A glass-ceramic comprising, in weight percent on an oxide basis, of 50 to 70% SiO2, 0 to 20% Al2O3, 12 to 23% MgO, 0 to 4% Li2O, 0 to 10% Na2O, 0 to 10% K2O, 0 to 5% ZrO2, and 2 to 12% F, wherein the predominant crystalline phase of said glass-ceramic is a trisilicic mica, a tetrasilicic mica, or a mica solid solution between trisilicic and tetrasilicic, and wherein the total of Na2O+Li2O is at least 2 wt. %; wherein the glass-ceramic can be ion-exchanged.