Items Needed (Field Trip, or get some sand at the beach for classroom)
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Clear grains—glassy in texture
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Most likely quartz (near the ocean may be iron stained)
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Milky grains—variety of colors,
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Most likely feldspar
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pinkish, beige, white
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Flakes—not round grains
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Most likely micas (white mica—muyscovite, black mica— biotite)
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Dark grains
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Probably hornblende
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Other colors
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Depending on location, could be garnets, amethysts, etc.
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DIRECTIONS Spread a few grains of sand on a piece of paper. With the tip of your pencil, spread the individual grains into four piles.
60% quartz
30% feldspar
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8% hornblende
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= granite broken down into sand granules
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2% mica flakes
HOW TO TEST THE PHYSICAL PROPERTIES OF ROCKS AND MINERALS
Identifying rocks and minerals can be frustrating because many of them resemble others. It’s a little like solving a mystery story, because there are a lot of questions that have to be answered before you can narrow down the suspects.
One way to introduce rock identification is to ask students how their parents recognize them. They will say by their hair color, facial features, eye color, height, weight, etc. (maybe even their smell!)
It’s the same with rocks and minerals. Each has its own specific characteristics that make it what it is. In the following pages are tests geologists use to identify minerals. Physical properties such as crystal shape, hardness, color, magnetism, shine (or lustre), and cleavage (how a mineral splits or breaks), and streaking (what color mark it leaves), all help identify minerals.
Once students begin examining their rocks and minerals, they will soon see that a single test is rarely enough to identify a mineral. A geologist must look at many characteristics before making a decision.
COLOR IN ROCKS
We have all picked up a pretty rock and tucked it into our pocket. Rocks come in all kinds of beautiful colors, but color is the least dependable property when it comes to identification.
Some rocks and minerals are always one color—they are the easy ones to identify. For instance, pyrite and gold are always yellow. Basalt is always dark gray. However, some minerals and rocks come in different colors. An example is quartz, one of the most abundant minerals on earth. With just a trace of different chemicals, quartz, which in its pure form is clear like glass, can be found in just about every color in the spectrum.
Quartz can be purple (amethyst), pink (rose quartz), white (milky quartz), brown or black (smoky quartz), banded (agate), and many other hues and patterns. Yet each is still quartz! So color along is not a reliable way of determining what a mineral is. It is just a first step.
ACTIVITY Find rocks of all basic colors.
THE STREAK TEST
Items Needed: Rocks (various)
Did you know you do a “streak test” whenever you write on the blackboard with a piece of chalk? Using a pencil (graphite) on a piece of paper is also the same idea. Hopscotch on a sidewalk is another example of what a streak test accomplishes. The color of a powdered mineral may be different from the color of the mineral itself, so the streak is a more definite clue than color. To make a streak or to “powder” a mineral, use a
streak plate, which is a piece
of unglazed porcelain tile.
If the mineral is softer than the tile, it will rub off and leave a trail of powder. There may be some surprises in the streak color. Pyrite, which is called “Fool’s Gold” because of its resemblance to real gold, leaves a black streak on the streak plate. Gold leaves a yellow streak. A dark mineral may leave a streak that is much lighter. Biotite mica looks black but leaves a colorless streak. Chalcopyrite is gold but leaves a greenish-black streak. Try it with the children. What happens? Try another sample. Pull the rock or mineral specimen across the unglazed tile hard enough to leave a streak.
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Examples:
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MINERALS
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STREAK COLOR
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Calcite
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White
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Feldspar
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Colorless
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Galena
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Lead Gray
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Graphite
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Steel Gray
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Gypsum (when available)
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White
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Halite
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White
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Hematite
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Reddish-Brown
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Quartz
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C1ear
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Sulfur
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White
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Talc
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White
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HARDNESS ACTIVITY
Students can try to determine hardness of different rocks and minerals by scratching them with different objects—very soft rocks can be scratched with the fingernail.
A good assortment of rocks for this activity for a broad range of hardnesses are:
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Talc
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Gypsum—
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Calcite
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Feldspar
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Graphite
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(when available)
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Kaolinite
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Obsidian
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1. Calcite. It has a hardness of three, which is fairly soft. Begin by having the students try to scratch the calcite with their fingernail. It is too hard to scratch with a fingernail. Next try a penny. The penny will scratch it, so according to the Mohs Hardness Scale (see below chart), calcit as a hardness of three.
This hardness scale is arranged so that each mineral will scratch the minerals that have lower numbers and will be scratched by all the minerals having higher numbers. For instance, #4, Fluoride, will scratch Calcite, Gypsum, and Talc, but will BE scratched by Apatite, Feldspar, Quartz, Topaz, Corundum, and Diamond.
If you don’t have a copy of the Mohs scale with you, another way to check hardness is by using three common things: your fingernail, a copper penny, and a piece of steel.
¥ A FINGERNAIL SCRATCH HAS A HARDNESS SLIGHTLY OVER 2.0:
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All minerals that can be scratched by your fingernail are less than 2.5.
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¥ A PENNY SCRATCH HAS A HARDNESS OF ABOUT 3.5:
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A penny will scratch all minerals less than 3.5.
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¥ A PIECE OF STEEL SCRATCH HAS A HARDNESS OF ABOUT 5.5:
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A piece of steel will scratch all minerals that are less than 5.5.
Next, have the class try the quartz specimen. Quartz resembles calcite but is at the opposite end of the hardness scale. Again, begin with the fingernail test. Next, try the penny, then a nail. Next, try to scratch the nail with the quartz. The quartz should scratch the nail which means it has a hardness of 7. In other words, it is a pretty hard mineral. It will even scratch glass. Ask students to bring in small glass jars to see which minerals will scratch glass.
This is a good activity for “pairing and sharing” to see if two students agree on results.
VINEGAR TEST
Items Needed
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(1) Medicine eye dropper or small squeeze bottles
(2) Quartz, limestone, kaolinite, calcite, marble, or chalk
Geologists use weak hydrochloric acid to determine the presence of calcium carbonate, the prime ingredient of many sedimentary rocks. Vinegar will also react with those same rocks. It is included in the kit and has the advantage of safety! Here’s how to test for calcium carbonate.
Using medicine dropper or squeeze bottles, drop two or three drops of vinegar on rock or mineral sample. If it fizzes, it contains calcium carbonate.
A good group of rocks to use in this experiment is quartz, limestone, kaolinite, calcite, marble, and chalk.
Limestone is a sedimentary rock found where ancient seas and lakes once existed. It is made up of the shells and bones of long dead animals that once lived in those seas. Natural chalk is a form of limestone. Both will react strongly to vinegar.
Kaolinite looks a lot like chalk but has only a small amount of calcium carbonate in it. It may react only slightly or not at all with the vinegar.
Calcite is pure calcium carbonate and should react strongly to the vinegar test.
Milky quartz, which resembles calcite, will not react at all. It is not sedimentary, and even though it resembles calcite, has no calcium carbonate in it at all.
Marble was once limestone. It has been changed (and metamorphosed) under great heat and pressure. It may react less strongly to vinegar than limestone does.
The class can try this test with other rocks brought in to see if they react to the vinegar test.
