TEACHING SHEET SILICATE MINERAL STRUCTURES WITH LEGOS®

The arrangement of tetrahedral and octahedral layers in sheet silicate mineral structures is easily described as building a structure with LEGOS®. After lecturing many times about these structures being just like LEGOS®, I decided to design an in-class exercise that teaches the structures using various colored blocks. A large collection of 6x1 LEGOS® must be amassed. I used white blocks for the tetrahedral layers, gray for the octahedral layers, and red for the cation layers. To represent di- and tri-octrahedral layers, the bumps of the LEGOS® are colored with a marker. If two out of three bumps on the 6-bump LEGOS® pieces are colored in then that is a dioctahedral layer.

I first give the students 8 different sheet silicate structures constructed from LEGOS®. Before they know that they are really working on mineral structures, I have them break apart the structures, describe them, and rebuild them. The students observe one structure that is just a gray block with all three bumps colored in or a structure containing the following blocks: white/gray/white/red/white/gray/red with two out of three bumps on the gray layer colored in. Once they have an idea of the permutations within the structures, they start assigning a mineral formula to each structure. They start with the octahedral sheet and given the following information: “Structure 1 is considered an octahedral sheet: two planes of OH- with either divalent or trivalent cations sandwiched in between the OH- planes. The mineral formula of this structure ideally is Mg₃(OH)₆ or Al₂(OH)₆. Structure 2 is the tetrahedral layer with an ideal formula of Si₂O₅^2-”. The students are told that in order to snap these layers together, an OH- must be removed from the octahedral layer to make room for the apical oxygen of the tetrahedral layer. Given the formulae for kaolinite and serpentine, the students can recognize which part of the formula is from the tetrahedral layer and which from the octahedral layer. The students then have to match each LEGO® configuration to a mineral formula and also identify the corresponding hand sample.

A student survey administered after the lecture-format of learning sheet silicates and then the year after the LEGO® exercise asked the students to rank comfort level with identifying biotite and muscovite. The average confidence level increased approximately 20%.