Summary: The incorporation of impurities in tridymites taken from a used silica brick have been studied by means of a high-resolution microprobe, X-ray fluorescence, X-ray diffractometry, and optical microscopy. The bulk impurity content of the brick changes strongly from the hot zone (4.7 wt%) to the colder part of the brick (7.6 wt%), indicating material transport along the temperature gradient. The transport medium for migration processes is probably a melt occurring in narrow veins between large tridymite crystals at high temperatures. The average impurity contents of homogeneous tridymite single crystals are 0.49 wt% in the hot zone of the brick and 0.81 wt% in the cold zone. Al₂O₃, TiO₂, and Na₂O are main impurity constituents; the tridymites do not contain significant amounts of Fe₂O₃ or CaO. The a parameters of tridymites decrease by about 0.26% from 4.9837 Å to 4.9709 Å from the hot zone to the cold zone of the brick and the c dimension shows a smaller decrease (0.11 %) from 8.2023 Å to 8.1933 Å.