Electron exchange coupling in a naturally occurring tetramangano cluster in the mineral helvite, (Mn4S)(SiBeO4)3

The mineral helvite, (Mn₄S)(BeSiO₄)₃, contains discrete tetrahedral Mn₄S⁺⁶ clusters in which the S^?2 is tetrahedrally coordinated and each Mn(II) is in a distorted tetrahedron of one S^?2 and three oxygens; the cluster is situated within an encompassing lattice of SiO₄^?4 and BeO₄^?6 tetrahedra. Mn₄S⁺⁶ centers provide an interesting model for comparison to the polynuclear manganese center that is associated with photosynthetic water oxidation. Magnetic susceptibility data between 77 and 298 K have been measured for a natural helvite sample containing principally Mn₄S⁺⁶ centers but with significant contamination from Mn₃FeS⁺⁶ and Mn₃CaS⁺⁶. The data exhibited Curie-Weiss behavior with μ_eff = 5.969 B.M. and θ = 178.3 K. An analysis of the magnetic susceptibility, based on Van Vleck's formalism, demonstrated the presence of antiferromagnetic coupling, with a coupling constant J = ?5.83 cm^?1. Mössbauer spectra of Mn₃FeS centers in helvite and of Fe₄S centers in the related mineral danalite have also been recorded. Isomer shifts show little temperature dependence and lie in the range 1.23–1.43 $\text{mm}\text{sec.}$. This range is typical of tetrahedrally coordinated Fe(II) in several ionic crystals but is significantly above that of Fe(II) in ferredoxins and below that in the [quinone-Fe(II)-quinone] complex of the photosynthetic bacterium,Rhodopseudomonas sphaeroides. Quadrupole splittings are highly temperature dependent, ranging from 2.4 $\text{mm}\text{sec}$ at 4.2 K to less than 0.5 $\text{mm}\text{sec}$ at 248 K.