Developing new rare-earth ion-doped optical fibres for power amplifiers and lasers requires continuous improvements in fibre spectroscopic properties. To overcome some limitations inherent to silica glass, it is proposed to embed rare-earth ions in dielectric nanoparticles. In this article we focus on the modifications of the Er 3+ ion spectroscopy in Mg-silicate nanoparticles doped into optical fibre preforms. Through EXAFS and fluorescence measurements, we demonstrate that different local environments are experienced by Er 3+ ions, attributed to the depolymerization of the phosphate network. These results gain insight into the tailoring of luminescence properties.