As clay minerals represent the most abundant mineral phases that form under surface and near-surface environments of the Earth, they are of key importance for unravelling the processes of weathering and diagenesis. The classic approach of most studies is to characterize the type and abundance of minerals formed in paleo-soil and rock assemblages of the geological record and to attempt to recognize their origin. In this presentation, a geochemical approach is outlined which uses mineral phase chemistry to search and match a database containing well-studied mineral varieties of known composition and origin. The database currently consists of >150 samples from published studies covering ca. 39 types of clay minerals, including pure and mixed-layered phases. Most have been well purified and contain low concentrations of contaminating minerals. The method uses the most abundant elements (Si, Al, Mg, Fe, K, Ca, Na and Ti; expressed in oxide %) of any unknown clay mineral phase to find the best match in the databank using a correlation function. The database is designed to use either geochemical analyses (e.g. XRF, ICP-methods) of separated and purification monominerallic clay fractions (if available) or EDX-determinations of particle chemsitry by election microscopy (SEM, TEM). Additionally, a Clay Mineral Composite is presented, representing the average composition of all clay minerals phases in the database following normalization to 100%. Water or hydroxides ions that prove difficult to reliability quantify are not included. Compared to an average continental crustal composition, the CMC has notably higher Al, Mg, Fe and K and lower Si, Ca, Na and Ti. This pattern is discussed in terms of the underlying leaching-precipitation reactions that form clay minerals and links to global scale processes are highlighted. A number of examples are also given on how to use the CMC database as a tool for studying alteration processes relevant to continental weathering and diagenesis in a range of geological and climatic environments.