Relationship between cation substitution and hydrogen-bond system in hydrous pyroxenoids with three-periodic single-chain of SiO4 tetrahedra: pectolite, murakamiite, marshallsussmanite, serandite and tanohataite

Abstract

Relationships between cation substitution and hydrogen-bond system in hydrous pyroxenoids with a twisted chain consisting of three SiO4 tetrahedra are investigated by means of chemical and X-ray single-crystal structural analysis. Minerals with the common crystal-chemical formula M3(Me1þ)M1,M2(Me2þ)2[Si3O8(OH)], Me1þ = Na, Li and Me2þ = Ca, Mn, Mg, comprising pectolite, Li-bearing pectolite, murakamiite, marshallsussmanite, serandite, and tanohataite were studied. In pectolite and murakamiite two structurally independent octahedrally coordinated sites (M1 and M2) are occupied by Ca. In marshallsussmanite, serandite, and tanohataite, the site occupancies at M1 and M2 are M1[Ca0.764(7)Mn0.236]M2[Mn0.910(8)Ca0.090], M1[Mn0.758 (4)Ca0.242]M2[Mn1.0], and M1[Mn0.87(2)Ca0.13]M2[Mn0.943(9)Mg0.057], respectively. The Ca and Mn contents primarily influence and distances and hence the unit-cell parameters. The a, b, and c dimensions increase linearly with Ca content. Moreover, high Li concentration at M3 reduces the cell dimensions. The incorporation of smaller Liþ ions for larger Naþ ions at M3 modifies the coordination polyhedron due to the shift of the atomic position, and it decreases the coordination number of M3. The oxygen atoms at O3 and O4 host the hydroxyl group, and both oxygen sites may act as donor and acceptor of the hydrogen bond. With increasing Mn content the O3···O4 distance reduces from 2.487(3)A in murakamiite to 2.447(7)A in tanohataite. A short O3···O4 distance also facilitates disordered hydrogen bonds. The hydrogen-bond system in pyroxenoids with three-periodic single-chain of SiO4 tetrahedra depends on (1) the Na and Li position at the M3 site and (2) the O3···O4 distance.