Pegmatopia

The association between trace metals and other components of melt, including added ligands of B, F, and P, is testable through an experimental design that we employ to study diffusion (Fig. 3). We ascertain if the dissolution of a particular trace element in melt induces changes in concentration of the other components of the melt in the dissolution aureole (Fig. 4). If there is no interaction between the added trace element and other melt components, then the concentrations of those melt components will decrease in the proportion of added trace element, i.e., by dilution. Some melt components do exhibit this behavior, but typically the concentrations of one or more melt components increase toward the dissolving trace-element phase, indicating that this component is diffusing toward the dissolving phase. We interpret this behavior to reflect the formation of a melt species, or complex, that lowers the energy contribution of the added trace element and hence promotes its solubility.

This NSF-funded project has sufficient scope to encompass one or two Ph.D. dissertations. The initial trace-elements of interest include Be, Cs, and Ta. The added ligands are B, F, and P. The goal is to determine if and to what extent speciation occurs between the added trace elements and other components of the melt.


	The association between trace metals and other components of melt, including added ligands of B, F, and P, is testable through an experimental design that we employ to study diffusion (Fig. 3). We ascertain if the dissolution of a particular trace element in melt induces changes in concentration of the other components of the melt in the dissolution aureole (Fig. 4). If there is no interaction between the added trace element and other melt components, then the concentrations of those melt components will decrease in the proportion of added trace element, i.e., by dilution. Some melt components do exhibit this behavior, but typically the concentrations of one or more melt components increase toward the dissolving trace-element phase, indicating that this component is diffusing toward the dissolving phase. We interpret this behavior to reflect the formation of a melt species, or complex, that lowers the energy contribution of the added trace element and hence promotes its solubility. This NSF-funded project has sufficient scope to encompass one or two Ph.D. dissertations. The initial trace-elements of interest include Be, Cs, and Ta. The added ligands are B, F, and P. The goal is to determine if and to what extent speciation occurs between the added trace elements and other components of the melt.
			Fig. 3. Diffusion gradients frozen in glass. Al K alpha x-ray map showing Al concentration profile away from dissolving corundum. Bright dots in glass are polishing abrasive. From Acosta et al. (2002).


			Figure 4. Experimental determination of speciation in melt. Mineral (MIN) dissolves into granitic liquid (MELT). The concentration of constituents of MIN (blue curve) decrease with distance from the crystal surface. Elements that do not interact with constituents of MIN (red curve) are diluted toward the crystal surface. Elements that do interact with constituents of MIN to form melt species diffuse toward MIN (green curve), forming a profile referred to as “uphill diffusion.”