 1 to 10, with a diamond being a 10 (the hardest). The chrysacolla shown above has a hardness of about 2.6, and ice is similar, with a hardness of 2.5. Apparently a mineral’s hardness has a lot to do with its suitability as a gemstone…corundum (which occurs as either red—a ruby, or blue—a sapphire) has a hardness of 9, which is twice as hard as topaz (8) but only ¼ as hard as a diamond. Feeling up to one more fact for today? That hardness scale was developed in 1812 and is courtesy of the German mineralogist Friedrich Mohs. I checked to see if Mohs also had any sort of mineral named after him, but couldn’t find one. Having your name attached to *all* minerals though, that just might be sufficient enough.


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