In situ voltammetric observation of transitions above the liquidus in silicate melts |
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| Authors: | Russell O Colson |
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| Institution: | (1) Department of Anthropology and Earth Science, Minnesota State University Moorhead, Moorhead, MN, USA |
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| Abstract: | Reduction potentials (E
0) and diffusion rates (D) for divalent nickel in a variety of silicate melts and as a function of temperature have been measured in situ using voltammetric
methods. These measurements provide an indirect window into structural changes in melts at temperatures above the liquidus.
Slope of the reduction potential versus temperature (related to entropy) and slope of the diffusion rate versus 1/T (related to diffusional activation energy) remain roughly constant or change only gradually above the liquidus, with the
exception of a sharp break in slope typically observed about 60–160°C above the liquidus. This sharp break is consistent with
a structural transition in the melt rather than with a gradual change in melt structure. A correlation between the transition
temperature and the liquidus temperature, combined with the qualitative effect of prior crystallization, suggests that the
transition may be related to the melt “preparing” for crystallization roughly 100°C above the liquidus. These observations
may have implications for understanding melt activities, partitioning between melt and crystalline phases, and liquid immiscibility. |
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