Scapolite is a metamorphic aluminosilicate mineral that can be described by the general formula (Na,Ca,K) 4 (Al,Si) 6 Si6O 24 (Cl,CO 3 ,SO 4 ). Two common end-members are called marialite (Na 4 ClSi 9 Al 3 O 24 ) and meionite (Ca 4 CO 3 Si 6 Al 6 O 24 ). Variations in scapolite composition can be described by two independent substitutions, NaSi(CaAl) -1 and NaCl(CaCO 3 ) -1 . Twenty eight natural scapolites in the present study exhibit a range of compositions from X EqAn [(Al-3)/3] = 8% and X Me [Ca/(Na+K+Ca)] = 7% to X EqAn = 82% and X Me = 90%. Several coupled exchange reactions can be identified in some inhomogeneous samples (e.g., Na 1.49 SiCl 0.47 [Ca 1.44 Al(CO 3 ) 0.43 ] -1 , Na 1.69 SiCl 0.58 [Ca 1.55 Al(CO 3 ) 0.50 ] -1 , Na 1.91 SiCl 0.79 [Ca 1.75 Al(CO 3 ) 0.69 ] -1 ). The extent of coupling between the two substitutions is controlled by the crystallization environment (P, T, and mineral assemblages). Electron diffraction patterns suggest that the symmetry of scapolite with X Me up to 18% is I4/m, whereas that for intermediate scapolite from X Me = 18% to at least X Me = 90% is P4 2 /n. Under darkfield observation (g = hkl, h + k + l = odd) using a transmission electron microscope (TEM), the P4 2 /n samples have anti phase domains of various sizes, the presence of which provides evidence for an I-P phase transition. A wide compositional range of scapolite solid solutions should have an I4/m symmetry at the time of formation.