Equilibrium properties of granite pegmatite magmas Available to Purchase

Revision of the quasi-crystalline model has proceeded through a refinement of the cryoscopic equations for the major felsic melt components NaAlSi₃O₈ (ab), CaAl₂Si2O₈ (an), KAlSi₃O₈ (or), and Si₄O₈ (qz), which has permitted more definitive identification and evaluation of the major speciation reactions that occur in the anhydrous and H₂O-saturated melts of the unary and binary subsystems of the system Ab-An-0r-Qz-H₂O. Application of this revised thermodynamic model to the investigation of melt speciation behavior in the system Ab-0r-Qz-Sil(sillimanite)-H₂O indicates the occurrence of a sil- and qz-consuming reaction to produce Al_1.455Si_2.91O₈ (dpy = dehydroxylated pyrophyllite). Similarly, application of the model to the system Ab-Qz-Eu(eucryptite)-H₂O at 2.0 kbar indicates the occurrence of a eu- and qz-consuming reaction, which forms a petalite-like species (pe) and greatly influences the positions of the liquidus field boundaries. Also, theoretical investigations of the shift in the haplogranite liquidus minimum upon addition of the volatiles F and B have led to the conclusion that these volatiles complex mainly with Na in the melt to form a cryolite-like and a sodium tetraborate-like melt species, respectively.

The effects of the proposed speciation reactions on the activities of the melt components in the expanded system Ab-An-Or-Qz-Co(corundum)-Sp(spodumene)-H₂O-F-B have been incorporated into the calculation of crystallization paths of the Spruce Pine, North Carolina, and Harding, New Mexico, pegmatites at 2.0 and 5.0 kbar. The resulting calculated sequences of crystallization show a strong correspondence with the change in major mineralogy of the inward succession of zones in these pegmatites, indicating magmatic control of the major mineralogy within each zone.

The compositions of chloride-free aqueous fluids reacted with Spruce Pine and Harding pegmatites and their melts up to 10 kbar indicate that the alkalis complex in these fluids with Al and Si in large polynuclear species similar to those in the coexisting melts. The addition of Cl, however, drastically depresses the aluminosilicate solubility and enhances that of the alkalis. The formation of perthite hoods and albitic lower zones in pegmatites can be attributed to the efficacy of transport of alkalis by a Cl-bearing aqueous phase in which the proportions of transported alkalis are determined by the equilibrium assemblage of each zone.