Abstract
Microcrack distribution patterns on the microscopic scale and splitting planes in Inada granite and Kurihashi granodiorite were examined using a fluorescent technique and image analysis. Using the fluorescent technique, microcracks which could not be detected under natural light were visualized under ultraviolet light. Characteristics of the microcracks such as orientation and total length per unit area were measured by image analysis. Microcracks observed in the specimens were divided into three groups, intracrystalline cracks, intercrystalline cracks and grain boundary cracks. Inada granite was characterized by three weak planes intersecting perpendicularly with each other, the rift plane, grain plane and hardway plane in the order of brittleness, while Kurihashi granodiorite showed no distinctive weak plane.
In the Inada granite, the rift plane was dominated by the predominant orientation of intercrystalline cracks the total length of which was shorter than that of intracrystalline cracks and grain boundary cracks, and the grain plane was coincident with the orientation of intracrystalline cracks. The grain boundary cracks showed no leading orientation. The numbers of intercrystalline cracks due to the rift plane in Kurihashi granodiorite were much smaller than that in Inada granite. However, Kurihashi granodiorite is estimated to contain the second rank of hidden splitting plane, because a predominent orientation of intracrystalline cracks and grain boundary cracks was found.
