  A detailed analysis of the magnetic properties of a vanthoffite type mineral
Na6Mn(SO4)4 basedon dc magnetization, low temperature neutron powder
diffraction and theoretical calculations is reported. The mineral crystallizes
in a monoclinic system with space group P21/c, where MnO6 octahedra are linked
via SO4 tetrahedra. This gives rise to super-exchange interaction between two
Mn2+ ions mediated by two nonmagnetic bridging anions and leads to an
antiferromagnetic ordering below 3 K. The magnetic structure derived from
neutron powder diffraction at 1.7 K depicts an antiferromagnetic spin
arrangement in the bc plane of the crystal. The magnetic properties are
modelled by numerical calculations using exact diagonalization technique, which
fits the experimental results and provides antiferromagnetic ground state of
Na6Mn(SO4)4.
