The author of this note has repeatedly emphasized the important role of ferruginous chlorites in the composition of the ores of a number of tin deposits. However, as prospecting practice shows, the existing information is still clearly insufficient. At present, there are sufficient grounds to assert that ferruginous chlorites are specifically characteristic minerals for a large group of tin deposits belonging to the so‑called cassiterite‑sulfide formation. For example, in almost all ore districts of the vast northeastern metallogenic province, we now know representatives of the “chlorite” variety of cassiterite‑sulfide deposits. The value of chloritization as a prospecting indicator is further enhanced by the fact that chlorite‑rich deposits are almost indisputably among the highest quality concentrations of tin, both in terms of reserves and ore grade. In any case, the proportion of deposits of industrial interest in this group is very high. Thus, although ferruginous chlorites are not exclusive to tin deposits, their value as a prospecting indicator for tin ores remains almost in full force.