Phase equilibria of the MgO-Cu2O-CuO system have been determined in ai
r at temperatures ranging from 850 to 1100 degrees C. The starting com
positions ranged from 0 to 100% Mg in (Mg1-xCux)O. CuO was quite solub
le in MgO. The solubility of CuO in MgO at 850 degrees C was about 9%
and at 1050 degrees C was about 20%. The only known ternary Guggenite
phase had a composition of Cu2MgO3. The Guggenite phase exists in thre
e forms, namely, Guggenite A (G(A), orthorhombic, space group Pmmn(59)
), Guggenite B (G(B), orthorhombic, space group I(0)), and Guggenite X
(G(X), monoclinic). Guggenite A was obtained as a single phase in air
at x(CuO) = 67% (i.e., 67% CuO and 33% MgO) and 1000 degrees C. It un
dergoes a phase transition to the Guggenite X phase when it is heated
above 1050 degrees C. The Guggenite B phase was obtained in a mixture
at x(CuO) = 75% and at temperatures above 1050 degrees C. MgO was not
found to be soluble in CuO. For samples with x(CuO) greater than or eq
ual to 67%, the Guggenite A phase coexisted with tenorite (CuO, monocl
inic) at temperatures less than or equal to 1000 degrees C and above 1
021 degrees C, it was in equilibrium with cuprite (Cu2O, cubic). For C
uO compositions between 20% and 67%, a mixture of Guggenite A phase an
d the periclase phase (MgO, cubic) was obtained at temperatures less t
han or equal to 1000 degrees C. The decomposition of Cu2MgO3 (Guggenit
e A) phase was determined at several oxygen partial pressures and the
Gibbs free energy of formation of this phase was also calculated. Copy
right (C) 1997 Elsevier Science Ltd.