Mineral assemblages formed in Oxford Clay fired under different time-temperature conditions with reference to brick manufacture

Time-temperature-transformation (TTT) diagrams have been determined for hig h-lime (10.2%) and moderate-lime (5.2%) bulk raw materials from the Peterbo rough Member of the Oxford Clay Formation of King's Dyke [TL 525 297] and O rton [TL 517 293] brickworks. Study of the chemistry and mineralogy of the brickclays provides the starting point for 79 firing experiments on each ra w material, at temperatures between 800 and 1100 degreesC, and for firing t imes at maximum temperature from 15 minutes to 336 hours. Quantified X-ray diffraction of each fired briquette enables individual TTT diagrams to be c onstructed for each phase. Finally, a summary diagram shows the distributio n of mineral phase assemblages in time-temperature space. The original mine ralogy (quartz, illite, kaolinite, chlorite, calcite, aragonite, K-feldspar , albite, pyrite, gypsum, anatase and apatite) changes on heating to a mixt ure of calc-silicates (anorthite, melilite and pyroxene) with quartz, hemat ite, anhydrite and glass in commercial bricks and also wollastonite and cor dierite when longer firing times and temperatures are used. The mineralogic al changes can be followed in quantified equations using the actual composi tions and quantities of the phases. The calc-silicates are produced by lime (from calcite) reacting with the clay minerals or their breakdown products such as metakaolinite. The glass has granitic affinities. The data provide a case for considering shorter commercial firing times.