MY FAVORITE DEMONSTRATION: Classroom Demonstrations for the Earth Sciences: Silicate Mineral Structures
By Greg Mead, Santa Fe College, Gainesville, FL
The Earth's crust is around 75% oxygen and silicon, and silicate minerals (consisting of those two elements plus a smattering of other ions) are by far the most abundant minerals in the crust. Silicates (like olivine, Figure 1) have a wide variety of crystal structures, all based on the silica tetrahedron, which is a complex ion shaped like a foursided pyramid, with four oxygen atoms at the corners and a silicon atom at the center, During my classroom discussion of this structure, I pass around two different examples of tetrahedra: Dungeons and Dragons dice (Figure 2) that I find at various stores in town, available in a variety of shapes, many of which correspond to crystal shapes, and a back massage device (Figure 3) that I found at a yard sale. This is useful because I also point out that the wooden balls correspond to oxygen atoms, and the silicon atom would be at the center where the steel bars cross. (Incidentally, these feel great on your back if you can find one!)
Individual tetrahedra (isolated tetrahedra), as represented by these initial two models, are found in such minerals as olivine. I point out that these negatively charged complex ions are held together with positive metal ions in between the tetrahedra and that the corners of some of the tetrahedra are very close to each other.
Other ways in which silica tetrahedra are organized in silicate minerals are more complex and involve joining tetrahedra together. For these examples, I use a model that originated with a gift my sister gave me several years ago. This construction set consists of twenty-eight ~0.5" diameter steel balls and forty-two ~1" plastic rods with powerful rare-earth magnets at the ends (Figure 4). [Go to the online extras—see the table of contents page for instructions—for information on ordering these types of construction sets.]
Using parts from the construction set, I start with the construction of multiple single tetrahedra (Figure 5). I then join several of them in front of the students by removing steel balls from some corners and "polymerizing" the tetrahedra together to form single chains (Figure 6). I discuss how this forms "single chain silicates" (the pyroxene group of minerals). From there, I bring two chains together to form a "double chain silicate" (the amphibole group of minerals) (Figure 7). Unfortunately, I don't have enough of these rods and balls to create "sheet" (mica and clay) or "framework" (quartz and feldspars) silicates, but with enough balls and rods, these could be made as well.
I'm a strong believer in the idea that if you can draw something accurately, you'll be well on the way to understanding it, and while I make these models, I also draw them on the board so that the students can see what's happening. I encourage the students to draw the structures themselves. These models and the drawings together go a long way to helping the students understand the structure of these typical silicate structures.
This magnetic ball-and-rod construction kit could undoubtedly be used to model other mineral structures as well, as these structures are easier to manipulate and are less permanent than the classic wooden ball with holes/steel rod models used in many classrooms.Without recommending any particular product, I was able to find similar sets with more balls and rods at varying prices online:
Geomag - MASTER BOX YELLOW - 248-Piece Magnetic Building Set, Certified STEM Construction Toy, Safe for Ages 3 and Up
PlayMaty 306 Pieces Magnetic Building and Construction Blocks Toys Magnet Stick Build Toy for Kids Block Stacking Game Include