Low-temperature magnetic investigation of ankerite

The magnetism of a very Fe-rich natural ankerite, Ca (sub 1.00) (Fe (sub 0.68) Mg (sub 0.23) Mn (sub 0.04) Ca (sub 0.04) )(CO (sub 3) ) (sub 2) , was investigated from 300 to 1.7 K. While the crystallographically related siderite orders antiferromagnetically below approximately 38 K, static long-range magnetic order does not appear for ankerite down to 1.7 K. The main cause of this different behaviour is thought tentatively to be due to the geometries of the Fe position in the two minerals. In siderite each Fe has six Fe neighbours at 3.73 Aa in the form of an elongated octahedron; in (ideal) ankerite each Fe again has six neighbours, but now at 4.83 Aa and in the form of a regular coplanar hexagon.