Authors: A. Sazonov, V. Kaiser, G. Heger, M. Meven, V. Hutanu

Abstract
Olivine, (Mg,Fe)2SiO4, is an important mineral of the upper Earth’s mantle. Therefore, the properties of olivine are of considerable interest in geophysics, geology and crystal chemistry. Well known examples of natural olivine-type silicate are fayalite (Fe2SiO4, with paramagnetic Fe2+ ions) and forsterite (Mg2SiO4, with diamagnetic Mg2+ ions). There are a few natural members with a mixture of diamagnetic/paramagnetic ions, e.g. kirschsteinite (CaFeSiO4).

Synthetic Co2SiO4 also crystallizes in the olivine-type structure. Investigations on the magnetization of Co2SiO4 single crystals in dependence of crystal orientation and applied magnetic field were performed by W. Lottermoser and H. Fuess [2,3]. At about 50 K an antiferromagnetic phase transition occurs. In the antiferromagnetic state at 20 K the Co(2) spins are collinear while the Co(1) spins are canted.

Using a mirror furnace a large single crystal Co2SiO4 was grown by the zone melting method. Crystal and magnetic stuructures of the compound were determined by means of neutron diffraction measurements (HEiDi, FRM-II) at room temperature and 2 K. These studies are a part of the complex investigation of Co-olivines.