NaxCa1-xAl2-xSi2+xO8 plagioclase solid solutions (0 <= x <= 1) were synthesized under sub-solidus conditions using a solid-state reaction technique. The plagioclase formation and the sintering temperature decreased with an increase in x from the anorthite (CaAl2Si2O8; x = 0) to the albite (NaAlSi3O8; x = 1). Microwave (MW) dielectric measurements revealed that slow-cooled (P (1) over bar) anorthite exhibited higher Q x f values than fast-cooled (I (1) over bar) anorthite. Slow cooling also considerably improved the Q x f values of the sodium-rich NaxCa1-xAl2-x Si2+xO8 solid solutions (0.8 <= x <= 1), where the highest Q x f value of 17600 GHz; was obtained for slow-cooled Na-0.8 Ca0.2Al1.2Si2.8O8. The temperature coefficient of resonant frequency (tau(f)) approached zero for 0.8 <= x <= 1.