  The crystalline garnet Mn3Fe2Si3O12 and an amorphous phase of the same
nominal composition are synthesized at high pressure. The magnetic properties
of the two forms are reported. Both phases order antiferromagnetically. The
crystalline phase exhibits a Curie-Weiss theta of -47.2 K, with a sharp
ordering transition at 12 K. The glassy phase exhibits a larger
antiferromagnetic Curie-Weiss theta, of -83.0 K, with a broad ordering
transition observed at 2.5 K. Both phases can be classified as magnetically
frustrated, although the amorphous phase shows a much higher degree of
frustration. The amorphous phase exhibits spin-glass behavior and is determined
to have an actual composition of Mn3Fe2Si3O13.
