EDGY GROWTH: PRELIMINARY ASSESSMENT OF SECTIONED HOPPER MICROSTRUCTURE USING SYNTHETIC BISMUTH

Hopper or edge-growth microstructure occurs in some igneous magmas subjected to undercooling during crystallization, typically forming intergrowths with adjacent phases. To aid the assessment of intergrowths in thin sections of rocks with such textures, we are characterizing the sectional morphology of relatively large, isolated three-dimensional hopper crystals in the hope of finding useful parameterization techniques. Bismuth metal readily forms macroscopic hopper structure in its melt, with crystals that are easily isolated and sectioned. We melted bismuth metal at temperatures exceeding 521°C in an aluminum foil-covered steel crucible on a hotplate, then crystallized the molten metal in a separate container in a sand bath at temperatures near 180°C (ΔT≈340°C), using a suspended steel wire as a nucleation point. We grew four nicely developed 1 cc crystals that crystallized downward from the upper surface, which we refer to as the "base" of the crystal. These were oriented and cut into 1 mm thick sections using a low-speed saw. We imaged each side of each section at 1200 dpi on a flat-bed scanner. Images were rendered background transparent in Corel Photo-Paint and further processed in ImageJ to produce outlines of threshold images of each section side. We used the on-board particle analysis tools in ImageJ to determine perimeter (P) and area (A) to calculate the Form Factor (FF=4π A/P²), a proxy for outline irregularity. We also employed the fourier2.5 java plugin by Boudier and Tupper to determine Elliptical Fourier Descriptor (EFD) coefficients, another useful outline parametrization. We plotted the first two (of 30) EFD cycles for comparison among sections and orientations.

Preliminary findings indicate that for sections on (0001) and (101̄1), the FF are high (>0.5) at the base, become lower (0.17-0.3) at 3-4 mm, and increase again towards the outer margin of the crystal. A crystal cut parallel to (121̄1) exhibited the opposite relationship. Plots of EFD1 and EFD2 produce a "pathway" starting outward from the basal or near-basal section. Findings show that (0001) and (101̄1) have initial positive slopes that become convoluted loops roughly halfway through the crystal. The crystal cut parallel to (121̄1) exhibited a negative slope that progresses to a convoluted loop again roughly at the crystal midpoint.