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Building a Rock and Mineral Collection

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Author: Marc Rosner

Minerals are inorganic natural substances containing one or more elements.

Rocks are mixtures of minerals blended and bonded by ancient geological processes.

Sandstone is a sedimentary rock. Sedimentary rocks are formed from the buildup of sediments deposited on riverbeds and ocean floors, and often contain fossils. Marble is a metamorphic rock. Metamorphic rocks are altered by heat and pressure deep in Earth's crust, and often have light and dark bands. Obsidian is an igneous rock. Igneous rocks are produced when magma (molten rock beneath Earth's surface) reaches the surface through volcanoes, flows as lava, and cools. Obsidian, a natural form of glass, is smooth and shiny.

You can easily build a rock and mineral collection with samples gathered from your yard and neighborhood, and from trips to other places. You can also add specimens purchased in science stores and gift shops, many of which are available for a dollar or less. As you study your samples, you will fill in important information about each sample, much as a detective solves a mystery. After determining the physical properties of a sample, you will be able use to your field guide and try to identify it.

Materials

  • mineral and rock samples
  • pencil, paper, and metric ruler
  • mineral hardness kit (You can purchase a mineral hardness kit, or create your own kit with samples of the following minerals: talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum, diamond; or use a pin, a knife, and a piece of glass.)
  • white, unglazed piece of tile to use as a streak plate (The back of a spare ceramic bathroom or kitchen tile works well.)
  • safety goggles
  • hammer and anvil, or bench vise
  • electronic or pan balance, or postal scale
  • graduated cylinder wide enough for the sample to fit into (A small measuring cup or a rain gauge with milliliter markings can be used in place of a graduated cylinder.)
  • water
  • directional compass (optional)
  • ultraviolet light (optional)
  • rock and mineral field guide
  • local geology guide (from your bookstore or library)
  • clear plastic display box (optional)

Procedure

  1. Spread out your mineral and rock samples on a work surface.
  2. Use a pencil, paper, and ruler to create a table like the one shown (the first row is completed as an example).
  3. Begin by filling in the origin of your first sample—where you found it. Be as specific as possible about location and position.
  4. The first property to measure is mineral hardness. Test each sample using your mineral hardness kit and the following mineral hardness scale (Mohs' scale). According to Mobs' scale, any mineral can be scratched by a mineral of greater hardness.

Mohs'Scale

  1. talc (crushes easily between fingers, feels powdery)
  2. gypsum (can be scratched with a finger-nail)
  3. calcite (can be scratched with a pin)
  4. fluorite (can be scratched with a knife blade)
  5. apatite (hard to scratch with a knife)
  6. orthoclase (can be used to scratch a knife)
  7. quartz (will scratch glass)
  8. topaz
  9. corundum
  10. diamond

To determine where the mineral falls on the scale, test its hardness against the known samples in your kit. For instance, if a mineral can be scratched with a knife blade but not with a pin, record the hardness as 3.5, since it is between 3 and 4 on the hardness scale. A mineral can scratch any mineral with a lesser hardness value, and can be scratched by any mineral with a greater hardness value.

Building a Rock and Mineral Collection

Building a Rock and Mineral Collection

Magnetite will attract iron filings and move a compass needle. Franklinite glows eerily under UV light. Sulfur smells like rotten eggs.
  1. Next, determine the streak by rubbing the mineral against a streak plate to see what color it leaves behind. Note whether the streak is colorless, white, gray, or another color.
  2. Cleavage is the tendency of a mineral to break in the direction of its crystal system (see chapter 18, "Growing Crystals"). You can often determine the cleavage just by looking at the sample. Quartz is a hexagonal crystal, with six sides. Halite is cubic, with four sides. Mica breaks into thin sheets. If you have extra samples of a rock or mineral, you can put on your safety gog gles and break them with a hammer and anvil Cor crush it in a vise) to see how they split.
  3. The specific gravity can be found by calculating the sample's density (D, which is the ratio of mass (m) to volume (V). D = m/V). First, measure the mass of your sample in grams with an electronic balance. You can use a home office postal scale and multiply ounces by 28 to get grams. Next, fill a graduated cylinder halfway with water and note how much the liquid rises when you drop the rock in it. For instance, if the water rises from 5.0 mL to 7.2 mL, the volume of the rock is the difference, 2.2 mL. If you are using a measuring cup with ounces, you can convert ounces to milliliters by multiplying by 28. Finally, divide the mass by the volume. Use the number you get as the specific gravity—how much denser your rock is than water.
  4. Note any other special properties your sample has, using your senses and other equipment you might have at your disposal. Does it feel greasy? Does it have a distinct odor? Is your mineral magnetic? If so, it will affect the needle on a compass. Will it glow under ultra- violet (black) light? If so, it is phosphorescent. Phosphorescence is the process by which a substance absorbs electromagnetic radiation at one wavelength and then releases it over time at another, as glow-in-the-dark materials do.
  5. Use a good rock and mineral field guide and a local roadside geology book for the region where you live to identify the samples in your collection. You can purchase a clear plastic display box with compartments in hobby and craft stores, and label your collection for display. Add to your collection when you travel to distant places.

References

A Field Guide to Rocks and Minerals, 5th ed. (peterson Field Guides) by Frederick H. Pough and Roger Tory Peterson (Boston: Houghton Mifflin, 1998).

Rocks and Minerals (Eyewitness Books) by R. F. Symes (fopeka, Kans.: Econo-Clad Books, 1999).

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