During the fine comminution of clay minerals and layered silicate materials with relative large volume of specific surface area appreciable changes in chemical and minerological compositions can be observed. In this paper is described the mechanochemical investigation of conventional clay minerals from „Lenti” region (Hungary), having natural BET specific surface larger than 35-40 square metres per grams. The laboratory experiments made by author had shown an intensive reduction of specific surface volume at the beginning of grinding, further relatively slow growth and reduction of BET and Langmuir surfaces were observed. The content of mineral Muscovite–2M1 – (K, NH4Na)Al2(SiAl)4O10(OH)2 – has decreased and the contents of Quartz – SiO2 – and Illite-1M (NR) – KO.7Al2(SiAl)4O10(OH)2 – have increased considerable depending on grinding time. Increasing the milling time the amount of Albite – NaAlO.91Si3O8 – and other mineral components were also not stable, but their content have varied not so strong as Muscovite–2M1 or Quartz and Illite–1M. The examined clay minerals had a certain proportion of amorphous particles as well, but in this contribution only the changes in proportions of crystalline particles are described.