Magnetic susceptibility and exchange coupling in the mineral ardennite

Ardennite, a rare silicate mineral, contains about 19 wt.% manganese. Some of the manganese atoms are in positions which are close enough to allow negative exchange and hence a reduction of the total magnetic susceptibility. It is shown that the susceptibility can be accounted for approximately by the treatment of Earnshaw and Lewis (1958) for S =52 and a Hamiltonian H = −2gμH_b−2JS₁·S₂.