The magnetic properties of silicates are compiled in subvolume 27I. Silicates are very complex systems, covering large classes of minerals as well as synthetic samples. Individual chapters are dedicated to orthosilicates, sorosilicates, cyclosilicates, inosilicates, phyllosilicates and tectosilicates. Due to the huge amount of data these chapters have to be spread over several subvolumes I1, I2, etc.. – The present subvolume 27I1 deals with orthosilicates. The different groups of minerals and synthetic silicates were distinctly analyzed in various sections, presenting data on crystal structure, magnetic properties (such as magnetizations, susceptibilities, ordering temperatures), nuclear gamma resonance, nuclear magnetic resonance, transport properties, thermal properties, as well as dielectric and optical data.