Vapor pressures of Ca(OH)2(g) and Al(OH)3(g) were measured using the transpiration technique, employing the reactions CaO(s) + H2O(g) half-arrow-right-over-half-arrow-left Ca(OH)2(g); 2CaAl2O4(s) + H2O(g) half-arrow-right-over-half-arrow-left CaAl4O7(s) + Ca(OH)2(g); and CaAl4O7(s) + 3H2O(g) half-arrow-right-over-half-arrow-left CaAl2O4(s) + 2Al(OH)3(g). From these vapor pressures, enthalpy and entropy changes in the above reactions were determined for the average experimental temperature (1650 K). This, combined with literature thermodynamic data, gives the heats of formation and entropies of the hydroxide gas species at the standard state (1 atm and 298.15 K): DELTA-H(o) = -615.0 +/- 7.1 (2-sigma) kJ/mol and S0 = 284.5 +/- 7.9 (2-sigma) J/mol.K for Ca(OII)2(g), and DELTA-H0 = -1004.2 +/- 15.0 (2-sigma) kJ/mol and S0 = 313.8 +/- 7.9 (2-sigma) J/mol.K for Al(OH)3(g). The Si-tetrahydroxide molecule, Si(OH)4, was produced in the reaction SiO2(s) + 2H2O(g) half-arrow-right-over-half-arrow-left Si(OH)4(g) by application of the same technique. This is the first report of a stable Si-hydroxide gas species. Its heat of formation and entropy at 1600 K and 1 atm was determined to be -1342.7 +/- 2.7 (2-sigma) kJ/mol and 592.5 +/- 1.0 (2-sigma) J/mol.K, respectively. Using these values for the gas species investigated and literature thermodynamic data for other gas species, "volatilities" (the total of vapor pressures of gas species containing the element in question, over the pure condensed oxide of that element) of the five major planet-forming elements Fe, Mg, Si, Ca, and Al were calculated for a range of physical and chemical conditions (P, T, bulk system composition) appropriate to the solar nebula. The results indicate (1) volatilitics of all the elements investigated are significantly enhanced if the formation of hydroxide species by reaction with H2O(g) is taken into account, compared with previous calculations which neglected these species; (2) generally, hydroxide gas species predominatc at relatively low temperatures and/or in an oxidizing atmosphere, while monatomic and undersaturated-oxidized gas species are most abundant at high temperatures and/or in a reducing atmosphere; (3) hydroxide species of Ca and Al are relatively stable even at high temperatures compared to those of Si, Mg, and Fe; (4) these effects explain how Ca and Al could have evaporated from Ca,Al-rich inclusions in carbonaceous chondrites while Si, Mg, and Fc condensed onto them during the preaccretion alteration of CAIs.
