Chemistry and symmetry on crystal growth of scapolite

Abstract

Scapolite, [(Na,Ca,K)4Al3(Al,Si)3Si6O24(Cl,CO3,SO4)], is an aluminosilicate mineral found in a wide variety of metamorphic rocks. Several scapolites show remarkably sector zoning and concentric oscillatory zoning. In the present study, sector-zoned samples and its morphology were carefully examined from the view point of growth rate and a degree of disequilibrium. The results suggest that systematic compositional differences between non-equivalent sectors can be due to not only growth rate anisotropies, but also different surface structures. Moreover, electron diffraction and micro- structural analyses were also performed. Systematical extinctions of reflections in diffraction patterns suggest that the symmetry of scapolite with intermediate composition is mostly P42/n. However, another P-lattice phase (P4/m) was confirmed in a small part of the sector-zoned samples. An atomic ordering manner of the P4/m phase is quite different from that of a P42/n phase. Surface structure not only plays an important role in the differential partitioning of elements but also influences the ordering of atoms.