The generalized method of quantitative description of the uniaxial anisotropy in weak ferromagnet rhombohedral calcite type structure crystals with S-state ions

► The method for investigation of the anisotropies in rhombohedral FeBO3 crystals has been developed. ► The energy contributions to the hexagonal anisotropy were analytically treated. ► The temperature dependence of the hexagonal anisotropy is analytically treated. ► The anisotropy constants for FeBO3 crystal was obtained by using of the obtained results.