On non-isothermal oxidation of ytterbium nitrogen woehlerite (Yb4Si2O7N2), also known as the J-phase, a previously unknown and possibly metastable modification of Yb2SiO5 was detected. It is isostructural with the A-type RE2SiO5 modifications (RE = rare-earth element). The crystallographic data and stability of this new modification of Yb2SiO5 are discussed and compared with available data on RE2SiO5 compounds. The present results suggest that, in general, oxidation of rare-earth oxynitride phases might prove to be a fruitful method to obtain phases that are metastable or cannot be prepared by standard methods since their low-temperature stability range would require unrealistically long firing periods.