This research is devoted to closely related problems on iron silicate minerals: their crystal structure, chemical composition, geological background and history. The investigation was carried out by microscopy, chemical analysis, X-Ray diffraction, X-Ray fluorescence and differential thermal analysis. All the mineral specimens from Chamoson and related areass are oomposed of iron silicates with several specific structural types always present. These specimens can be divided into: A. the ooliths in which a ferrochlorite, a nontronite and a ferrokaolin are the main constituants, with other compounds as limonite and siderite; B. the matrix surrounding the ooliths, which may be an original deposit or, in some cases, an alteration product of disrupted ooliths. This ground mass contains ferrochlorite and ferrokaolin. The SiO₂ content is generally high, allowing the presence of nontronite and the possibility of silica as an amorphous minor component. Although X-Ray diffraction work on a mixture of ferrokaolin and ferrochlorite is hampered by the large number of reftections which am common to both, it is reasonable to conclude that both types are present. There is some evidence that an intermediate structure may be present, as a mid-way kaolin-chlorite structure, due to the migration of Si0₄ tetraedra between the two very similar structural types. In general, the main minerals and their structures are in reasonable agreement with silicates found in oolithic ironstone minerals elsewhere and differences of chemical composition can be accounted for, by the conditions of deposition and subsequent geological history, without any need for new principles of origin of those mineral types.