CRYSTAL-STRUCTURE OF MOGANITE - A NEW STRUCTURE TYPE FOR SILICA

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 AB2 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(l), c = 10.715(2) angstrom, beta = 90.08(3)-degrees, space group I 1 2/a 1, Z(SiO2) = 12, D = 2.55 g cm-3. 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 (1011BAR)-slices through right- and left-handed quartz - which form a three dimensional framework of comer-sharing SiO4-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 I4-positions of quartz. Besides the 6- and 8-rings, the structure contains 4-rings of SiO4-tetrahedra.