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

Ishii, Eiichi   ; Sanada, Hiroyuki; Iwatsuki, Teruki  ; Sugita, Yutaka ; Kurikami, Hiroshi  

In a massive siliceous mudstone at the Horonobe Underground Research Laboratory site, a "hard part" showing remarkably high unconfined compressive strength was observed at the top of the siliceous mudstone (opal-CT zone) bounding overlying diatomaceous mudstone (opal-A zone). The "hard part" corresponds to a transition zone where opal-A and opal-CT coexist. Bulk chemistry and SEM images indicate that the "hard part" results from the stronger framework/cementation by the coexistence of opal-A and opal-CT. A transition zone can be generally stronger and more brittle and, if it is deformed, have significant impact on fluid flow.