Bismuth analogs for microstructure
Undergraduate research and EURECA student, Jared Steger, has grown bismuth crystals for use in a study on rock microstructure. Jared labored to find a suitable technique for crystal growth in our lab. He determined that crystals were best grown with significant superheating and an elevated but undercooled quench accompanied by constant agitation of the surface.
Following growth, Jared sectioned these crystals for into 1 mm thick slabs. He digitized the slabs sequentially and analyzed the images for morphology parameters. His data set provides numerical parameters in two dimensions.
The crystals are neat, but we are using them to constrain crystal growth in magmatic rocks.
The key feature of these crystals is their step-like morphology. Rapidly crystallized bismuth crystals grow their edges faster than faces, a phenomenon known as hopper structure. This common growth morphology is found in other materials, notably quartz-feldspar intergrowth (granophyre) in shallow plutonic felsic melts. An example in cross-polarized light is shown on the right; patches of clear quartz in turbid feldspar have irregular margins and common extinction (goes dark). Like bismuth hopper structure, granophyric texture arises from melt undercooling. The microstructure is easily assessed in sections of rocks, but its three-dimensional nature is obscured by the opacity of the grains. Jared is hoping to numerically compare sections of these three-dimensional bismuth crystals to the strictly two dimensional sections of granophyre. Ultimately, the research will aim to model quartz intergrowths in three dimensions.