The magnetism of a very Fe-rich natural ankerite, Ca1.00(Fe0.68Mg0.23Mn0.04Ca0.04)(CO3)2, was investigated from 300 K down to 1.7 K. While the crystallographically related siderite, FeCO3, orders antiferromagnetically below ca. 38 K, static long-range magnetic order does not appear for ankerite down to 1.7 K. The main cause for this different behaviour is tentatively thought to be due to the geometries of the Fe arrangements in the two minerals: In siderite each Fe has six Fe neighbours at 3.73 Å in the form of an elongated octahedron, in (ideal) ankerite each Fe has again six Fe neighbours, but now at 4.83 Å and in the form of a regular coplanar hexagon.

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