New Mexico Mineral Symposium — Abstracts

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The results presented here represent a joint effort by Dr. R. H. Jahns (Stanford University), Dr. P. M. Fenn (University of California, Davis), and myself. The research was conducted at Stanford University and Los Alamos Scientific Laboratory. We acknowledge support from the National Science Foundation (Jahns and Fenn) and from the Department of Energy (Luth) through Los Alamos Scientific Laboratory. The bulk composition used in this study was prepared by Drs. R. H. Jahns and:-C. Wayne Burnham (Pennsylvania State Univ.) and is based on extensive diamond drilling and geologic mapping. It is a volumetrically weighted average of the various zones of the entire pegmatite, derived on the basis of the three-dimensional information.

The experimental study focused on the determination of equilibrium phase relations at a confining pressure of 5 kb (0.5 GPA) as a function of temperature and amount of water added to the essentially anhydrous bulk composition. The preliminary experimental results are illustrated below. In addition to the phases noted on the diagram (Q = quartz, ab = altiitic feldspar, Kf = potassic feldspar, Mu = muscovite, L = silicate liquid as quenched glass, and V = aqueous vapor, fluid, or gas phase) spodumene and beryl were observed sporadically and in small amounts.

The phases encountered in the experimental study were analyzed using the CAMECA electron probe microanalyzer at Los Alamos Scientific Laboratory. Of particular importance was the change in composition of the equilibrium liquid accompany¬ing crystallization at constant confining pressure. Such infor¬mation is of considerable utility in evaluating the closed system crystallization of granitic compositions. Considerable difficulty was encountered in analyzing the liquids, represented as hydrous silicate glasses at the completion of the experiment, because of sodium mobility under the electron 'beam. However, an empirical correction procedure was developed which yields both precise and self-consistent results.

The experimental study of equilibrium phase relations also provided the base for additional studies of crystal nucleation, growth, and textural development in this particular silicate-water system. It was possible to simulate the various stages leading to the development of the graphic texture observed frequently in pegmatitic rocks.