The phase equilibrium study on
x (Co
3Al
2Si
3O
12)-(1-
x) (Y
3Al
5O
12) series (
x being the weight fraction) showed that garnet was the only stable phase in the composition range of
x=0.0-0.30; garnet coexisted with spinel, cristobalite, and Y
2O
3-SiO
2 mineral in the range of
x>0.30-0.46; and spinel, cristobalite, olivine and Y
2O
3-SiO
2 mineral were the stable minerals for
x=0.49 (T. Noda and M. Ushio, this journal, 75, 330 (1967).
In the present paper, the pressure effect on the stability region of garnet or other minerals of this series are reported. Glasses or gel mixtures corresponding to
x (Co
3Al
2Si
3O
12)-(1-
x) (Y
3Al
5O
12) series (
x being the weight fraction) were heattreated at pressures up to 40 kbar, using a sgeezer or a modified girdle type high pressure apparatus.
The increases in temperature from 385° to 800°C and in pressure up to 40 kbar resulted in the extension of the region of garnet formation to a larger value of
x. At relatively lower temperatures, however, the
x-value of garnet formation region was not found to exceed a certain value even if high pressure was applied. Cobalt garnet proper could not be synthesized below 800°C at pressure even above 25 kbar. As was already shown,
dp/
dT for the reaction of cobalt garnet formation was positive, and even in the stability region of the garnet, reaction of the garnet formation hardly proceeded at temperatures below 1000°C (T. Noda and M. Ushio, this journal, 75, 125 (1967)).
The results obtained in the present experiments that the
x-value of garnet formation region increased with the increase in temperature are probably due to the sluggish rate of reaction at low temperatures. Therefore, the observed region of formation is not considered to be in the true equilibrium state.
With heat treatment at 1200°C cobalt garnet was obtained at pressures above 21 kbar. The
x-value of the garnet formation region increased almost linearly with increase in pressure. The experimental results obtained at 1200°C are likely to show relationships in the equilibrium state.
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