Mechanical strength of the transition zone at the boundary between opal-A and opal-CT zones in siliceous rocks

In a massive siliceous mudstone at the site of the Horonobe Underground Research Laboratory in Hokkaido, Japan, a “hard part”, having a very high unconfined compressive strength, occurs at the top of siliceous mudstone (opal-CT zone) at the boundary with the overlying diatomaceous mudstone (opal-A zone). Bulk chemical data does not indicate additional silica cementation or differences in mineralogy in the “hard part” compared with the remainder of the siliceous mudstone. The “hard part” corresponds to a transition zone between opal-A and opal-CT, where the two phases coexist. SEM images show that microcrystalline opal-CT precipitates within pore spaces in the pre-existing, intricate diatom-supported framework of the transition zone. Below this zone, the framework is replaced by aggregations of relatively fine and equigranular opal-CT. The strength of the “hard part” probably reflects the strong framework/cementation due to the coexistence of opal-A and opal-CT. Consequently, the transition zone between opal-A and opal-CT is generally strong and brittle, and if it tectonically deformed it could act as a fluid conduit.