Abstract Phase relations for the the bulk composition Na2O·4CaO·8MgO·Fe2O3·12SiO2·2Al2O3 + excess H2O have been determined as a function of fluid pressure (P), temperature (T), and oxygen fugacity (fO2) using conventional hydrothermal apparatus and the oxygen buffer method. Magnesiohastingsite, NaCa2Mg4Fe3+ Si6Al2O22 (OH)2, is stable over a wide range of, P and FO2 conditions. Al-clinopyroxene + olivine + nepheline + spinel ± magnetite ± melt ± fluid, all of variable compositions, are produced upon dehydration of the amphibole at high temperature. Physical properties of the anhydrous phases indicate that increasing amounts of aegirine and forsterite components are present in the clinopyroxene and the olivine, respectively, with increasing fO2. Magnesiohastingsite breakdown in the presence of excess fluid occurs under the following conditions (see Tables 1 and 2 for abbreviations and fO2 equations of oxygen buffer equilibria):

The P-T curve for the beginning of melting intersects the amphibole reaction curve at a minimum fluid pressure of 420 bars at 1002 °C (HM buffer). Nepheline probably dissolves in the silicate liquid at temperatures and pressures slightly in excess of the investigated range. Magnesiohastingsite is thus stable well within the magmatic range.

Standard enthalpy and entropy of reaction for the equilibria Mh = Cpx + 01 + Ne + Sp ± Mt + F range from 50 to 30 kcal/mol and 37 to 35 cal/K/mol, respectively, from low to high fO2 (see Table 3 for abbreviations for synthetic phases).

Because of their extensive pressure-temperature stability fields, Mh-rich hornblendes should occur in igneous and high-grade metamorphic rocks whose norms contain major magnesian Cpx + 01 + aluminous phases. The rarity of amphiboles from such rocks indicates that low H2O fugacities (50 to 500 bars, approximately) and reducing conditions attended their crystallization.

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