COMPOSITION AND I4/M-P42/N PHASE TRANSITION IN SCAPOLITE SOLID SOLUTIONS

Abstract

Scapolite is a metamorphic aluminosilicate mineral that can be described by the general formula (Na,Ca,K)(4)(AI,SI)(6)Si6O24(CL,CO3,SO4). Two common end-members are called marialite (Na4ClSi9Al3O24) and meionite (Ca4CO3Si6Al6O14). 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 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., Na1.49SiCl0.47 [Ca1.44Al(CO3)(0.43)](-1), Na1.69SiCl0.58[Ca1.55Al(CO3)(0.50)](-1), Na1.9iSiCl0.79[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 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 dark-field observation (g = hkl, h + k + l = odd) using a transmission electron microscope (TEM), the P421n 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.