Geoscience in Two-year Colleges > Activities > Observational Skills in a Simple Crystal Growing Experiment

Observational Skills in a Simple Crystal Growing Experiment

This page and activity is authored and developed by Rich Gottfried,Frederick Community College.
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Summary

The purpose of this activity is to acquaint students with the basic geometry of crystals using an inquiry-based format that will include detailed observations and interpretations of those observations. Students grow selected crystals and observe their growth using a stereo-microscope. This allows for a more accurate determination of the geometry of the crystals grown. Students are then introduced to the geometric properties of each crystal system and use their observations to determine the system for each solution. Students may comment on the symmetry content of each solution observed if possible.

Learning Goals

Goals for this activity include:
  1. Recording of visual observations of various crystals and the use of the observed geometry to identify crystal systems. Comment on the symmetry content of each solution observed if possible;
  2. Identifying and testing hypotheses about the various processes that control crystal growth;
  3. Describing how this experiment models at least two types of rock-forming processes and evaluate each model.

Context for Use

This can be done as a class activity (or as a demonstration with appropriate projection equipment) or as a lab. A full three-hour time slot should be enough time to do any pre/post-lab discussion and have the students observe the crystals as they grow. I usually do this activity as the first lab in my Physical Geology classes. I have also used version of this lab in a sophomore-level physics class where the mathematics (tensors, linear transformation, etc) of this exercise were stressed.

The solutions should be made in advance (Ward's sells a kit for this, but home-prepared solutions work better). The only other materials needed are the stereo-microscopes and paper models (see web-links) or a set of wood models.

As this topic includes concepts from math, chemistry, physic sand geology, this activity can be used in a variety of settings. So this activity can be kept very basic for grades 7-12 or can be modified as appropriate for undergraduate classes to include higher-level concepts.

Description and Teaching Materials

Materials: Prepared saturated solutions, microscope slides and a several stereo-microscopes. Optional: Wood or paper crystal models.

Procedure:
  1. Place one drop of one of the solutions on a slide that is placed on the stage of the stereo-microscope.
  2. Observe and record the growth of the crystals. Describe the crystallization process for each of the crystals. Comment on your observations.
  3. Find single crystals that are oriented in a way that allows you to estimate the relative length and angles of their axes. Use these crystals to determine the crystal system.
  4. Note the pattern of crystallization (e.g., where are the crystals forming within the drop?)
  5. Repeat this until you have done this for all the solutions provided.
Teacher Handout/ Notes for Crystal Growing Lab (Microsoft Word 45kB Jun15 10)
Forms for Paper Models (Acrobat (PDF) 31kB Jun15 10)

Teaching Notes and Tips

Teacher Notes:
  1. Make sure students do not mix the solutions. A separate dropper should be provided for each solution.
  2. Some solutions like the dichromate can stain (even skin). Make sure you inform students of any potential hazards.
  3. It is best if all students prepare slides and observe all solutions. However, if scopes are limited, you can set up one slide per scope and let the students move around to each station.
  4. The crystals may not grow enough in the time allotted. In this case, you can leave them on the slide and view them in a subsequent class.
  5. Slides should not be agitated when viewing. Agitation may cause some redissolution.
  6. The time for the first crystals to appear may vary. Many factors determine the rate and nature of crystal growth. These factors include:
  • the temperature and pressure gradients of the system
  • the chemistry and structure of parent phase (solution, etc)
  • chemical and heat transport properties of the crystal
  • the lattice symmetry of the crystal
  • the presence of defects on the growing face
  • changing chemical gradients
  • density and viscosity of the solution
  • the area of the solid-solution interface
  • the thickness of the boundary layer

Assessment

  1. Students submit a lab summary of their results including the answers to the analysis questions and any other items assigned with the lab (e.g., web-related follow up on symmetry or other research question.
  2. At least one assessment item (a wood model or a station with a microscope slide) is included on the appropriate lab practical.
  3. A student tutorial is provided on the course web-site for student self-assessment.

References and Resources

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