Rock Cycle - Minerals (5A)
 Lab 

   
OBJECTIVES:
  • Identifying mineral characteristics.
  • Determining the hardness of minerals
VOCABULARY:
  • cleavage
  • fracture
  • characteristic
  • hardness
  • mineral
MATERIALS:

Students will test different minerals for their hardness.


copper is 3.0 on Mohs hardness scale

BACKGROUND:

The obvious descriptive characteristics of a mineral such as size, shape, and color are not the only features that can identify that mineral. Geologists recognize a number of useful key characteristics for mineral identification. These include:

  1. CRYSTAL FORM - The natural growth (shape) of a mineral.
  2. FRACTURE AND CLEAVAGE -The way a mineral breaks. Fracture is irregular breakage. For example, quartz has a conchoidal fracture; it breaks along hollowed and rounded, uneven surfaces. Cleavage is a regular breakage that follows the atomic structure of a mineral. Cleavage results in smooth, planar surfaces. Different minerals may have one, two, three, four, or six cleavages.
  3. HARDNESS - The mineral’s resistance to scratching. It is controlled by the strength of atomic bonds within the mineral. Mineral hardness is rated from 1 (soft) to 10 (hard) on the Mohs hardness scale.
  4. SPECIFIC GRAVITY - The density of a mineral relative to water.
  5. STREAK - The color of a powdered mineral sample.
  6. LUSTER - The way that a mineral reflects light. There are two types of luster. Metallic minerals look like shiny or rusted metal. Nonmetallic elements reflect light like glass or pearls or glue.
  7. TASTE - Certain minerals like halite (salty) and sulfur (bitter) have characteristic "flavors."
  8. MAGNETISM - The attraction of a mineral to a magnet.
  9. REACTION TO ACID - The mineral reacts by "fizzing" with dilute HCl reacts with carbonate minerals.

This lab will deal with "hardness" which is one of the easiest characteristics to test. Hardness helps geologists determine the identity of some minerals when they are doing field work. For instance, if a steel knife cannot scratch a white or clear mineral, it is likely quartz. Hardness also can tell us something about the composition of minerals (how tightly the elements are bonded together). Formally, the hardness of a mineral is ranked by Mohs Hardness Scale (named after Austrian mineralogist Friedrich Mohs), which lists 10 reference minerals that are arranged in increasing order of hardness. Note that this is a relative hardness scale; diamond is actually over four hundred times harder than talc.

Mohs Hardness Scale

1 = talc
2 = gypsum
3 = calcite
4 = fluorite
5 = apatite
6 = microcline (feldspar)
7 = quartz
8 = topaz
9 = corundum
10 = diamond
PROCEDURE:
  1. Before lab, set up mineral specimen collections for each student group. You will need samples of gypsum, olivine, mica, fluorite, calcite, apatite, hematite, talc, feldspar, and quartz. Use the Rock Cycle - Minerals (5A) samples or samples of your own. 
     
    Do not include rocks with this lab. Since most rocks are composed of a variety of minerals, they cannot be classified on the Mohs scale. If you have not tested mineral hardness using a penny or a nail, you may wish to experiment before the lab starts. This will also help you guide the students to use the right amount of strength and pressure when scratching the minerals.  You should limit it to one scratch or the specimens will not last long.  
      
  2. Explain the lab procedure from the student workbook. Let the students be geologists (or mineralogists) and create their own hardness scales using fingernails, pennies, and steel nails. You might want to demonstrate how hard it is to scratch the minerals. Instruct students to  "test" the mineral specimens. Sometimes samples can vary, so you should "discover" a key for the specific samples in advance.
      
  3. Tally the students' results on the board and determine a hardness scale for the whole class. Emphasize that their results may differ because some people scratch harder than others and many minerals are "in between" soft and hard.
      
  4. GENERAL ANSWER using Rock Cycle - Minerals (5A): softness to hardest: talc, gypsum, mica, calcite,  fluorite, apatite, hematite,  feldspar, olivine, quartz

  [Dictionary]  
[Back to Rock Cycle Grid]   [Back to Minerals (5)]