Abstract

Phase relations for the bulk composition LiAlSi5O12 (system LiAlSiO4–SiO2–H2O) have been investigated experimentally over the range 340–950°C and 0.5–6.0 kbar P(H2O). Stability relations among eucryptite, spodumene, petalite, β-spodumene (tetragonal), and virgilite (hexagonal) have been determined for quartz-saturated subsolidus conditions. The result is a quantitative P–T phase diagram that is applicable to quartz-saturated lithium aluminosilicate assemblages in pegmatites and some volcanic rocks. Thermochemical data derived from these experimental results are internally consistent and in close agreement with values obtained from calorimetry.

Under the quartz-saturated conditions that prevail in pegmatites, stability relations among the lithium aluminosilicates are a function of P and T and are largely independent of the nature and proportions of other phases in the chemically complex pegmatite system. Thus, the lithium aluminosilicate phase diagram constitutes a petrogenetic grid from which P–T conditions of emplacement and crystallization can be ascertained for many lithium aluminosilicate-bearing pegmatites.

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