Classifying Crystals by Shape

© Kenneth Fuller 2005
Crystallography plays an important role in chemistry, and even more so in mineralogy.  Each chemical element, and each chemical compound has a particular shape to its crystals.  Since a mineral is a chemical element or compound, each mineral is characterized by the shape of its crystals.  So, the shape of the crystals in a rock are an important clue to identifying minerals which make up the rock, and in turn helps us to identify the kind of rock.

Crystals are classified into six classes by shape.  The classes of shapes are technically defined in terms of the relationships among the axes.  This may be a difficult concept for students to grasp (it took me a while).  The two dimensional diagrams in most books are not a great deal of help.  So, I provided my classes with three dimensional, visual and tactile manipulative models to assist them with a kinesthetic means of  conceptualizing the shapes.  Having students build the models achieved several collateral objectives as well.

When reading about the crystal classes, I point out to the students that, in the face centered forms an axis is a straight line connecting the centers of opposite faces.  In the body centered forms, an axis is a straight line connecting diagonally opposite corners.  Since axes are abstract, I redefine the classes in terms of the faces, which are concrete, and much easier to identify and understand.

Face centered forms:
face centered crystal shapes
Isometric (cubic) - All faces are square and the same size.
Tetragonal - All faces are rectangular, two are square.
Orthorhombic - All faces are rectangular, none are square.
Monoclinic (leans one way) - Four sides are rectangular, you can place it on the table so that its sides are vertical.
Triclinic (leans three ways) - No sides are rectangular, no matter how you place it, the sides are never vertical.
Hexagonal - Two faces are hexagons, six are rectangular.

Body centered forms:body centered crystal shapes
Isometric (cubic) - All faces are equilateral triangles.
Tetragonal - All faces are identical isosceles triangles, it has a square cross section.
Orthorhombic - All faces are isosceles triangles, but different shapes, it has a rectangular cross section.
Monoclinic - Four faces are scalene triangles, four are isoscelese triangles, it has a rectangular cross section.
Triclinic - All faces are scalene triangles, it has no rectangular
cross section.
Hexagonal - All 12 faces are isosceles triangles, it has a hexagonal cross section.


Primary objective: To provide a concrete, kinesthetic familiarity with the shapes used in the classification of crystals (minerals, compounds, elements).

Collateral Objectives:
Improve by practice and reinforcement, skills of;
    Reading and following directions.
    Cutting and pasting.
    Spatial perception, the visualization of 3 dimensional shapes from 2 dimensional diagrams.
    Understanding verbal (written) descriptions.

Provide an example of a scientific classification system (taxonomy).  Some books would have you believe that taxonomic skills and concepts are important only in biology.

Provide a boost to self esteem by successfully completing a model, and by understanding what the book means.

Provide samples of student work for display.

Teacher Preparation:
Print the patterns. (Probably best to print one copy and make class sets on a duplicating machine.  By using a photocopy machine the size of the patterns can be adjusted as desired.)
    Click on the link.
    Use the "print" button on your browser.  Each should print three pages with two patterns on each.
       [face centered patterns]         [body centered patterns]
    In the same way, print the charts for mounting the models.

        [face centered chart]            [body centered chart]
    Or better, I can e-mail  the Word document (12 pages) in the form I used in my classroom.
        [Crystal Patterns.doc]
    I sometimes duplicate each pattern on a different color paper, just to add visual interest.
    The heavier the paper the better, card stock is ideal, but expensive.

Construct one of each model, in order to anticipate student questions and problems.

Glue sticks will work, but I prefer white glue.  However, it does require that students learn the difference between spreading a small drop into a thin film of glue with a little finger, and saturating the paper.

Cover the subject of crystal classes, at least briefly, so the activity will have relevance for your students.

Student Preparation:
Briefly review crystal classification.

Each student should have;
    A copy of each of the crystal patterns.
    A copy of the appropriate chart.
Student instructions;
    Carefully, cut out the pattern on the solid lines.
    Lay the pattern on the table, printed side up.
    Fold upward and crease the model on each dotted line.  This will put all the print on the inside.
    Carefully, glue the tab marked "A" to the INSIDE of the edge marked "A".
    Carefully glue each other tab INSIDE its corresponding edge.
    Select the least attractive (most messed up) face of the model, glue that face to the proper space on the chart.

[Lesson Plans]