A THEORETICAL EVALUATION OF MINERAL STABILITY IN DON-JUAN POND, WRIGHT VALLEY, VICTORIA LAND

Don Juan Pond, located in the Wright Valley, Victoria Land, is unique for several reasons. It is the most saline of the Antarctic lakes, bei ng a near-saturated CaCl2 solution. As a consequence of this high sali nity, Don Juan Pond generally remains unfrozen in winter, even at temp eratures below -50 degrees C. Don Juan Pond is the site where antarcti cite (CaCl2 . 6H(2)O) was first identified forming naturally. The obje ctive of this paper is to demonstrate the utility of a chemical thermo dynamic model (FREZCHEM) by developing theoretical stability diagrams for ice, halite (NaCl), hydrohalite (NaCl . 2H(2)O), and antarcticite in Don Juan Pond, using experimental data collected on 34 days between 1961 and 1983. The composition of Don Juan Pond at the calculated eut ectic temperature (-51.8 degrees C) was CaCl2 = 3.72 mol kg(-1) and Na Cl = 0.50 mol kg(-1), which is similar but not identical to a pure NaC l-CaCl2-H2O system. The low eutectic temperature and high CaCl2 concen trations of Don Juan Pond account for lack of freezing during winter. The model is compatible with the experimental data, and predicts the f ormation of ice during rare high water periods, halite, and antarctici te. These solid phases have all been reported from Don Juan Pond. The model also predicts the formation of hydrohalite at subzero temperatur es; hydrohalite has never been observed at Don Juan Pond, but this may simply reflect that most sampling was done during the summer when hal ite is thermodynamically more stable than hydrohalite. The FREZCHEM mo del may prove useful in elucidating the physicochemical behaviour, the origin of salinity, and the evolution of Antarctic lakes.