Minerals have specific properties that allow us to identify them. In this activity students will split into small groups and become class "experts" regarding each of the three physical property tests covered in this lesson. Once the small groups have mastered each of the tests, the groups will split and reform using each "expert" to determine an unknown specimen.
Allow students to use common tests to determine different rock types.
Objectives: Students will be able to:
Explain to other students one of the following tests, hardness, acid or streak.
Identify an unknown rock or mineral specimen.
Explain the difference between a rock and a mineral in their own words.
Set up: 30 minutes
First round of tests: 30 minutes
Solving unknowns: 45 minutes
Extension: one week homework
Rock and mineral samples
Mohs hardness scale
PDF mineral ID chart
Mohs Hardness Scale
Rocks are made of one or more minerals. Minerals are pure, solid, inorganic (nonliving) materials found in Earth's crust. Minerals are made of one or more elements. Elements are the most basic, naturally occurring substances on Earth. Elements cannot be broken down (except by radioactive decay).
If you think of a cookie as a rock, the flour, sugar, and chocolate chips are like the minerals that make up the rock. Depending on the recipe, you get different kinds of cookies. It is the same with rocks because each type of rock has a different combination (or recipe) of minerals.
Minerals all have chemical compositions and physical properties unique to that specific mineral. (A chocolate chip in an oatmeal cookie is the same as the chocolate chip in a peanut butter cookie). Even rocks with the same mineral ingredients may be different due to variations in the amounts of minerals (more flour, fewer chocolate chips) and the processes by which they are formed such as being burned, doughy, or just right. Common rock-forming minerals are feldspar, quartz, calcite, mica, and hornblende.
All minerals have value, but their value varies. The more rare a mineral is, the more valuable it is. The same goes with mineral use. If a mineral is used for many of different things like copper, it becomes valuable. Some minerals are mined for their beautiful properties, such as diamonds. Some are so valuable they are used for jewelry or decorations, like gold and platinum.
We use different characteristics to identify people, such as eye color, hair color, language, etc. Geologists do the same thing, using specific properties to identify rocks and minerals. Geologists use the following tests to distinguish minerals and the rocks they make: hardness, color, streak, luster, cleavage and chemical reaction.
A scratch test developed by a German mineralogist Fredriech Mohs in 1822 is used to determine mineral hardness. He developed a hardness scale that helps to identify mineral properties. The scale measures hardness on a scale of 1-10. One being the softest mineral (talc) and 10 being the hardest mineral (diamond). Common objects of known hardness can be used to determine mineral hardness. These common objects are: your fingernail (2.5), a penny (3), a piece of glass (6) and a knife blade or nail. For example, if your fingernail can scratch the mineral, it has a hardness of less than 2.5, which is quite soft. If the mineral can scratch glass it has a hardness of greater than 6, which is very hard.
Color Color can sometimes be helpful when identifying minerals. However, some minerals have more than one color, like quartz. Quartz can be blue, brown, pink, red, purple, and almost any other color, or it can be totally colorless. Therefore, geologists have developed a better way of using color as an identifying property. This property is called a streak.
Streak Streak is the name given to the colored residue left by scratching a mineral across an abrasive surface, such as a tile of unglazed porcelain. The streak may not always be the same color you see in the hand specimen. A mineral with more than one color will always leave a certain color of streak. Hematite is a mineral that can be red, brown, or black, but it will always leave a characteristic reddish brown streak.
Another mineral property that geologists use to identify minerals is luster. Luster is the way in which the surface of a mineral reflects light. There are two main types of luster: metallic and nonmetallic. A metallic luster is shiny and similar to the reflection from a metal object, such as a faucet. A mineral that does not shine like metal has a nonmetallic luster. For example, the wall has a nonmetallic luster. There are many types of nonmetallic luster. A glassy luster is bright and reflects light like a piece of glass. A greasy luster has an oily appearance. An earthy luster is a very dull and looks like dirt. Waxy luster looks like the shininess of a crayon.
