Smectites and kaolinite are seldom found.
Sepiolite predominates over the rest of clay minerals.
Genetic conditions which are necesary for growing fibrous minerals are revised. In our case, we have observed a straight relation between dissolution of dolomite, silicated gel formation and development of sepiolite and palygorskite fibers from it. It is situated at the top of the Miocene filling of an cratonic SW-NE basin in the southeastern part of the Tertiary Duero Basin. The age of the bed is upper Miocene. A complementary study is also made for other sections and outcrops located in the neighbourhood of the Bercimuel deposit.