Why bother with minerals?
Here are the two main reasons I like to know about them:
- Few features help us identify rocks better than knowing which minerals compose them
- Minerals often form beautiful crystals such as the quartz crystal at the right, and it's fun to know what kinds of crystals we find (they're often visible in average sand and as parts of rocks)
What are minerals?
|DIFFERENCES BETWEEN ROCKS & MINERALS
Minerals have fairly definite chemical formulas and in their natural state usually occur in much smaller quantities than "rock size." Rocks are usually composed of one or more minerals in varying proportions. Unless a rock is composed of a single mineral, typically you wouldn't think of it as having a definite chemical formula.
Minerals are, according to my old mineralogy book, naturally occurring, homogeneous solids, inorganically formed, with a definite chemical composition and an ordered atomic arrangement. Another definition I've seen is that minerals are stony mixtures of one or more of the 92 more-or-less stable elements. When I read these definitions, I get the impression that it's hard to define what a mineral is in a way that average people can understand it!
The way I think of a mineral is that if you look at the Periodic Chart on our Periodic Chart page, which lists all known elements, a few of those elements, such as gold and silver, form minerals to begin with, but the others combine with one another to form minerals, which are "stony" when you hold them in your hand.
As with plants and animals, the world of minerals can be divided into neat categories and subcategories. Among plants and animals, the various divisions are based on genetic relationships. In the mineral world, categories and subcategories are based on chemical composition, as indicated in the chart at the right. . Because of this, two closely related minerals may share many features of color, hardness, how they combine with other chemicals, etc. Therefore, as among plants and animals, when we know a little about a mineral category, then automatically we know a little about each mineral found in that category.
What can we do with minerals?
THE 8 GREAT CLASSES OF MINERALS
(mineral example names in red, formulas in blue)
1. Native Elements
metals (gold Au, copper Cu, etc.)
semi-metals (arsenic As, tellurium Te, etc.)
non-metals (sulfur S, diamond C, etc.)
galena PbS, pyrite FeS2, etc.
3. Oxides & Hydroxides
hematite Fe2O3, corundum Al2O3, etc.
halite (salt) NaCl, fluorite CaF2, etc.
5. Carbonates, Nitrates & Borates
calcite CaCO3, dolomite CaMg(CO3)2, etc.
6. Sulfates, Chromates, Molybdates
gypsum CaSO4·2H2O, anhydrite CaSO4, etc.
7. Phosphates, arsenates, Vanadates
turquoise CuAl6(PO4)4(OH)8·4H2O, etc.
quartz SiO2, opal SiO2·nH2O, etc.
If we just deal with backyard minerals, most of us won't be able to do much with them. However, most of us do have places in the general neighborhood where rocks, gravel or sand are exposed, and most of us travel sometimes, so if that's the case with you, there's plenty you can do.
The first thing to do (just as with plants and animals) is to acquire a field guide -- a portable book that will help you identify your minerals and tell you something about them. Also very much as with studies of plants and animals, we almost have to have a good magnifying glass, especially the kind known as a hand lens -- such as the one pictured on our Magnifying Glasses page. Minerals can be identified by such traits as the kinds of crystals they produce, their color (gold is gold colored, silver is silver colored), hardness, weight, luster, the manner by which they break, crack or bend, whether they fizz when acid is applied, and many other features.
Once you're equipped with these, then it's hard to resist collecting the minerals you identify. You've probably seen how beautiful such collections can be. Part of Australia's Albert Chapman Mineral Collection is accessible online, as is part of the Smithsonian Gem & Mineral Collection.
One of the most fascinating parts of mineralogy is that dealing with crystals -- crystallography. You might enjoy taking a look at Mike & Darcy Howard's Introduction to Crystallography and Mineral Crystal Systems. Once you learn to identify the most common crystals -- like the difference between quartz and calcite -- your efforts to identify rocks will become much easier. And nothing is more pretty and interesting than a collection of fine crystals.
|WHAT ARE CRYSTALS?
Crystals are smooth-faced, angular forms assumed by minerals when they solidify from a molten state (as when volcanic lava cools), or separate from solutions in open spaces (as when salt crystals form as seawater evaporates). Crystals reflect the arrangement of atoms composing them.
By the way, the crystals at the top of this page are quartz crystals from Arkansas. Quartz is the most common of all minerals, and the main constituent of sand. The crystal is hexagonal -- trigonal trapezohedral, which means that if you were to cut across it, it would have six sides. A salt crystal, which also is clear and glassy looking, has -- as you know -- only four sides.
Article courtesy of Jim Conrad of Backyard Nature.