[en] Search is going on for geological evidence of natural smectite clay materials that have been exposed to conditions that are similar to those radioactive in repositories. Cases in which heating to 90 degree C or more for long periods has taken place, are of particular interest. The report describes two bentonite layers, one of Miocenic age located at central Sardinia (Busachi), and the other of Ordovician age, forming a basal stratum of southern Gotland, (Hamra), Sweden. They both serve as excellent examples of the survival potential of montmorillonite-rich clays. The more than 10 m thick Sardinian bentonite bed was very significantly heated when the magma moved in and covered it. The upper meter was heated to more than 200 degree C for several days, while at more than 4 m depth, the temperature did note exceed 80 degree C. The test show that the smectite content was not reduced to less than 60 percent in any part of the layer sequence, while slight cementation was caused by precipitation of heat-released silica in the uppermost layer. The 0.3 m thick bed on Gotland is presently located at 515 m depth. Various investigations indicate that it has been exposed to an effective pressure of 300 MPa and a temperature of 110 degree C for several million years due to burial under almost 3 km of Devonian sediments. The content of smectite is around 25 percent of the bulk material, and 30-40 percent of the clay fraction. Illite appears to have been neoformed in small voids of the smectite matrix and the identified apparent I/S material is suggested to consist of mixed-layer minerals with hydrous mica and Ca or Na locked in instead of K, which would be the conventional interpretation. The earlier developed alteration model appears to be valid and it is extended in the present report on the basis of the findings. (28 illustrations, 9 tables)