The biotites of the crystalline massif of Catalonia (Spain) are weathered into a complex assemblage of clay secondary
minerals. This is mainly formed by hydrobiotites (Iv) with very different compositions. The K2O of these phyllites changes
from 7.2 to 2.8%. After this stage, the evolution is characterised by the genesis of protochlorites (Sm-C), then chlorites.
Illites are less abundant in the studied profi les; while kaolinites are uncommon. The diversity of secondary minerals can be
observed on a very small scale, often a centimetre and sometimes in one crystal of biotite only. A hydrobiotite - mineral
close to chlorite- chlorite continuum occurs especially in the retromorphosis zone, not subject to weathering and these
minerals are inherited in the saprolite formations. However, some chlorites can be the consequence of superfi cial