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Gestures for Mineralogy: Silicate Structures

Carol Ormand, Kinnari Atit, and Barb Dutrow

Introduction:

In this exercise, you will use gestures to convey information about silicate mineral structures. Think of these gestures as linguistic “shortcuts.” Gesturing provides a tool to analyze mineral structures and makes it easier for you to visualize and remember crystal structures.

Self-assessment:

At the conclusion of this exercise, you should be comfortable using gesture to convey information about isolated tetrahedra and single- and double-chain silicate structures. For each exercise, one person gestures while the other person watches. The person watching analyzes the gesture for the information conveyed. Then you trade roles and repeat the exercise once.

Silicate structures

Part 1: Individual tetrahedra

In a silica tetrahedron, four oxygen ions surround the central Si cation. If you connect the centers of the oxygens, and draw planes, the shape of a tetrahedron is created. Thereis an oxygen ion at each of the four corners and a siliconion in the center. To gesture a silica tetrahedron, use the tip of your thumb, index finder, and middle finger to represent oxygen ions. The fourth oxygen ion is atthe point (approximately speaking) where all of those digits come together. You have to imagine a silicon ion in the empty space between your fingers and thumb.

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Part 2: Shared oxygen ions

In a mineral, when silica tetrahedra share an oxygen ion, the other six oxygen ions tend to be as far apart from each other as possible. For example, if you think of the two tetrahedra that are sharing an oxygen ion as being stacked on top of one another, their two triangular bases are likely to be rotated 60 degrees with respect to each other. (See figures below.) Using both hands, show how two adjacent silica tetrahedra can share a corner (a single oxygenion). Have your partner confirm that your tetrahedra are oriented correctly with respect to each other.

Side view (Cross section)View from above (Plan view)

Part 3: Single and double Si-chain structures

With your partner, using all four of your hands, show how silica tetrahedra can share two oxygen ions (as in a single chain silicate) or three oxygen ions (as in a double chain silicate). Compare your gestures to the diagrams below to make sure that your tetrahedra are oriented correctly with respect to each other.

For the double chain silicate, you only have enough hands to show a tetrahedron that shares three oxygen ions and the three tetrahedra with which it shares them. If you would like, join forces with another pair of students to complete more of the double chain structure.

Single chain, perspective view: Double chain, perspective view: Top views, chain silicates:

(Looking down the chain)(Looking down the chain) single double