A highly crystalline sample of the impurity stabilized phase y-Y2Si2O7, generally known as yttrialite, has been formed from the melt of a glass with a nominal composition of 62(SiO2)-10(Al2O3)-28(Y2O3) mol%. Powder X-ray diffraction patterns were collected using in-house instrumentation and the 11-BM diffractometer at the Advanced Photon Source, Argonne National Laboratory, Argonne, IL. Rietveld refinements were carried out on the patterns using two structural models. On patterns collected using in-house instrumentation the correct structure assignment was difficult to determine; however, the extremely high-quality data afforded by the 11-BM instrument showed conclusively that the sample was found to crystallize in the monoclinic system (SG=P2(1)/m) with lattice parameters a=5.03032(6), b=8.06892(6), c=7.33620(6) angstrom, and beta=108.673(1). Furthermore, simulations have shown that it is likely that this structure model can be used to describe natural yttrialite or yttrialite that is formed at low temperatures, though the possibility that such materials are paracrystalline is also discussed.