Silurian bentonites from Sweden and the eastern Baltic are correlated using XRD analysis of sanidine composition. Identifying the same bentonite in different sections has constrained correlations between the conodont, graptolite and chitinozoan zonations. Three bentonites correlate the Cyrtograptus murchisoni Zone with the upper part of the Pterospathodus amorphognathoides Zone and both the Lower and the Upper Pseudooneotodus bicornis conodont biozones at their type locality. Thus the base of the Llandovery–Wenlock boundary stratotype can be correlated with a level in the upper part of the Cyrtograptus murchisoni graptolite Biozone. Identifying the Y Bentonite (Gotland, Sweden) in eastern Baltic drill-cores shows that the base of the Eisenackitina lagena chitinozoan Biozone is close to the base of the K. patula conodont Biozone, which is coeval with an upper part of the C. rigidus graptolite Zone. Sanidine data from the Grötlingbo Bentonite fit well with previous correlations. Some bentonites from Gotland with distinct sanidine compositions and bulk geochemistries have not yet been identified elsewhere.