Breaking Down Rock
OverviewThe first process of rendering an ore or rock to its composite materials is crushing and separating. Since rocks and ores are made of many composite materials, it is important to break the rock down so the pieces can be mechanically separated. Once a rock is crushed it can be sieved and examined. Using a magnet, iron can be removed.
Preparing Materials
Students can crush ores using a “stamping mill.” The best types of rocks for this exploration are described below.
• A large galvanized cap and short piece of PVC pipe can serve as the anvil. A smaller pipe and two end caps forms the pounder.
Alternatively, the rock sample can be placed on concrete and pounded with a hammer. The rock to be crushed can be covered with heavy material such as the leg of an old pair of jeans to control the scattering of rocks and chips. Although these methods are safe, be sure to wear eye protection with either method.
• A series of screens can be used to separate the various sized materials in the mix. A canning jar with a ring lid works well with various screens made of gauze or window screen. Again, wear eye protection.
• A magnet in a plastic bag can be used to attract iron particles from the crushed rock
Exploration
Choose a piece of granite, gneiss, or gabbro for this exploration. These are igneous rocks that have several mineral ingredients and work well to crush and to separate minerals. If you cannot find these exact rocks, find any rock that has more than one mineral and that can be crushed without extraordinary effort.
Granite is an igneous rock found commonly in mountainous areas. The black crystals are mica or biotite, the clear or milky ones are quartz, and the colored ones are typically feldspar. Depending on the kind of granite, there can be different amounts of each of these minerals.
Gneiss has the same components as granite but it appears in bands. When granite is under a great deal of pressure over time, the minerals can line up and form this metamorphic rock.
Gabbro may look similar to granite or gneiss but typically has darker minerals like feldspar, mica, and very small amounts of quartz. It is the most common kind of rock as it makes up much of the earth's crust. Gabbro forms when magma cools slowly and crystals form. Basalt forms when magma cools quickly.
• Take a small piece of rock and crush it in the mill.
• The crushed rock should be screened or sieved so that the material has the same "grade" or size. Use a sieve jar with a fine screen to shake the material onto a white piece of paper.
The sample of crushed rock can then be separated into different piles of similar minerals using a small paint brush to sweep grains together. By measuring the mass of each pile or the relative size of the pile (See Process Hint), the ratios of amount of each mineral can be computed.
• Use a hand lens or magnifier to examine the crushed rock samples. Describe the crystals. Use the STEM Notebook to record findings. Draw approximate sample sizes and write about the nature of the materials using descriptive words of color, size and texture.
• Take a strong magnet and place it in a plastic baggie. Run the plastic baggie through the crushed rock and look for any surprising results. The plastic barrier is important because the iron filings are difficult to remove from a magnet. Removing the magnet from inside the bag allows the iron particles to fall away easily.
Alternatively, leave the crushed sample on white paper and place a magnet under the paper. By placing the magnet under the paper, an interesting demonstration of a magnetic field can be shown.
Use the STEM Notebook to record the results of using the magnet. The quantity of nature of iron can be described. A drawing of the magnetic field can be included.
