A MINERALOGICAL STUDY OF ROCK FRACTURE PATTERNS UNDER THERMAL LOADING

Understanding the mechanism for fracturing rock under thermal stress could have important implications in mining, petroleum, geothermal, and civil engineering. In the ancient times, fire setting was a widely used mining method in which rocks were heated with fire then cooled by water flushing. This heating weakened the rock and allowed excavation using only basic tools. The decrease in rock strength is assumed to be the result of fractures generated by thermal stresses opening in the rock. However, few studies have been conducted on the mineralogical changes that occur during the fracturing process, and almost all of the previous studies were conducted on granite.

The objective of this study is to compare the results of induced thermal loading (by microwave) on rocks of different mineralogical compositions. Based on the maximum temperature reached and the duration of exposure to irradiation, the various minerals in the rock will respond differently, in terms of degrees of fracturing and rates. The study has tested three types of rocks: granite, granodiorite and quartzite. Each rock type was heated according to the same protocol. By varying the time of exposure the maximum temperature reached was controlled.

Results of mechanical tests (UCS and p-wave velocity measurements) show a decrease in rock strength up to 47% for an exposure of 180 seconds. The mineralogy and fracture patterns of treated samples were examined with optical microscopy and scanning electron microscopy. Net differences in the cracking patterns (fracture dimensions and morphologies, and intra- versus inter-grain fractures) were documented in the different rock types. They are likely related to thermo-physical properties such as the coefficient of thermal expansion, heat capacity and thermal diffusivity. Future work will extend this study to other types of rocks with different mineralogy (e.g. limestone, gold-mineralized quartzite), and the results will be compared to rocks that have undergone natural thermal loading (e.g. skarn). This research could help explain how rocks fail during natural thermal events, which has important implications for forming fluid pathways.