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Geosciences LibreTexts

7: Matter and Minerals

  • Page ID
    5566
  • Learning Objectives

    After completing this chapter, you should be able to:

    • Know the definition of a mineral.
    • Understand the many different physical properties of minerals, and how to apply them to mineral identification.
    • Be able to distinguish mineral cleavage from mineral fracture.
    • Identify 18 minerals.

    • 7.1: Introduction
      Have you used a mineral yet today? While many people may initially say no, answer these questions: Have you brushed your teeth? Have you eaten anything that might contain salt? Did you put on make-up this morning, or do you have painted fingernails or toenails? If you have done any of those things, you have used at least one mineral, and in many cases, you have used a great number of minerals. Minerals are very useful and common in everyday products, but most people do not even realize it.
    • 7.2: Physical Properties
      Identifying a mineral is a little like playing detective. Minerals are identified by their physical properties. For example, look at Figure 7.2. How would you describe it? You may say that it is shiny, gold, and has a particular shape. Each of these descriptions is actually a physical property (shiny=luster, gold=color, shape=crystal form). Physical properties can vary within the same minerals, so caution should be applied.
    • 7.3: Lab Exercise (Part A)
      Your HOL Lab Kit contains 18 numbered mineral samples, separated into 3 bags (labeled as Mineral Bag 1, 2, or 3). Use these instructions to test and identify them. You will test for different properties after learning about them, then work on identification at the end of the lab. The HOL kit has been specifically tailored to this class – make sure that you are using the kit required by this class, as other rock and mineral kits will not work.
    • 7.4: Crystal Form
      This property refers to the geometric shape that a crystal naturally grows into, and is a reflection of the orderly internal arrangement of atoms within the mineral. If minerals have space to grow when they are developing, they will display their crystal form. These ideal growth conditions do not always occur, however, so many minerals do not display their ideal crystal form due to crowded conditions during growth.
    • 7.5: Cleavage
      As minerals are broken (such as with a rock hammer, for example), some may cleave, or break, along smooth flat planes known as cleavage. These flat surfaces are parallel to directions of weakness within the crystal. All the bonds among the atoms within a mineral may not be of the same strength so that when a mineral is broken, it breaks along these zones of weakness. This results in flat cleavage planes.
    • 7.6: Fracture
      When minerals do not break along cleavage planes, but rather break irregularly, they are said to fracture. Commonly fracture surfaces are either uneven or conchoidal, a ribbed, smoothly curved surface similar to broken glass.
    • 7.7: Lab Exercise (Part B)
      Take out Minerals Bag 2 and lay the six mineral samples out on a white sheet of paper. We will first examine cleavage and fracture, along with hardness, from these six samples, and will answer more questions about them later in the lab. Look closely at each of the minerals, using the hand lens to observe them. In this bag, you have the following minerals (not listed in order): Pyroxene, Muscovite Mica, Halite, Amphibole, Calcite, and Biotite Mica. They are numbered 7-12.
    • 7.8: Luster
      Luster refers to the appearance of the reflection of light from a mineral’s surface. It is generally broken into two main types: metallic and non-metallic. Minerals with a metallic luster have the color of metal, like silver, gold, copper, or brass. While minerals with a metallic luster are often shiny, not all shiny minerals are metallic. Make sure you look for the color of metal, rather than for just a shine.
    • 7.9: Streak
      Streak is an easily detectable physical property. It refers to the color left behind on an unglazed piece of porcelain when a mineral is rubbed along its surface. A streak plate is included in your rock and mineral kit to test this property. Often a mineral will have a streak of a different color than the color of the mineral (for example, pyrite has a dark gray streak).
    • 7.10: Special Physical Properties
      Several minerals have unique properties that aid in their identification. Tenacity refers to the way a mineral resists breakage. If a mineral shatters like glass, it is said to be brittle (like quartz), while minerals that can be hammered are malleable. Minerals may be elastic, in which they are flexible and bend like a plastic comb, but return to their original shape. Sectile minerals are soft like wax and can be separated with a knife (like gypsum).
    • 7.11: Lab Exercise (Part C)
      Take out Minerals Bag 3 and lay the six mineral samples out on a white sheet of paper. It should appear like Figure 7.21. We will first examine several properties, including streak, from these six samples, and will answer more questions about them later in the lab. Look closely at each of the minerals, using the hand lens to observe them. In this bag, you have the following minerals (not listed in order): Magnetite, Graphite, Copper, Sulfur, Hematite, and Pyrite. They are numbered 13-18.
    • 7.12: Student Responses
      The following is a summary of the questions in this lab for ease in submitting answers online.

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