Name Class Date

Identification of Minerals Activity: Luster, Cleavage and Fracture

Background Information: Luster is the quality and intensity of the light that is reflected from the surface of a mineral. Luster is described as metallic or non-metallic. Non-metallic luster is more common and further described as glassy, or earthy (which appears silky or pearly). Cleavage is if a mineral tends to break along smooth or lustrous surfaces. Cleavage is described by the number of planes and the quality of the break. For example Mica cleaves with one perfect plane (like a stack of paper) and halite cleaves with three perfect planes (like a cube). If a sample does not cleave when it breaks, it is called fracture. Fracture is an irregular break pattern and it can be described as uneven, conchoidal (shell-like), splintery, or earthy (like clay).

Objectives: To observe and describe the luster and cleavage of various minerals.

To use the properties of luster and cleavage to classify and identify mineral samples.

Procedure:

  1. Collect mineral samples and spread them out on the lab table.
  2. Select one sample and describe, with details, its luster.Use a magnifying lens and comparison materials to help in your description.
  3. Observe the sample closely and decide if the sample exhibits cleavage or fracture.
  4. For cleavage samples: Observe the sample closely and count how many flat surface planes there are. Record the number of cleavage planes in the data table. (If there are no planes, record “zero” and check fracture.)
  5. Describe the quality of the cleavage planes (perfect, good, poor) in the data table.
  6. For fracture samples: observe the sample closely and describe the type of fracture (uneven, conchoidal, splintery, earthy, sandy)
  7. Answer the discussion questions.
  8. Use the Mineral Identification Key to try and determine the name of your samples.

Data:

Sample / Luster / Cleavage or Fracture / # of Planes / Quality of Planes / Type of Fracture
1 / Cleavage
Fracture
2 / Cleavage
Fracture
3 / Cleavage
Fracture
4 / Cleavage
Fracture
5 / Cleavage
Fracture
6 / Cleavage
Fracture
7 / Cleavage
Fracture
8 / Cleavage
Fracture
9 / Cleavage
Fracture
10 / Cleavage
Fracture
11 / Cleavage
Fracture
12 / Cleavage
Fracture

Discussion Questions:

  1. Was the luster of each sample easy to identify?Describe why the luster of some samples were easier, or more difficult, to determine.
  2. Based on the samples you studied, describe what you think the most common type(s) of luster are. Why do you believe these are the most common types of luster?
  3. What was the most difficult aspect of determining the cleavage, or fracture, of the samples?
  4. Based on the samples you studied, describe what you think the most common type(s) of cleavage or fracture are. Why do you believe these are the most common types of cleavage or facture?
  5. What are some factors that could alter the luster of a mineral sample?
  6. What are some factors that could alter cleavage or fracture of a mineral sample?
  7. Based on your response to question 5, do you consider luster a good tool for the identification of minerals? Why/Why not?
  8. Based on your response to question 6, do you consider cleavage/fracture a good tool for the identification of minerals? Why/Why not?