Crystal structure of moganite: a new structure type for silica

Abstract

Structural characterization of the microcrystalline silica mineral moganite, using a combination of transmission and scanning electron microscopy, X-ray powder diffraction, infrared spectroscopy and chemical analyses, reveals that it represents a new AB 2 structure type which is closely related to quartz. Moganite typically contains up to 3 wt% of water which is not a constituent of the structure. Rietveld refinement of the X-ray diffraction data leads to profile R-values R=3.5%, R w =4.5%, GOF=8.0. Moganite is essentially monoclinic with a=8.758(2), b=4.876(1), c=10.715(2) A, β=90.08(3)°, space group I 1 2/a 1, Z(SiO 2 )=12, D=2.55 g cm #753 . The crystallites frequently display a triclinic superstructure which doubles the volume of the unit cell. The crystal structure can be thought of as consisting of alternating layers―resembling (1011)-slices through right- and left-handed quartz―which form a three dimensional framework of corner-sharing SiO 4 -tetrahedra. The structural principle is thus a periodic twinning according to the Brazil-law on a cell dimension scale. The topology of the quartz structure is transformed to the moganite topology if 1/6 of the Si-atoms occupies I 4 -positions of quartz. Besides the 6- and 8-rings, the structure contains 4-rings of SiO 4 -tetrahedra