Mineral Crystal Shapes

Introduction

In this activity, paper crystals will be constructed to illustrate and enhance students’ knowledge of different crystal shapes. The constructed paper crystals will then be compared to actual mineral samples.

Concepts

• Unit cell
• Minerals
• Crystals
• Lattice

Materials

Safety Precautions

Wash hands thoroughly with soap and water after handling mineral samples. Follow all laboratory safety guidelines.

Disposal

Assembled paper crystal shapes, paper crystal worksheets and minerals may be saved for future use.

Prelab Preparation

Make as many copies of the unit cell lattice model worksheets on white or colored paper as needed. Cut out and assemble each unit cell lattice model using the patterns and assembly instructions provided.

Assembly Instructions for Crystal Lattice Models

1. Before cutting out the models, score the solid lines accurately with the back of a table knife or a ballpoint pen. Press firmly, but do not cut the paper. To do this, place over an index card on a hard surface and use a straight edge.
2. Color the models if desired. Use colored model paints or felt tip markers or colored pencils. The paints are bright and waterproof and stiffen the model. If colored paper is used, then coloring is unnecessary.
3. Cut neatly into all corners along the dotted lines to form tabs.
4. Accurately fold up along the scored lines.
5. Fasten the tabs with glue, tape or a good quality paper cement.
   1. Where two tabs come together at a corner, make an inside butt joint. This forms a rib edge inside the model as shown:
      {11943_Preparation_Figure_1_Butt joint}
   2. A single tab meeting an edge without a tab must be glued with an overlapping joint. The tab underlies the face, as shown:
      {11943_Preparation_Figure_2_Lapped joint}
6. The white copies of the lattice models will copy easily on any quality copier.

Procedure

1. Display each mineral sample alongside its corresponding paper cut-out models for each lattice to be illustrated.
2. Point out the angles, properties, similarities and differences among the different lattices.
3. Allow students to develop an appreciation for the differences among the crystal lattices and their usefulness in the identification of minerals. Avoid dwelling on the math or on the angles presented.

Teacher Tips

• This kit contains two samples of each mineral presented. The classroom may be divided to smaller groups in order to give the students a closer look at each mineral.
• Minerals may be labeled, or numbered with a piece of tape for future reference.
• If student involvement is desired, only assemble and display one of the lattices to the class. Copy the remaining cell lattice models and have students assemble the cut-outs for the other crystal systems. Hands-on construction will allow students to observe the properties of each lattice more closely.
• This activity may also be done as an open-ended type lab. Have students assemble the paper unit cells and match the correct unit cell to the mineral sample.
• Use the following table to further identify the mineral specimens.
  
  {11943_Tips_Table_1}

Discussion

Minerals crystallize in a series of distinct crystal lattices based upon the sizes of the atoms, molecules or ions present, their shapes, charges and the number of each present. These crystal lattices are formed by a regular repeated pattern of atoms which are connected together called unit cells. Unit cells are the basic repeated building blocks of minerals. In many materials, the arrangements of atoms are very random. Minerals, however, crystallize in a highly regular and ordered manner. Because of this ordered repetitive nature, minerals take on very distinct and interesting shapes. Individual crystal lattices can be classified using a relatively small number of basic unit cells. The following table shows the basic unit cell structures and how they are formed. A mineral example for each unit cell is also given.

References

Gross, G.; Colangelo, E.; Holzer, M. A Demo A Day™—A Year of Earth Science Demonstrations; Flinn Scientific Inc.: Batavia, IL, 2001; pp 106–113.