Thirteen glasses of the general formula (M1, M2)(9.33)Si14Al5.33O41.5N5.67 where M1 = La or Nd and M2 = Y or Er have been prepared with M1/(M1+M2) fractions of 1, 0.75, 0.5, 0.25, and 0. Data for molar volume (MV), glass compactness (C), Young's modulus (E), microhardness (H), glass transition temperatures (Tg), and dilatometric softening temperatures (T-d) have been recorded. In addition, temperatures at which crystallization exotherms arise have also been determined as well as crystalline phases present after the glasses had been heat treated to 1300 degrees C in nitrogen. The results clearly demonstrate that glass properties vary linearly with effective cation field strength (CFS) of the combined modifiers (M1, M2), which is calculated from the atomic fractions of M1 and M2 and their associated CFSs. Glass stability in both the La-Y and La-Er systems reaches a maximum at M1 and M2 fractions of 0.5 because of the relative stability of different oxynitride and disilicate phases with changes in ionic radius. Furthermore, La appears to stabilize the alpha polymorph of yttrium disilicate because of combined La-Y ionic radius effects.