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Crystal Systems
by Jami Britt
Physical Geology
Spring 2008
Six Crystal Systems
What are crystals?
A crystal is a mineral whose internal, geometrical repeating pattern of atoms is consistent throughout the entire structure. This pattern is what gives the crystal its shape.
Where are crystals found?
Crystals are found everywhere, all around us. There are crystals in the kitchen called salt and sugar. There are crystals at the beach called sand. Mountains are full of crystals. Snowflakes are water vapor frozen into crystals. There are crystals inside caves, hanging from the ceiling and coming up from the floor. Crystals are found everywhere, if you are paying attention.
Why are they important?
Crystals are used in many types of tools from pencils to computers. Some crystals, such as diamonds, rubies, emeralds and opals are called gemstones and can be worn as jewelry. But, diamonds can also be used as cutting tools because of the hardness. Much of modern technology depends on the use of crystals. Quartz crystals are used in watches. Crystals of silver salts, which are light sensitive, are used in photography. Liquid crystals are used in calculators and clocks for number displays.
How are they categorized?
Crystallography has developed terminology which, when applied to crystals, can describe their structure, symmetry and shape. Using the type of symmetry a mineral species possesses, we can categorize the mineral into one of six crystal systems. These systems include the isometric, hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic systems. (The hexagonal crystal system is sometimes broken down into hexagonal and rhombohedral.)
Isometric Tetragonal Orthorhombic Monoclinic Triclinic Hexagonal
Illustrations from www.mineralogicalassociation.ca/young/recognize.php
Modification of the basic shapes
The six basic shapes can all be modified by cutting off corners or edges. For example, here are some of the ways that the isometric crystal can be shaped:
Crystal class illustrations from Manual of Mineralogy and Petrology, by James D. Dana, 1889
Let’s take a quick look at each system
Isometric - This system comprises crystals with three axes, all perpendicular to one another and all equal in length.
Basic wooden model Halite (salt)
Tetragonal - This system comprises crystals with three axes, all perpendicular to one another; two are of equal length.
Basic wooden model Apophyllite
Orthorhombic - This system comprises crystals with three mutually perpendicular axes, all of different lengths.
Basic wooden model Golden Topaz
Monoclinic - This system comprises crystals with three axes of unequal lengths, two of which are oblique (that is, not perpendicular) to one another, but both of which are perpendicular to the third.
Basic wooden model Gypsum
Triclinic - This system comprises crystals with three axes, all unequal in length and oblique to one another.
Basic wooden model Orthoclase
Hexagonal - This system comprises crystals with four axes. Three of these axes are in a single plane, symmetrically spaced, and of equal length. The fourth axis is perpendicular to the other three.
Basic wooden model Sapphire
How to teach the six crystal systems:
Crystals Lesson plan
Target Grade Level: 7th
Allotted Time: 45-50 minutes Subject/Content: Earth Science/Crystals
Learning Objectives: The student will be able to understand the six basic shapes of crystals and be able to cut their own “crystal” with 100% accuracy.
AZ State Standard:
Science Strand 6 Earth and Space Science, Concept 1 Structure of the Earth, PO 1 Classify rocks and minerals by the following observable properties: grain, color, texture, and hardness.
Purpose/Rationale: To make the students aware of the six classic shapes which crystals can be found on Earth.
Materials Needed:
chunks of cheese (most any type without holes will work well),
plastic knives, handout, textbook or Internet access,
mineral samples (if samples not available, use pictures on/from the internet),
clay models (if you don’t have models of the crystals, print out paper cut-outs by © Kenneth Fuller 2005 found at www.learn-science.20m.com/xtl.html.)
Anticipatory Set: For the last couple of weeks, we have been talking about rocks and minerals. Today, I want to show you what one type of mineral, a crystal, looks like and the different shapes that they come in. We are going to make a crystal model.
Procedures: Pass out the handout “Six basic types of Crystals”. Divide students into groups of two or three. Give each group a hunk of cheese and a plastic knife. Tell the students that they are going to choose, as a group, which crystal type they want to try to make. Each group can choose one person to do the carving or they can each take turns. The students will also need to pick an sample mineral from their textbook or Internet. You can also provide pictures taken from the Internet if there is no access to computers in your classroom.
Closure: Once the students have finished their “crystal”, ask for volunteers who would like to show the class which shape they chose and to name a mineral that is the shape that they chose.
Evaluation/Assessment: By observation, do they understand that there are different shapes or only one shape?
Comments: You will need to know if there are any students in your class that are lactose intolerant. This project can also be done with sponges, potatoes or modeling clay.
Six Basic Types of Crystals Worksheet
Isometric Tetragonal Orthorhombic
Monoclinic Triclinic Hexagonal
The shape I/we chose: _____________________
A mineral that is found in this shape: _______________
Models of the six crystal systems:
The outlines for the paper cut-outs are found at www.learn-science.20m.com/xtl.html. The paper cut-outs are a great way for the students to understand the exact shapes, as it is difficult to grasp the concepts from a two dimensional picture. But, this takes some time and the finished product can easily be squashed.
For the clay models, I shaped the modeling clay in the approximate shapes that I wanted. Once they dried thoroughly, I trimmed and sanded them until they looked just right. (It took a good 48 hours for the thicker models to dry completely.) The modeling clay shapes are something that the students can handle and not get broken (or damage something or someone if dropped or thrown).
References
Crystal illustrations from
www.mineralogicalassociation.ca/young/recognize.php
Crystal Class Illustrations from Manual of Mineralogy and Petrology, by James D. Dana, 1889.
Cut-out Crystal Forms from © Kenneth Fuller 2005 found at
www.learn-science.20m.com/xtl.html.
Mineral photos courtesy of R. Weller/Cochise College
copyright 2008-R.Weller found at
http://skywalker.cochise.edu/wellerr/mineral/minlist.htm
Wooden models courtesy of R. Weller/Cochise College
copyright 2008-R.Weller found at
http://skywalker.cochise.edu/wellerr/crystals/crystalL.htm