Young’s modulus of illitic clay in the temperature region of quartz transition

Quartz in traditional ceramics brings about microcracking during firing due to its modification transition at 573 °C. The resulting microcracks are usually studied after firing by optical methods. Therefore, the exact temperature at which the microcracking occurs is unknown, and hence, the underlying conditions cannot be identified. In this study, in situ measurements of Young’s modulus (YM) and acoustic emission (AE) were used for studying cracking in the temperature region around the α ↔ β quartz transition. Experimental samples were prepared from natural clay (36% illite, 10.5% kaolinite, 5% chlorite, 25% quartz, 11% K-feldspar, and 3.8% carbonates) using plastic extrusion. The samples were subjected to linear heating (up to 900 °C, 1000 °C, and 1100 °C) and cooling. Quartz transition was manifested by a local minimum of YM at around 570 °C during cooling. This minimum was more pronounced for higher firing temperature, i.e., when a higher fraction of glassy phase was developed. A strong correlation between YM and AE was observed, giving evidence of the involved cracking. Heating and cooling of the fired samples revealed a hysteresis in YM, caused by a reversible quartz transition. These results imply that not only the intrinsic properties of individual phases but also microcracks significantly influence YM.