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The enthalpies of drop solution in lead borate (2 PbO*[B.sub.2][O.sub.3]) of four Mg-staurolite samples, synthesized at 720 [degrees] C and pressures between 2 and 5 GPa, were measured by high-temperature oxidemelt calorimetry at 702 [degrees] C. Staurolite compositions, determined by electron microprobe analysis, Karl-Fischer titration, and thermogravimetry, are: [Mg.sub.3.71][Al.sub.18.17][Si.sub.7.60][O.sub.44.31][(OH).sub.3.69], [Mg.sub.3.87][Al.sub.17.65][Si.sub.7.75][O.sub.43.68][(OH).sub.4.32], [Mg.sub.3.66][Al.sub.17.76][Si.sub.7.68][O.sub.43.31][(OH).sub.4.69], and [Mg.sub.3.58][Al.sub.18.05][Si.sub.7.43][O.sub.43.01][(OH).sub.4.99]. The enthalpy of drop solution of the bulk samples (as well as the calculated values for the enthalpy of formation from the elements of Mg-staurolite) are strongly correlated to the H content of the samples. The enthalpy of formation from the elements is best described by the linear relation [[DELTA].sub.f][H.sup.0.sub.298] (Mg-staurolite) = (-25357.58 + 87.35 N) kJ/mol, where N = number of H atoms per formula unit in Mg-staurolite. The enthalpy of drop solution of two partially dehydrated Mg-staurolite samples is in a good agreement with the linear relation. Phase-equilibrium data for Mg-staurolite (Fockenberg 1998) were recalculated using the stoichiometric formula [Mg.sub.3.5][Al.sub.18][Si.sub.7.75][O.sub.44][(OH).sub.4]. Based on these mineral equilibria and the internally consistent data set of Berman (1988), a mathematical programming analysis of the thermodynamic data of Mg-staurolite gave [[DELTA].sub.f][H.sup.0.sub.298] [[Mg.sub.3.5][Al.sub.18][Si.sub.7.75][O.sub.44][(OH).sub.4]] = -25005.14 kJ/mol, and [S.sup.0.sub.298] [[Mg.sub.3.5][Al.sub.18][Si.sub.7.75][O.sub.44][(OH).sub.4]] = 937.94 J/(K*mol). Thus, for the first time, reliable thermodynamic data for Mg-staurolite, based on experimental constraints, are provided.

The stability field of MgMgAl-pumpellyite, [Mg.sub.5][Al.sub.5][Si.sub.6][O.sub.21][(OH).sub.7], was determined in the system MgO-[Al.sub.2][O.sub.3]-Si[O.sub.2]-[H.sub.2]O in reversal experiments at pressures between 34 and 100 kbar and temperatures in the range of 597 to 1050 [degrees] C. Brackets were obtained on five breakdown reactions (in order of increasing pressure): [Mathematical Expression Omitted] (1) [Mathematical Expression Omitted] (2) [Mathematical Expression Omitted] (3) (3) [Mathematical Expression Omitted] (4) [Mathematical Expression Omitted] (5) This phase becomes stable only at pressures of more than 34 kbar and temperatures up to 820 [degrees] C. Thus, MgMgAl-pumpellyite may be an [H.sub.2]O-containing phase at depths greater than 100 km in the coldest parts of subduction zones.

First principles molecular dynamics simulations have been carried out at various temperatures and pressures starting with either Ca2+ or CaO in a reactive volume of 63 H2O molecules. In the case of aqueous Ca2+, the ion is surrounded by six H2O molecules in the first hydration shell at 300 K/0.3 GPa, with rare exchange between first and second hydrations shells. At 900 K/0.9 GPa, the coordination number in the first hydration shell fluctuates between six and eight, the average being 7.0. CaO immediately reacts with the surrounding H2O molecules to form Ca2+ + 2OH-. The hydroxyl ions form transient Ca(OH)+ and Ca(OH)2 complexes and have a mean residence time in the first coordination shell of Ca2+ of 6 ± 4 ps at 500 K and 3 ± 3 ps at 900 K, respectively. At 500 K/0.5 GPa, the time-averaged relative concentrations of the transient Ca2+, Ca(OH)+, and Ca(OH)2 species are 14%, 55%, and 29%, while at 900 K/0.9 GPa, they are 2%, 34%, and 63%. [ABSTRACT FROM AUTHOR]