Cannibal Crystals: An Experimental Analog of Crystallization in Magmas (2009)

Undergraduates: Jackie Ratner, none none none

Faculty Advisor: Allen Glazner
Department: Geology

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Analyzing the crystalline texture of an igneous rock allows geologists to infer the history of the magma and environment from which the rock came. Since it is difficult to observe crystallization processes in magmas, this experiment used a material comprised of melt and three crystalline phases, (ammonium thiocyanate, ammonium chloride, and diammonia tetrathiocyanato cobaltate), to represent magmatic crystallization processes. The solution was prepared on microscope slides and cooled on a temperature-controlled microscope stage. High-energy initial crystallization took the unstable form of dendrites, which then coalesced into larger, shapelier grains over time. The resulting grains tended to be oriented in the same direction as the dendrites’ spines. No nucleation of grains beyond the initial dendrites was observed, and accretion was a minor process for grain augmentation. Diffusion of material from smaller particles with higher surface energy to larger particles with lower surface energy, (Ostwald ripening), was the primary process by which grains matured. Small grains shrank and disappeared over time, especially when proximately located to larger grains. Grains were not observed to exhibit intergrowth, except in rare circumstances of full accretion. A better understanding of the nucleation, accretion, and Ostwald ripening processes would allow geologists to decipher the cooling history of rocks, as suggested by crystal textures.