THERMODYNAMIC PROPERTIES OF AQUEOUS GE(IV) HYDROXIDE COMPLEXES FROM 25-DEGREES-C TO 350-DEGREES-C - IMPLICATIONS FOR THE BEHAVIOR OF GERMANIUM AND THE GE SI RATIO IN HYDROTHERMAL FLUIDS/

The stoichiometry and thermodynamic properties of Ge(IV) hydroxide com plexes were generated from both solubility and potentiometric measurem ents. The solubility of the tetrahedral germanium oxide (GeO2(tetr)) w as measured at temperatures from 25 to 350 degrees C in acid to alkali ne solutions at the saturated vapor pressure of the system (P-sat). Po tentiometric measurements were performed on GeO2-KOH aqueous solutions at temperatures from 21 to 200 degrees C and P-sat using a pH solid-c ontact glass electrode. Results indicate that Ge(OH)(4)degrees(aq) is the dominant Ge-bearing species at concentrations up to at least 0.05 m over a wide range of pH (0 - 8) and temperatures (20-350 degrees C). GeO(OH)(3)(-) forms in significant amounts only in alkaline solutions (pH > 8-9). These results were combined with the available low-temper ature solubility data on the hexagonal germanium oxide (GeO,(hex)) and the thermodynamic properties of GeO,(tetr) and GeO,(hex) to generate Ge(OH)(4)degrees(aq) and GeO(OH)(3)(-) thermodynamic parameters within the framework of the revised HKF equation of state (Helgeson et al., 1981; Tanger and Helgeson, 1988). Calculations carried out using these parameters indicate that the distribution of Ge hydroxide species as a function of pH and temperature is similar to that of silicon hydroxi de complexes. However, the significant differences between Ge(OH)(4)de grees(aq) and Si(OH)(4)degrees(aq) enthalpies of formation and heat ca pacities can lead to large variations with temperature of Ge/Si ratios in solutions in equilibrium with Ge-bearing silicates. For example, c alculations show that the Ge/Si ratio in a fluid in equilibrium with a Ge-bearing wollastonite (Ca(Si,Ge)O-3) increases by an order of magni tude when temperature is raised from 25 to 500 degrees C. This can be responsible for the high values of Ge/Si ratios measured in high tempe rature crustal fluids. Copyright (C) 1998 Elsevier Science Ltd.