Observational Skills in a Simple Crystal Growing Experiment
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.
- 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;
- Identifying and testing hypotheses about the various processes that control crystal growth;
- Describing how this experiment models at least two types of rock-forming processes and evaluate each model.
Context for Use
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
- Place one drop of one of the solutions on a slide that is placed on the stage of the stereo-microscope.
- Observe and record the growth of the crystals. Describe the crystallization process for each of the crystals. Comment on your observations.
- 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.
- Note the pattern of crystallization (e.g., where are the crystals forming within the drop?)
- Repeat this until you have done this for all the solutions provided.
Forms for Paper Models (Acrobat (PDF) 31kB Jun15 10)
Teaching Notes and Tips
- Make sure students do not mix the solutions. A separate dropper should be provided for each solution.
- Some solutions like the dichromate can stain (even skin). Make sure you inform students of any potential hazards.
- 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.
- 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.
- Slides should not be agitated when viewing. Agitation may cause some redissolution.
- 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
- 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.
- At least one assessment item (a wood model or a station with a microscope slide) is included on the appropriate lab practical.
- A student tutorial is provided on the course web-site for student self-assessment.
References and Resources
- Crystallography and Minerals Arranged by Crystal Form. Includes paper model forms and 3D-rotating models