Waco Gem and Mineral Club, Waco, Tx

1.1 Learning the Characteristics of Minerals

 

1.1: Learning the characteristics of minerals

Definition: A mineral is a solid chemical element or compound which:

occurs naturally

is inorganic (not a product of biological or life forces)

has a definite chemical composition

has an orderly atomic structure often expressed in a crystal form

For instance, graphite and diamonds (made of carbon) are minerals but coal (also made of carbon) is not because coal is an organic product that formed from fossil remains of plants, or once-living organisms.

Kids should learn to identify several common minerals using simple tests of physical properties such as color, streak, luster, crystal shape, cleavage, fracture, hardness, chemical reactivity, and weight, or specific gravity. To help them, you should direct them to a rock and mineral guidebook. Many good ones are available to purchase or to borrow from a library.

Some include:

Pellant‘s The Complete Book of Rocks and Minerals

Zim and Shaffer‘s Rocks and Minerals: A Golden Guide

Fuller‘s Pockets Rocks & Minerals

Simon & Schuster‘s Guide to Rocks and Minerals

Pough‘s Rocks and Minerals: Peterson Field Guide

Chesterman‘s National Audubon Society Field Guide to North American Rocks and Minerals

Roots, Willis, and Brett-Surman‘s The Nature Companion’s Rocks, Fossil and Dinosaurs

The official book for use in naming and labeling minerals entered into Federation-sponsored competitions is Fleischer’s Glossary of Mineral Species (from The Mineralogical Record).

Farndon‘s e.guides Rock and Mineral (2005) combines the traditional print content of a book with links to web sites offering interactive multimedia, games and quizzes, and downloadable images.

Finally, any number of web sites have sprung up to assist with mineral identification. One fun one is Yup…Rocks.

Another, offering links to all sorts of web sites and resources, is out of the Education Technology Center of Kennesaw State University. And one mineral identification web site which has become a standby is the Mineral Database.

Here are some basics of mineral identification:

Color can be the most striking aspect of a mineral, and some can be identified by color. For instance, malachite is always green, azurite is blue, realgar is red. But color alone is usually not enough. For example, quartz occurs in many colors, caused by minute impurities, and may be clear, cloudy (milky quartz), yellow-orange (citrine), purple (amethyst), pink (rose quartz), a sparkly green (aventurine), etc.

Streak is the color left when a mineral is scratched on an unglazed tile plate. This can surprise you in that it is sometimes very different from the mineral‘s outward color. For instance, gray hematite leaves a red streak and golden pyrite a green-black streak.

Luster is a reflective property of mineral surfaces. The way a mineral reflects light may make it look hard and shiny or dull or waxy. A mineral may be metallic (pyrite), vitreous or glassy (quartz), silky (gypsum), waxy (jade), resinous, pearly, earthy, etc.

Crystal shape is the characteristic appearance of a crystal, usually determined by the underlying atomic structure. Crystal shape may be cubic (pyrite or galena), octahedral (fluorite), rhombohedral (calcite), hexagonal (quartz), etc. For more on crystal shapes, see Crystal Shapes.

Cleavage is the tendency of some minerals to split or break along characteristic planes corresponding to directions of minimum cohesion. For instance, mica cleaves in thin sheets, a form known as basal cleavage. Other common forms of cleavage include rhombohedral (calcite), cubic (galena), and octahedral (fluorite).

Fracture is the manner in which a rock or mineral breaks if it doesn‘t exhibit cleavage. For instance, a break may be conchoidal (curved like a clam shell, as in breaks on obsidian), uneven (with a rough surface, e.g., lepidolite), or fibrous (splintery, e.g., ulexite).

Hardness is the resistance of a mineral to scratching. The Mohs‘ scale is a relative measure of this property, comparing the hardness of ten different minerals from softest to hardest: 1 – talc, 2 – gypsum, 3 – calcite, 4 – fluorite, 5 – apatite, 6 – feldspar, 7 – quartz, 8 – topaz, 9 – corundum, 10 – diamond. To arrive at approximate hardness, you can use some common tools: a fingernail is hardness 2.5, a penny is 3, a pocketknife blade or steel nail is 5, glass is 5.5, a steel file is 6.5.

Chemical reactivity. Some minerals will chemically react. For instance, a good test for carbonates (calcite, limestone, dolomite, etc.) is a drop of acetic acid, or vinegar. If it fizzes, it contains calcium.

Weight, or Specific Gravity. To determine the weight, or specific gravity, of a mineral requires special equipment. Specific gravity (SG) is technically defined as the density of a mineral compared to the density of water. The light mineral borax has SG 1.7, whereas the heavy mineral gold has SG 19.3. For most purposes, kids can just judge the relative weight, or heft, of a mineral, whether heavy, light, or in-between.

Darryl Powell (aka Diamond Dan) has prepared a wonderful variety of mineral identification resources you may wish to purchase to use with your club‘s kids in learning about minerals. These include The World of Minerals & Crystals (a coloring book introducing minerals from A to Z, with commentary on physical properties, forms, and uses in everyday life) and Earth Digger Clubs (a series of mineral-identification exercises in kits of one-hour activities, complete with patches as rewards for kids who complete an activity; kids learn about individual minerals such as calcite, pyrite, quartz, gypsum, or fluorite, as well as about properties of minerals such as hardness, color, crystal formation, etc.). These resources may be purchased from: Diamond Dan Publications, c/o Darryl Powell, phone (585)-278-3047.

For those who like to play games that also educate, check around for the Smithsonian Institution‘s What Do You Know About Rocks, Minerals, and Gems? Quiz Deck. It‘s a deck of cards, each with a colorful photograph and question on one side and answers on the back. It‘s published by Pomegranate Communications, Inc.

See the accompanying chart that provides you with a cheat-sheet of a wide variety of minerals and their various characteristics. In addition, we‘ve provided a blank chart you can copy and give to kids to fill in with different minerals they wish to test.

Rock Collection Forms