Study of trioctahedral mica by neutron and X-ray beams

The trioctahedral mica phlogopit and lepidolith were investigated with neutron and X-radiation. The structure of the phlogopit is distinguished by a large regularity of the distances and angles within the individual layers. The O-H distance of the overage nuclear position is 1,003(8) A. The thermal vibrations of the protons represents approximately a flattened rotation ellipsoid in c*-direction. The lepidolith exhibits a partly ordered cation distribution in the octahedral layer. The 2d position is nearly exclusively occupied with lithium whereas the 4h position is occupied statistically with Al-Li, Li-Al and Al-Al in the same proportion. As the cations of the octahedral layer are of different valency and size, a tetrahedron tilt occurs which effects a rippling of the base oxygen layer (ΔZ = 0.08 A). A Li-Al order in the tetrahedron could not be detected in either phlogopit or lepidolith. Due to the small OH content and to the only partly ordered octahedron positions of the lepidolith, the proton could not be exactly localized. (orig./LH)