Here are the two main reasons I like to know about 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 in the Universe, a few of those elements, such as gold and silver, form minerals to begin with, but the other elements combine with one another to form minerals, which have specific features such as hardness, color, luster, specific gravity, etc. (More on that, below)
Below you see a sedimentary limestone rock coated with the rusty-red mineral known as cinnabar. It was found next to an abandoned mercury mine.
Rock coated with the reddish mineral known as cinnabar,
known chemically as mercury sulfide, or HgS
THE 8 GREAT CLASSES OF MINERALS
(mineral example names in red, formulas in blue)
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.
MINERALS HAVE DEFINITE CHARACTERISTICS
color (Mica comes in several colors)
streak -- color in powdered form, as when scratched across a hard, white plate (Mica's streak is white)
luster (Mica is pearly)
diaphaneity -- (Mica is transparent to translucent)
cleavage -- how it breaks, depends on crystal faces (Mica exfoliates into flat sheets)
hardness (Mica's hardness is 2 to 4)
specific gravity -- weight per unit volume (Mica's is 2.7 to 3)
crystal system -- (Mica's is monoclinic)
You can look up a mineral's properties at Geology.Com
The above sample of mica was found in southern Canada where the Canadian Shield outcrops. Mica is one of the easiest to identify of all minerals because it occurs in flat sheets, like fused-together sheets of cellophane or glass. Often thin sheets of mica are easy to separate from one another -- they easily exfoliate. Before glass became easy to manufacture, people who had access to mica deposits looked for big sheets of mica to serve as windowpanes in their houses.
In modern industry mica is very important because of its amazing insulation ability, both thermal and electrical. In other words, both heat and electricity have a hard time passing through it. Mica maintains its electrical insulation properties even at very high temperatures, which is unusual.
For this reason mica is used in delicate machinery where there's high voltage, such as turbo-generators and electrical motors.
Mica belongs to the silicate class of minerals, so it's composed mostly of SiO2 In nature, mica forms veins in rock, typically in association with other silicate minerals, particularly quartz and feldspar.
Several mica varieties are recognized but here are the two best-known ones:
India, China, the US, South Korea and Canada are home to the greatest deposits of mica.
The white part of the rock in my hand is quartz. The dark stains are "impurities" of other minerals. The white, massive quartz here has the same chemical formula as the clear quartz crystals at the top of this page -- SiO2. The difference is that the molecules of SiO2 are arranged differently in the two quartz forms. This occurred because the quartz crystals formed in one environment, and the white, massive quartz in my hand formed in another. Therefore, you can see that identifying minerals entails far more than knowing their chemical formulas. Also, it is clear that when we find minerals in nature often we can deduce from their appearance the geological environment in which they formed.
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.
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). Crystal structure reflect the arrangement of atoms composing them.
One of the most fascinating corners of the big field of mineralogy is that dealing with the study of crystals -- crystallography. Once you learn to identify the most common crystals -- can recognize the differences between quartz crystals and calcite crystals, for example -- your efforts to identify rocks will become much easier. That's because rocks are composed of minerals. Nothing is prettier and more interesting than a collection of fine crystals.You might enjoy taking a look at Mike & Darcy Howard's Introduction to Crystallography and Mineral Crystal Systems.
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.