A MOSSBAUER STUDY OF FE2SIS4, HAVING THE OLIVINE STRUCTURE - COMPARISON WITH FAYALITE AND RELATED MINERALS

Fe2SiS4 has the olivine structure Pnma with n = 1.2410 nm, b = 0.7200 nm, and c = 0.5814 nm at room temperature. The Mossbauer spectrum at 2 95 K consists of two doublets having centroid shifts (relative to alph a-Fe) and quadrupole splittings for the two metal positions Fel (point symmetry (1) over bar) and Fe2 (point symmetry m) of CS(1) = 0.88 mm s(-1), QS(1) = -3.21 mm s(-1), and CS(2) = 0.92 mm s(-1). QS(2) = -2.7 5 mm s(-1). A point charge calculation for the nearest S octahedra giv es the Mossbauer asymmetry parameters eta = 0.65 and eta = 0.98 for Fe 1 and Fe2 respectively, and V-zz < 0 for both positions. At approximat ely 130 K there is an antiferromagnetic transition, and a second trans ition to a ferromagnetic or ferrimagnetic structure takes place at app roximately 30 K. Both magnetic regions show complicated Mossbauer spec tra, which require the full Hamiltonian for their analysis. Comparison s are made with Mossbauer results on the related minerals Fe2SiO4 (fay alite), Fe-3(PO4)(2) (iron sarcopside), and Fe2GeS4.