Abstract
Single crystals of kyanite (Otavi-Mine, South Africa) were heated gradually up to 1500°C and investigated at room temperature by means of optical microscopy, SEM and X-ray methods. The aim of the present paper is to look for preferred orientations of precipitated mullite that possibly occur during the thermal treatment. This may give insights in the decomposition mechanism. The decomposition of kyanite proved to be strongly dependent on the furnace atmosphere. In alkali-free atmosphere mullite and vitreous silica develop with a distinct texture of mullite with six orientations that can be explained by common [A1O6]octahedra of kyanite and mullite. Lattice constants measurements suggest the composition 3 Al2O3·2 SiO2 (3∶2-mullite) for the precipitated mullite. In the presence of Na-ions corundum appears together with a series of transition phases similar to δ-,γ- and ϑ-Al2O3 with rather good orientations. Occasionally oriented cristobalite occurs. All these orientations are interpreted as due to the pseudo-cubic oxygen packing of the kyanite structure. At 1500°C all transition phases disappear and only corundum and secondary formed mullite with preferred orientations are present.
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