TotBlocks: exploring the relationships between modular rock-forming minerals with 3D-printed interlocking brick modules

Abstract. Many rock-forming chain and sheet silicate minerals, i.e., pyroxenes, amphiboles, micas, and clay minerals, are built from shared chemical building blocks known as T-O-T modules. Each module consists of two opposing chains of vertex-sharing silica tetrahedra (T), which vertically sandwich a ribbon of edge-sharing metal–oxygen octahedra (O) in a T-O-T configuration. These minerals are both abundant and diverse in the lithosphere because T-O-T modules are chemically versatile (incorporating common crustal elements, e.g., O, Si, Al, Fe, and Mg) and structurally versatile (varying as a function of module width and linkage type) over a wide range of chemical and physical conditions. Therefore, these minerals lie at the center of understanding geological processes. However, their diversity leads to the minerals developing complex, 3D crystal structures, which are challenging to communicate. Ball-and-stick models and computer visualization software are the current methods for communicating the crystal structures of minerals, but both methods have limitations in communicating the relationships between these complex crystal structures. Here, we investigate the applications of 3D printing in communicating modular mineralogy and crystal structures. The open-source TotBlocks project consists of 3D-printed, T-O-T interlocking bricks, based on ideal polyhedral representations of T and O modules, which are linked by hexagonal pegs and slots. Using TotBlocks, we explore the relationships between modular minerals within the biopyribole (biotite–pyroxene–amphibole) and palysepiole (palygorskite–sepiolite) series. The bricks can also be deconstructed into T and O layer modules to build other mineral structures such as the brucite, kaolinite–serpentine, and chlorite groups. Then, we use the T-O-T modules within these minerals to visually investigate trends in their properties, e.g., habit, cleavage angles, and symmetry/polytypism. In conclusion, the TotBlocks project provides an accessible, interactive, and versatile way to communicate the crystal structures of common rock-forming minerals.