We have examined a series of sheet silicates, both in single crystal form in an applied magnetic field (biotite, chlorite, vermiculite) and as powders in the magnetically ordered state (greenalite, berthierine). It is found that VZZ is negative in the 1 : 1 minerals, that η ≤ 0,2 and that the Z-axis is close to the c-axis of the silicate. These results indicate a |0> ground state for the Fe2+ ion and a c-plane of easy magnetization. In the 2 : 1 minerals however, η = 0.9(1). The sign of VZZ was not determined, but the Z axis is again close to the c-axis.