Sudoite, Al-rich Li-Mg-bearing chlorite, clinochlore and kaolinite as alteration products of biotite in Visean K-bentonites from Belgium

Optical microscopy, XRD, EPMA and ion microprobe techniques have been used to study the alteration products of biotite flakes found in some Visean K-bentonites of the Namur-Dinant basin, S Belgium; alteration has induced bleaching and swelling of the flakes. The former biotites are now partially or completely colourless with low birefringence, and contain chlorites or kaolinite. Two types of Al-rich chlorite were identified: sudoite and a Li-Mg-bearing chlorite chemically intermediate between sudoite, cookeite and donbassite; clinochlorite is rarely present. The Li content of the bulk rock (Li (sub 2) O up to 0.28 wt.%) is considered to be a geochemical tracer for a magmatic origin.