Behavior of Sillimanite Crystals in Molten Glass

Abstract

The sillimanite grains, consisting of the aggregation of fibrous crystals are generally included as a minor constituent of heavy minerals in quartz sand which are produced from Seto region, Aichi prefecture. The content of the grains is very low. The glass stones caused by the presence of sillimanite grains, however, are commonly found in the bottle glasses manufactured from the quartz sand. The reaction between sillimanite crystals and molten glasses was studied by the use of cullets of bottle glass and rectangular rods of sillimanite aggregations. The rods were prepared by grinding fibrous and tablet crystal aggregation of sillimanite. Thin sections were made from the reacted samples. The optical analysis, EPMA and X-ray diffraction analysis were used to observe the reaction zones. The structure of reaction zones was a function of reaction temperatures. The diffusion of sodium ions and change of colour were found in the reaction zones at low temperature. The needle crystals of corundum were observed at high temperature. The rate constant for sillimanite decomposition was calculated from the data of reaction test. The apparent activation energy for the decomposition of sillimanite was calculated to be 135kcal/mol. The sillimanite glass stones were confirmed by the comparison of structure found in glass stones with that in reacted sillimanite crystals. The behavior of sillimanite crystals in a tank furnace has been discussed from the structure of glass stones. Most of the sillimanite crystals, found in glass stones seem to decompose in the low temperature zone estimated to be about 1300°C.