Minerals: Distinguishing Physical Characteristics of Minerals (page 2)

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Author: Janice VanCleave

Try New Approaches

Would the size of a mineral sample affect its specific gravity measurement? Repeat the experiment two times. First, use a larger piece of quartz; second, use a smaller sample of the mineral. Science Fair Hint: Use the mineral samples and their calculated specific gravities as part of a project display.

Design Your Own Experiment

  1. Cleavage is the property of a mineral when it breaks along a flat surface called a cleavage plane. Minerals that break easily and cleanly are described as perfect. Less-clean breaks are described as distinct, indistinct, or none. Minerals can have cleavage in many different directions. Cleavage in one direction is called basal cleavage. Muscovite, the most common form of mica, is a mineral that has perfect basal cleavage. Examine a piece of muscovite. You should be able to peel the layers off with your fingers.
  2. Remove a thin layer of mica and break it in half. Observe the broken edges. This is a fracture (an irregular break, one that is not along a cleavage plane). Fracture surfaces are described as uneven, conchoidal (curved), hackly (jagged), or splintery (small, thin, and sharp or fibrous). Use a rock and mineral handbook to identify the type of fracture that muscovite has.

    Collect and display minerals having different types of cleavage and fractures. Use the handbook to find out more about how minerals break and to discover the identity of the cleavage and fracture type for each mineral specimen.

    1. The hardness of a mineral is its resistance to being scratched. The scale of hardness from 1 to 10 was devised in 1822 by Frederick Mohs (1773–1839), a German chemist. He arranged 10 common minerals from the softest to the hardest, giving the softest mineral, talc, the number 1, and the hardest mineral, diamond, the number 10. Minerals with higher Mohs' numbers will scratch those with lower numbers. Determine which has a higher Mohs' number, muscovite or quartz. First try to scratch the quartz with the muscovite. Then, try to scratch the muscovite with the quartz. Find the list of minerals for Mohs' scale of hard ness in a rock and mineral handbook. Prepare a display showing specimens and/or pictures of the minerals for each Mohs' number.
    2. Minerals: Distinguishing Physical Characteristics of Minerals

    3. Everyday objects such as the following can be used to represent minerals' hardness:
    4. fingernail 21/2
      copper coin (penny) 31/2
      paperclip 41/2
      pocket knife blade 51/2
      glass 6
      sandpaper 7
      steel file 71/2

      Make and display a chart of the Mohs' scale, similar to the one shown in Figure 11.2.

  4. The streak of a mineral is the color of the powder left when the mineral is rubbed against a rough surface that is harder than the mineral. An unglazed porcelain tile, called a streak plate, has a hardness of about 7 and is used to determine the streak of minerals with a hardness less than 7. Determine the streak of hematite by trying to scratch the plate with the edge of the hematite. Rub your finger over the powder left on the plate by the hematite and determine its streak color. Note that a mineral's color is not always its streak color.
  5. All minerals have the physical characteristics of specific gravity, crystalline structure, cleavage, fracture, hardness, and streak. Use the previous tests to determine these characteristics of minerals you wish to display. (See Chapter 12 for more information about crystalline structure.) For hints on how to label and display your minerals, see Janice VanCleave's Rocks and Minerals (New York: Wiley, 1996), pp. 80–83.

Get the Facts

Other than the previously listed physical characteristics, minerals have characteristics such as luster (the way a mineral reflects light) and color, and some have magnetism and fluorescence. A rock and mineral handbook is a good source to find out more about these characteristics. What determines a mineral's color or luster? What elements must be present in a mineral for it to have magnetic properties? What is phosphorescence?

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