Structural and magnetic properties in Mn-implanted, p -type Si were investigated. High resolution structural analysis techniques such as synchrotron x-ray diffraction revealed the formation of Mn Si1.7 nanoparticles already in the as-implanted samples. Depending on the Mn fluence, the size increases from 5 nm to 20 nm upon rapid thermal annealing. No significant evidence is found for Mn substituting Si sites either in the as-implanted or annealed samples. The observed ferromagnetism yields a saturation moment of 0.21 μB per implanted Mn at 10 K, which could be assigned to Mn Si1.7 nanoparticles as revealed by a temperature-dependent magnetization measurement.