Abstract: A series of clay minerals with a 7A series of reflections has been shown to exist in the MgO-Al2O3-SiO2-H2O system. It has been demonstrated both on synthetic and natural samples that these minerals are polymorphous with the 14A-series chlorite minerals. Widespread occurrence of these minerals in nature is expected. The polymorphic relationship found clarifies the whole picture of the isomorphism of the chlorites. The problem of nomenclature of these 7A series minerals is examined and the term “septechlorites” hesitantly proposed.
On the basis of some experimental work it is shown that the currently adopted procedure for the identification of chlorites is not generally valid, and that the factors of time, temperature, composition and particle size are of crucial importance in such procedures.