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Mineral Testing (advanced student lab/journal)

How can you identify the minerals in a piece of rock? Geologists examine a variety of characteristics to identify minerals. You can perform several simple tests in the classroom to identify the minerals in your rocks.

Perform the following tests, record your findings, and compare your results against the Common Mineral Identification Chart to identify your mineral. Record the results of all of your tests in your Global Lab Journal.

A First Test: Color

Often you can identify a mineral by its color, though not all have distinctive colors.

Materials

Rock guide book (optional)

Procedures

1 Each teammate should carefully observe the color of the mineral, ideally under sunlight.

2 The team should discuss the mineral's color and agree on a final color.

3 Teammates should record the color in their journals.

 

A Second Test: Streaking

Another way to identify a mineral is to examine the color of the streak when a mineral is rubbed across a flat surface. Sometimes the streak has a different color than the mineral itself. 

Materials

A porcelain tile (the backs of porcelain tiles offer rough surfaces on which to streak minerals, other similarly rough surfaces will work)

Procedures

1 Rub the mineral across the back of a porcelain tile.

2 Have teammates examine the color of the streak, ideally under sunlight.

3 If the mineral is harder than the tile and leaves no streak, carefully scratch the mineral with a nail.

4 The team should discuss the streak's color (or the scratch caused by the nail) and agree on a final color.

5 Teammates should record the color in their journals.

 

A Third Test: Hardness (Mohs Test)

Although most minerals may feel equally hard to our touch, their hardness actually varies. In 1812, a German mineralogist, Friedrich Mohs, developed a scale of hardness with which to classify minerals. The scale ranges from 1 to 10, with 1 being talc and 10 being diamond.

The table to the right lists standard minerals by Mohs' scale and common items that are their equivalent in hardness.

Be careful if your test requires a nail, knife blade, or steel file. These can easily slip off the mineral being tested and cause injury!

 

Materials

Copper coin

Iron nail

Glass tumbler

Penknife blade

Steel file

Sandpaper

 

Procedures

1 Scratch your mineral with your fingernail. If your fingernail scratches the mineral, the mineral has a hardness of 2.

2 Otherwise, scratch the mineral with a copper coin. If your mineral is scratched, it has a hardness of 3.

3 Otherwise, continue scratching your mineral with increasingly hard objects (use the objects listed in the table above) until one scratches it. The corresponding hardness number in the first column is the hardness of your mineral.

4 Teammates should record the hardness of their mineral in their journals.

 

Mohs’ Common Equivalent scale Mineral

1 Talc none

2 Gypsum fingernail

3 Calcite copper coin

4 Fluorite iron nail

5 Apatite glass

6 Orthoclase penknife blade

7 Quartz steel file

8 Topaz sandpaper

9 Corundum none

10 Diamond none

 

Identify Your Mineral

Refer to the Common Mineral Identification Chart (PDF). Identify the mineral whose color, hardness, and streak characteristics are most like your results. This is probably the name of your mineral.

Note: The Common Mineral Identification Chart does not list all minerals, so if you do not find a mineral in the chart with the same color, hardness, and streak characteristics of your mineral, it is possible that your mineral is not listed here. If you cannot find a match, ask your teacher for help. You might want to ask a local amateur or professional geologist to help you identify it.

 

An Additional Test: The Carbonate Test

You can test your mineral to see if it is a carbonate, an important group of minerals that contain carbon and oxygen. The chief carbonate mineral is calcite, from which limestone and marble are made. Limestone and related carbonate rocks comprise 13 to 21 percent of the sedimentary rocks exposed on the surface of the land. Carbonates are soft and often whitish in color. Chalk, for example, is a carbonate. A distinguishing characteristic of carbonates is the way they react to acid. An acid solution, if strong enough, can dissolve carbonates and weaker solutions make them bubble and fizz, because of the release of carbon dioxide as the carbonate dissolves.

You can test your mineral for carbonates by dropping some vinegar (a mild acid) on it. You should also examine the mineral with a magnifying glass to observe reactions that are too faint for the unaided eye. You may want to practice the procedure on a piece of blackboard chalk.

 

Yet ANOTHER Additional Test: Magnetism

Hold a small magnet next to your mineral. Is the mineral magnetic? Record

your finding in your journal.

 

Special Characteristics

Look for other special characteristics of your mineral. Is it glassy? Metallic?

Silky? Does it feel greasy? Soapy? How does light interact with your mineral?

Is it translucent? Transparent?

Record your observations in your journal.


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