Chain silicate glass ceramics based on K-fluorrichterite (KNa-CaMg5Si8O22F2, KFR) have potential for use in restorative dentistry and biomedical applications because they may be cast to shape and, when cerammed, have high biaxial flexural strength and fracture toughness. Excess CaO in canasite-based chain silicate compositions is known to enhance their bioactivity. Therefore, two new glass ceramic compositions have been fabricated with 5 mol% CaF2 (Glass A) and 5 mol% CaO (Glass B) substituted for MgO in the KFR formula unit. The phase evolution of Glasses A and B was studied in detail using X-ray diffraction and transmission electron microscopy. In addition, their mechanical properties were assessed. For Glass A, the fracture toughness (FT = 2.66 +/- 0.02 MPa (.) m(1/2)) and biaxial flexural strength (BFS = 227.3 +/- 24.5 MPa) were optimized for samples heat treated at 900 degrees C for 4 h. In Glass B, however, the best FT (2.08 +/- 0.02 MPa (.) m(1/2)) and BFS (217.4 +/- 4.4 MPa) were obtained at 950 degrees C. In view of their excellent mechanical properties and castability, Glasses A and B are considered potential candidates for fabrication of custom medical devices in restorative dentistry and moderate load-bearing reconstructive bone surgery.