  The electronic and magnetic properties of clinopyroxene CaMnGe$_2$O$_6$ were
studied using density function calculations within the GGA+U approximation. It
is shown that anomalous ferromagnetic ordering of neighboring chains is due to
a "common-enemy" mechanism. Two antiferromagnetic exchange couplings between
nearest neighbours within the Mn-Mn chain and interchain coupling via two
GeO$_4$ tetrahedra suppress antiferromagnetic exchange via single GeO$_4$
tetrahedron and stabilize ferromagnetic ordering of Mn chains.
