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: 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: 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.
Objectives:
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.
Each student should have;
A copy of each of the crystal patterns.
A copy of the appropriate chart.
Scissors.
Glue.
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]
