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Abstract. Microcrystalline opal-CT and opal-C were investigated by 298i MAS NMR and 298i {1H} cross polarisation MAS NMR spectroscopy, X-ray small angle scattering, X-ray powder diffraction and infrared absorption spectroscopy. The results are compared with those for non-crystalline precious opal (opal-AG), non-crystalline hyalite (opal-AN), moderately disordered cristobalite and with well ordered low-cristobalite and low-tridymite.Opal-C is confirmed to be strongly stacking disordered low-cristobalite with about 20 to 30% probability for tridymitic stacking. More extensively stacking disordered opal-CT does not contain detectable domains of low-cristobalite or low-tridymite. The stacking sequence is close to 50% cristobalite and 50% tridymitic. The local order decreases with increasing stacking disorder, so that the structural state of microcrystalline opals lies between cristobalite, tridymite and non-crystalline opals.

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The structure of strontium barium niobate crystals SrxBa1-xNb2O6 is comprehensively studied in the whole range of the tetragonal tungsten bronze phase (x=0.32-0.82) using both powder and single-crystal X-ray diffraction measurements. Unit-cell parameters, density, site-occupancy factors and interionic distances show an explicit composition dependence which can be consistently explained using simple model calculations. The temperature dependence of the unit-cell parameters exhibits a remarkable anisotropy in a broad temperature region below the phase transition temperature. This proves that the electrostrictive contribution to the thermal expansion plays an important role in strontium barium niobate.

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 c10sely related to quartz. Moganite typically contains up to 3 wt% of water which is not a constituent of the structure. Rietveld refmement of the X-ray diffraction data leads to profile R-values R = 3.5%, Rw = 4.5%, GOF=8.0. Moganite is essentially monoc1inic with a = 8.758(2), b = 4.876(1), c = 10.715(2) A, ß = 90. 08(3)", space group I I 2/a I, Z(Si02) = 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 altemating layers-resembling (lOTl)-slices through right-and left handed quartz-which form a three dimensional framework of comer-sharing Si04-tetrahedra. The structural principle is thus a periodic twinning according to the Brazil-Iaw 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 14-positions of quartz. Besides the 6-and 8-rings, the structure contains 4-rings of Si04-tetrahedra.

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