Abstract
Orthoericssonite from the Hijikuzu mine, Iwate Prefecture, Japan, (Ba0.7Sr0.3) (Mn2+1.4Fe2+0.6)(Fe3+0.9Ti0.1)Si2O7(O, OH)2, is orthorhombic, space group Pnmn, with unit cell parameters a=20.230(5), b=6.979(2), c=5.392(2)Å and Z=4. The crystal structure has been determined based on three-dimensional Patterson and Fourier syntheses and refined to R=0.054 for 1626 independent reflections.
The structure contains composite sheets each built up of a pair of quasi-silicate sheets consisting of Si2O7 groups and square pyramids formed by oxygen atoms about Fe3+, intercalating between the pair a sheet of octahedra formed by oxygen atoms about Mn2+. The composite sheets are separated by (Ba, Sr) atoms. The orthoericssonite structure can be derived from a monoclinic subcell, which has dimensions a′=10.19, b′=6.979, c′=5.392Å, β 96°45′ and P2⁄m, after polysynthetically twinning them on {100}, with a glide reflection having the glide component of b⁄2 as twin operation. If the subcells are juxtaposed on {100}, with a glide of b⁄2, stepwise in one direction, the ericssonite cell is obtained. The relationship between orthoericssonite and ericssonite thus offers an example of cell twinning.
The minerals containing quasi-silicate sheets can be structurally classified in terms of different combinations of the stackings of various types of quasi-silicate sheets and intersheet cations or groups.
