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NREC Online Lesson - Rocks and Minerals in Nova Scotia

Main Lesson

Looking At Rocks and Minerals!

Different properties of minerals allow us to separate them or classify them into categories. With this series of tests we can determine exactly what the mineral is much like a doctor would diagnose an illness. When we are sick, a doctor an not “unzip” us to see what is wrong, so he or she must make observations about other things (temperature, how we feel, etc.), in order to make a diagnosis. We also know that the presence of one mineral often indicates the presence of another, for example, gold is often found in certain types of quartz and rust often indicates the presence of iron or other metallic minerals. If a Geologist locates fossils, we can often find coal.

When Geologists are out in the field looking for samples they need to have ways to narrow down what the mineral is that they are looking at. For example, fools gold may look like real gold but a Geologist knows it is not valuable because of several distinguishing features. If possible give the students several samples of rocks and ask them to sort them into groups with similar characteristics. After a few minutes discuss how the students sorted the samples. If samples are not available, simply ask the students how they could sort out a bunch of rocks if you gave them a huge pile.

The most common categories are:

  1. Color - Solid or streaked
    Soft colors (brown, pale, yellow) indicate gypsum
  2. Lustre - Is the way in which the surface of a mineral reflects light. Those minerals that look like metal are said to have a metallic lustre. Non-metallic lustres can be described as glassy, like Quartz, brilliant like gems or earthy as in dull or browns.
  3. Streak - Is the color of a mineral in its’ powdered form. To determine this, geologists scratch a piece of unglazed porcelain called a “streak plate” with the minerals. One drawback of this technique is that it usually does not help in identifying light colored or very hard minerals.
  4. Size and Density - Is the sample heavy for it’s size? (if the sample is heavy, but small, then it is said to be more dense then a sample that is larger but lighter). Are there crystals formed within the sample?
  5. Hardness - Is the most commonly used property of minerals used in identification. Hardness is expressed as a value of 1 (softest) to 10 (hardest). This scale, known as the Mohs Scale, is used to determine the hardness of various minerals compared to others. For example, a knife made of steel is 5.5 on the scale, a copper coin is 3 and a fingernail is 2.5. Hardness generally reflects the minerals structure rather than composition. For example, carbon forms one of the softest minerals (graphite with a hardness of 1) as well as diamond (with a hardness of 10), depending on the arrangement of its’ atoms.
  6. Magnetic - Can be tested using your compass by passing the sample near it. If the sample contains metal this will cause the compass needle to move. There is an instrument that Geologists use that resembles a pen that tests to see if the sample contains metal. It is called a pen magnet.
  7. Taste - Obviously students should not go around licking all of the rocks or samples that they find. However, a Geologist would be able to determine if a sample was salt or contained salt immediately by it’s taste.