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Minerals: Distinguishing Physical Characteristics of Minerals

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Author: Janice VanCleave

The Earth's lithosphere consists of the crust and the upper mantle. The lithosphere is made up mostly of minerals, which are naturally occurring inorganic solids with a definite chemical composition and physical characteristics.

In this project, you will learn a method to determine the specific gravity of a mineral and discover how heft can be used as a method of mineral identification. The difference between cleavage and fracture will be demonstrated. You will use minerals and everyday objects to demonstrate the hardness of minerals. You will learn how to test minerals for their streak and luster. All of this will then be used to prepare a display of minerals that represents their characteristics.

Getting Started

Purpose:   To determine the specific gravity of a mineral.

Materials

  • 2-quart (2-liter) bowl
  • tap water
  • 24-inch (60-cm) piece of string
  • fist-size sample of quartz, or any any mineral of comparable size
  • metric spring scale

Procedure

  1. Fill the bowl about three-fourths full with water.
  2. Tie the string around the mineral and make a loop in the free end of the string.
  3. Place the loop on the scale hook and measure the mass of the mineral in grams (see Figure 11.1A). Record this as mass 1 (M1).
  4. Minerals: Distinguishing Physical Characteristics of Minerals

  5. With the mineral hanging from the scale, lower the mineral into the water in the bowl (see Figure 11.1B). Do not allow the mineral to rest on the bottom or sides of the bowl. Record this as mass 2 (M2).
  6. Determine the mass of the water displaced (pushed out of the way) by the mineral by calculating the absolute difference between M1 and M2. Record the answer as mass 3 (M3).
  7. Example:

      M3 = M1M2
            = 530 g – 330 g
            =200g
  8. Use the following example to determine the mineral's specific gravity (sp. gr.), the ratio of the mass of the mineral in air to the mass of the water displaced by the mineral:
  9. Example:

      sp. gr. = M1 ÷ M3
                 = 530 g ÷ 200 g
                 = 2.65

Results

A method for determining the specific gravity of a mineral is used. The specific gravity of the mineral in the example is 2.65.

Why?

Most of the elements in the Earth's crust occur as minerals. A mineral is a single solid element or compound found in the Earth and has four basic characteristics: (1) it occurs naturally; (2) it is inorganic (not made from living things); (3) it has a definite chemical composition, meaning that it contains the same elements or compounds in the same proportions; and (4) it has a crystalline structure (atoms are arranged so that they form a particular geometric shape).

One way to distinguish one mineral from another is by the specific gravities of the two minerals. To calculate the specific gravity of a mineral, divide the mass of the mineral by the mass of the water displaced by the mineral. Specific gravity tells how many times heavier the mineral is than water. The mineral in the example is 2.65 times as heavy as the same volume of water. Most minerals have a specific gravity from 2 to 5. Since every mineral has a certain specific gravity, this characteristic can be used as a clue to the identity of a mineral.

Specific gravity is not always calculated with a scale. Instead, the heft of minerals is often used in identification. Heft is a subjective measurement. You measure heft by picking up minerals of equal volume and comparing their weights. Gold and pyrite are minerals that look alike, but the specific gravity of gold is 19.3 and the specific gravity of pyrite is 5.0. Thus, even though heft is not an exact measurement, the heft of gold is easily determined to be greater than that of pyrite.

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