Mg_3Al_2Si_3O_<12>(パイロープ組成)を持つMgO-Al_2O_3-SiO_2系の1気圧での相関系 [in Japanese] Phase relations for Mg_3Al_2Si_3O_<12>(pyrope composition)in the system MgO-Al_2O_3-SiO_2 at atmospheric pressure [in Japanese]
Access this Article
Search this Article
Phase relations have been determined for Mg<sub>3</sub>Al<sub>2</sub>Si<sub>3</sub>O<sub>12</sub> (pyrope composition) in the system MgO-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> at temperatures from 1000°C to 1550°C under atmospheric pressure. As increasing temperature the phase assemblage changes as follows: enstatite+cordierite+spinel; forsterite+cordierite+spinel; forsterite+cordierite+spinel+melt; forsterite+spinel+melt; spinel+melt; melt. The pyrope composition is represented by two subsolidus assemblages, viz., enstatite+cordierite+spinel and forsterite+cordierite+spinel. At temperature between 1000°C and 1025°C, the subsolidus assemblage changes from enstatite+cordierite+spinel to forsterite+cordierite+spinel through the univariant reaction: <br><br> 5Mg<sub>2</sub>Si<sub>2</sub>O<sub>6</sub>+2MgAl<sub>2</sub>O<sub>4</sub>↔5Mg<sub>2</sub>SiO<sub>4</sub>+Mg<sub>2</sub>Al<sub>4</sub>Si<sub>5</sub>O<sub>18</sub> (1)<br> En Spl Fo Crd<br><br>This indicates that enstatite and spinel is the low-temperature assemblage, and the forsterite-cordierite join is stable at high temperatures. Combining the present and previously published data, the entropy change of the reaction (1), Δ<i>S</i><sub>1</sub><sup>O</sup>, is evaluated as 7.59-7.66 cal/K, and we found the reaction boundary between enstatite, spinel, forsterite and cordierite has a slope of 2.0-2.5 bar/K in <i>dP</i>/<i>dT</i>. These results do not agree with the previous data obtained from the hydrothermal experiments.<br> At temperatures from 1025°C to 1370°C, cordierite is a stable subsolidus phase in equilibrium with forsterite and spinel. At the solidus temperature of 1380°C cordierite is consumed to form the melt by the following reaction:<br><br> forsterite+cordierite → spinel+melt<br><br>At 1400°C, cordierite disappears, and forsterite and spinel coexist with melt. At 1475°C euhedral spinels crystallize in the melt, and forsterite is not found. The liquidus temperature is to be 1500°C, and spinel disappears.
- Journal of Mineralogical and Petrological Sciences
Journal of Mineralogical and Petrological Sciences 93(1), 18-26, 1998-01-05
Japan Association of Mineralogical Sciences