Scapolite is a metamorphic aluminosilicate mineral that can be described by the general formula (Na, Ca, K)4(Al, Si)6Si 6O24(Cl, CO3, SO4). Two common end-member s are called marialite (Na4ClSi9Al3O24) and meionite (Ca4CO3Si6Al6O24). Variations in scapolite composition can be described by two independent substitutions, NaSi(CaAl)–1 and NaCl(CaCO3)–1. Twenty eight natural scapolites in the present study exhibit a range of compositions from XEqAn [(Al–3)/3] = 8%and XMe [Ca/(Na+K+Ca)] = 7%to XEqAn = 82%and XMe = 90%. Several coupled exchange reactions can be identified in some inhomogeneous samples (e.g., Na1.49SiCl0.47 [Ca1.44Al(CO3)0.43]–1, Na1.69SiCl0.58 [Ca1.55Al(CO3)0.50]–1, Na1.91SiCl0.79 [Ca1.75Al(CO3)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 XMe up to 18%is I4/m, whereas that for intermediate scapolite from XMe = 18%to at least XMe = 90%is P42/n. Under darkfield observation (g =hkl, h + k + l = odd) using a transmission electron microscope (TEM), the P42/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.

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