All crystallochemical classifications of chlorites published so far use conventional boundaries for contents of individual cations in chlorite structure. This article deals with a way of chlorite chemistry interpretation which is based on the determination of isomorphous series whose theoretical end-members are defined. It is a general approach based on vector representation of chlorite chemistry proposed by Wiewióra & Weiss (1990). This fact allows to transform trioctahedral as well as dioctahedral chlorites into a common basis. In the submitted paper, the principal classification criterion to interpret chlorite chemistry is the composition of octahedral sheets. The paper's second part deals with the effect of chlorite chemistry and polytypism on their X-ray diffraction pattern. Calculated diffraction patterns of chlorites of different chemical compositions have made it possible to derive diagrams which in turn allowed to estimate total contents of heavy atoms (e.g. Fe) in the structure and their distribution between octahedral sheets of 2:1layers and interlayers. The estimation is based on the intensity of basal diffractions 002, 004 and 001, 003.