Cleavage Cleavage is another property used to distinguish minerals. Cleavage is the tendency for minerals to break along flat planar surfaces. Cleavage is rated as good, fair and poor depending on the quality of the flat surface produced. Mica, for example, is a mineral that has good cleavage. It breaks into very flat sheets. Minerals that have very poor cleavage will only break along irregular surfaces. Quartz, for example, will break into pieces that have a seashell-like fracture plane. Others, like garnet, shatter with no distinguishable pattern. These are considered to have no cleavage at all.
A weak acid is used to tell if rocks or minerals contain calcium carbonate (CaCO3). If the specimen fizzes (giving off CO2) when it comes in contact with acid, it is considered carbonate rich. If it does not contain calcium carbonate, it will not fizz. Calcite and aragonite are two minerals that will always fizz. Not all of these tests will be used in the following activity. Use different mineral properties to distinguish which minerals you are studying.
Explain to the class differences between a rock and a mineral. In this activity we examine some properties that allow us to identify what mineral we have.
Explain to students why it is important to identify minerals. We use minerals all the time in everyday life. For example, you probably have copper pipes or wiring in your house. You use chalk (calcite) to write on the blackboard. Maybe you're wearing a silver or gold ring.
Place all hand specimens into two piles, a rock pile and a mineral pile. Have students make two lists, one for distinguishing characteristics of rocks, and another for distinguishing characteristics of minerals.
With the two lists, focus on the specimen properties the students observe (i.e. color, weight, feel). Explain that there is more to each mineral than meets the eye and that we will learn how to perform tests that dig a little deeper into mineral identification.
Divide the class into three groups. Each group will learn how to perform one of the following tests: A - Streak test - using porcelain streak plates. Students predict and then test streak color of various minerals within the three large groups. Place a streak plate on a flat surface and rub an edge of the mineral on the plate until a color appears or it is determined that no color will appear. B - Hardness test - Use the provided hardness objects to determine the hardness of a mineral. Write down the hardness of each mineral, i.e. harder than fingernail / softer than a penny. C - Acid test - ALL STUDENTS HANDLING ACID SHOULD WEAR SAFETY GLASSES. Set out various samples of minerals. Have students perform the test on each mineral. After performing the acid test on a few minerals have students attempt to predict which of the remaining minerals will react with the acid. Have students note whether the mineral fizzes or bubbles. If it fizzes it contains calcium carbonate (CaCO3). If there is no reaction (no bubbles or fizzing) place another drop of acid and observe, if there is still no reaction, there is no calcium carbonate present.
Once each person in the group has performed the test multiple times on different minerals, divide the existing groups into groups with at least one student from the original groups in the new groups. Each student in the new smaller groups now teaches the new group what they learned in the larger groups. Each student is now considered an expert for their test in the new group. The groups should have an acid test expert, a streak test expert, and a hardness test expert. These three "experts" are to pool their knowledge and teach the other two students in their group how to perform their test. Together the new groups of three experts will identify an "unknown" specimen. Remind students that they can refer to the keys they made at the beginning.
Give each group the necessary resources to complete each of the three tests, or have several stations set up around the room where the groups can go to perform each test. Give each group one "unknown". As you pass out the "unknowns" keep track of which mineral is given to each group. Allow for each group to determine which tests should be performed on the specimen allowing them to perform enough tests until they have identified their unknown. Have the list of unknowns and groups ready, as groups come to you with an answer you can congratulate them on their detective skills, or send them back to work with a little more guidance.
Once all groups have identified their unknown, each group should prepare a small presentation for the class, identifying their specimen and explaining which tests they used to determine what they had, and explain what each test told them.
For younger students: Simply have each original group rotate through all three stations and practice each test, instead of solving the unknown. As homework you can assign each student to choose one of the minerals they tested and do a small research project, looking for ways that they use that mineral in their homes. You may want to encourage parents to take students to home hardware stores or repair stores to find the different minerals in the items that make their house.
For advanced students: Explain to students the properties of luster and cleavage. Have them use these properties in addition to the others presented in the above activity to identify minerals.
After the experts determine and report the findings to the class regarding their "unknown," assign each student or pair of students any mineral, one that we tested or one of their choice and research where the mineral comes from, how it is we extracted, how it is used, if it has value, and where it can be found (raw or refined) in our homes or surroundings. Have each student write a small report answering these (or more) questions, and present their findings to the class.